Experienced eco-farmers have a far more advanced and complex understanding of fertilizer use than the simplistic NPK approach that dominates their industrial ag counterparts — but learning from and working with nature as a partner rather than an adversary is a never-ending process, and there are few better teaching tools than learning from mistakes. With that in mind, Acres U.S.A. asked some of the leading consultants in soil fertility and biological agriculture, “What are the top five mistakes you have observed in terms of fertilizer application?” Their responses present a wealth of information and considerations that every biologically correct grower will want to contemplate as they develop and refine their own fertilizer management programs.
Gary Zimmer Gary Zimmerheads Midwestern Bio-Ag, a manufacturing and consulting company that provides productsand services for sustainable agriculture.He is also an on-farm consultant, managesthe company’s learning center, and is apartner in Otter Creek Organic Farm. Heis the author of The Biological Farmer,the consultant’s bible for eco-growing.
Applying a fertilizer that is too soluble or too insoluble for the soil conditions is a common problem. A good example is a calcium. Many farmers overdo lime on soil with a neutral or high pH where not very much of that lime will become available to plants. Another problem I see is when farmers apply a fertilizer that is not balanced, so there are too many nutrients and not enough of others. There are four factors I like to see in a good fertilizer: a blend of soluble to slow-release nutrients; a balance of nutrients; concentration (putting fertilizer in the right place); and low pH.
2. Wrong amount.
Overdoing some nutrients will interfere with the uptake of others. For example, applying too much soluble nitrogen can wash available calcium out of the soil. Land that has been depleted of nutrients due to past farming practices and crop removal can have poor production if too little fertilizer is applied. Equipment calibration problems also fall into this category: if farmers miscalibrate their spreader they can get too much or too little fertilizer on their crop.
3. Wrong place.
If soluble fertilizer is placed on top of the ground it can volatize or erode, and those nutrients are lost. Placing too much soluble fertilizer or a fertilizer with a high salt index next to the seed can inhibit root growth or dry out the roots. Another problem I see is bulk spreading a starter fertilizer that would be better placed down the row where the crop can access it.
4. Wrong time.
A common mistake in timing is applying nitrogen in the fall when there isn’t a growing crop there to capture that nitrogen. Another timing issue I often see is applying a highly soluble fertilizer to a hay crop in the spring, which interferes with crop quality.
5. Not following a consistent program.
Some farmers are hit and miss with their fertilizer program. They’ll apply calcium, sulfur and boron some years but not others, when really those nutrients should be applied every year. If a farmer has a limited budget, they need to look at their major constraints and decide where to spend their dollars to do the most good for the crop and the soils. They should set up a program and determine the best time to take soil tests, then make their soil correctives and determine when and how much crop fertilizer to apply.
1. Test to find what nutrients are deficient and add them. 2. Apply soluble calcium, sulfur, and boron on all forages and legumes. 3. Use a crop fertilizer placed properly with a balance of all nutrients for all crops grown.
Neal Kinsey has been called a “consultant’s consultant.” Through the in-depth courses provided by his company, Kinsey Agricultural Services, Inc., he has trained hundreds of consultants and sophisticated growers in the methodology of soil element balancing utilizing cation exchange capacity. He is coauthor of the book Hands-On Agronomy and lectures frequently around the world.
1. Too many nutrients are only considered for their short-term effects and not the full benefit for both the life in the soil to do its work of digesting locked-up elements and the full benefit to the nutrient content of the plants being grown. The soil is the plant’s stomach. If we expect the plants we grow to supply our nutritional needs, then we must use the materials that aid its digestive system.
2. You can’t manage what you can’t measure. Test and properly feed the soil in order not to force the plant to take up elements it does not need instead of those it does, thus creating problems with excesses and deficiencies.
3. Failure to understand that when you add a nutrient to the soil something else will be lost if that material is to remain in the soil and available for plant uptake. Thus, adding too much of anything will assure there is not enough of something else.
4. Trying to average whole fields for fertilizer applications regardless of obvious differences in soil content and plant growth, assuming that one mix will provide all that is needed. For every difference you can observe in a field, there will generally be one or more different nutrient-availability problems that can be detected. If it is large enough to treat separately, supply the right fertilizer for that soil’s needs.
5. Failure to seriously study, learn and strive to apply what the real laws of soil fertility require to do the right job. Start small if it requires major changes, but determine to follow through on a nutrient-feeding program for that soil, testing and fertilizing accordingly, for at least three years. Consider the results each year, but make the final judgment by giving the program time to make the changes in the soil that matter most. Even then, when properly tested and fertilized accordingly, the soil and its production should continue to improve for years to the extent that such a program measures and provides the proper nutrients for the soil to feed the plants being grown there.
Phil Wheeler, Ph.D.
Dr. Phil Wheeler is an expert in blending Albrecht-method soil fertility balancing, Reams-style agronomy, and subtle-energy techniques including radionics and field broadcasters. His company, Crop Services International, provides advanced soil testing, technology, and products to enable farmers and growers of all sizes and types to improve the health of their soils, crops, and animals.
1. The biggest and oldest mistake made is not liming because of neglect, lack of knowledge, or the mistaken concept of pH as the basis for liming rather than percent base saturation. Since calcium is the prime nutrient in all living systems, this mistake has caused immeasurable loss of production and incredible amounts of economic and environmental damage when rescue chemistry had to be applied. The fact that calcium is not biologically available (as indicated by LaMotte soil tests) to the extent needed for nutrient-dense food under conventional farming techniques adds to the problem. The lack of fungi, in general, prevents the “holding” of calcium in the root zone.
2. Using high applications of nitrogen and potassium (from KCl) to push/ force a plant to grow instead of creating healthy biological soils. The excess N shuts down the natural processes of the cycle that fix atmospheric nitrogen. The KCl interferes with soil biology. Excess N also interferes with the most basic function of nutrient exchange involving mycorrhizal fungi. The fungi exude sugars to soil bacteria that exchange the sugars for macro-and micronutrients back into the plant. Again, the loss of the fungi or their function does not allow for nutrient-dense food.
3. Using highly concentrated, processed phosphates that tie up quickly with soil calcium, making it unavailable to the crop, and shut down mycorrhizal fungi. See numbers 1 and 2 for explanations.
4. Assuming that plants grow from nutrients, rather than the energy produced by the nutrients, the biology, the sun, the moon, the lightning, and the cosmic forces. Many growers don’t realize how much oil-based energy went into the production of those NPK fertilizers and that this energy is released back into the soil. A grower may also assume that larger amounts of this high-energy material will result in greater yields. That can be true, up to a point, but since this source of energy is tied to fossil fuel and overpowers free, natural sources of energy, the system is unsustainable. It also destroys soil carbon instead of sequestering it, resulting in the weakening of one of the most valuable sources of national wealth, our nation’s soil.
5. Assuming that just using starters every year is cost-efficient, when in fact it is very inefficient if you have major or minor mineral shortages that go uncorrected. Each major or minor mineral deficiency can cause loss of yield. About 99 percent of soil tests coming through our lab are severely short of boron, and about 95 percent are short of Zinc. Boron begins the whole process of moving silica and calcium into a crop. Zinc is responsible for growth functions. By planting each year without replacing or supplementing the crop in the start, a grower meets Einstein’s definition of “insanity”, i.e., doing the same thing over and over again and expecting a different result. Test your soil and replace your missing minerals!
Mike Amaranthus, Ph.D.
Dr. Mike Amaranthus has published more than 80 scientific papers on soil and fungi and has conducted research and presented invited scientific papers in England, Italy, Spain, Costa Rica, Australia, New Zealand, China, Mexico and Canada. His awards include the Secretary of Agriculture’s Highest Honors Award for scientific achievement in 1996. Dr. Mike is an associate professor (adjunct) at Oregon State University and the president of Mycorrhizal Applications, Inc. Farmers can reduce their fertilizer inputs and costs by managing the timing of fertilizer application and encouraging microbial life in the soil. When fertilizer availability is timed with plant nutritional needs and root activity, then fertilizer use efficiency is greatly improved. Microbes are like little sacks of fertilizer that store and slowly release nutrients when plants need them the most. Use carbon inputs and inoculants to improve microbial populations and activity. Remember, it is the microbial activity that makes phosphorus available, so don’t waste money on phosphorus fertilizers that get tied up immediately. Avoid overwatering and erosion, which result in fertility losses into surface and ground waters. The five most common mistakes would thus be
Jon Frank is a principal in the lab/ consulting group International Ag Labs, where he helps growers through phone consultations, fertility recommendations, and product development.
1. Not applying enough nitrogen in organic grain production. Organic grain producers are notoriously squeamish about applying nitrogen. Our standard recommendation for corn is 50 gallons of 5-1-1 liquid fish and 100 pounds Chilean nitrate. Cost is the biggest concern.
2. Trying to fix a calcium deficiency with NPK. Much of production agriculture focuses only on NPK, and consequently, most soils are low on available calcium. Most farmers don’t appreciate the value calcium brings to the volume of harvest. As a result, calcium is not properly addressed while production is pushed mostly with nitrogen and potassium.
3. Organic gardeners fertilize soils with far too much compost or manure. This is a classic problem with garden-scale growers. Since people know compost is good for the soil, most put on 100 times too much. The result is cumulative poorer quality, insect pressure, and dropping Brix — all because gardeners put on what they have plenty of rather than what the soil needs.
4. Not understanding the supreme role calcium plays as a soil amendment and crop nutrient. Calcium is what all other elements in the soil react against to create energy. Having the proper level and ratio of available calcium guards against wasting nitrogen that can be dissipated from the soil into the atmosphere. Adequate soil calcium is needed just as much by soil biology as by plants.
5. Trying to use biology to break loose locked-up nutrients rather than actually applying the needed nutrients. This is a classic approach taken by people “under the influence” of one too many compost tea applications. We have seen many farmers totally bankrupt phosphate levels, all the while stimulating soil biology to break loose more phosphates. To say that all any soil needs in biology and stimulation because all soils are well endowed with plenty of nutrients is a big mistake. Nature just doesn’t work that way. A better approach is to apply biology, stimulate biology, and apply moderate amounts of the nutrients in short supply.
The number of farmers in Australia has fallen 30 per cent in the last 20 years, with more than 10,000 farming families leaving the agricultural sector in the last five years alone. This decline is ongoing. There is also a reluctance on the part of young people to return to the land, indicative of the poor image and low income-earning potential of current farming practices.
Agricultural debt in Australia has increased from just over $10 billion in 1994 to close to $60 billion in 2009 (Fig.1). The increased debt is not linked to interest rates, which have generally declined over the same period (Burgess 2010).
The financial viability of the agricultural sector, as well as the health and social wellbeing of individuals, families, and businesses in both rural and urban communities, is inexorably linked to the functioning of the land.
There is widespread agreement that the integrity and function of soils, vegetation, and waterways in many parts of the Australian landscape have become seriously impaired, resulting in reduced resilience in the face of increasingly challenging climate variability.
Agriculture is the sector most strongly impacted by these changes. It is also the sector with the greatest potential for a fundamental redesign. The most meaningful indicator for the health of the land, and the long-term wealth of a nation, is whether the soil is being formed or lost. If soil is being lost, so too is the economic and ecological foundation on which production and conservation are based.
The soil carbon sink
In July 2009, the Portuguese government introduced an AUD$13.8 million soil carbon offsets scheme based on dryland pasture improvement, compliant with Article 3.4 of the Kyoto Protocol. The scheme will pay an estimated 400 participating farmers to establish biodiverse perennial mixed grass/legume pastures (upwards of 20 species) to improve soil carbon, soil water holding capacity, and livestock productivity in an area of approximately 42,000 hectares (Watson, 2010).
The Portuguese scheme has been designed to comply with Kyoto’s strict criteria of additionality and permanence. Coordinator of Project Extensity and Terraprima project leader, Professor Tiago Domingos, has calculated that the area of agricultural land in Portugal amenable to soil carbon offsets could collectively sequester more than the current Portuguese national emissions deficit under existing Kyoto arrangements (Watson 2010).
The Mediterranean-type climate of central and southern Portugal is very similar to that in many parts of south-eastern, southern, and south-western Australia. The Portuguese Terraprima data illustrated in Fig.2 show that under sown perennial pasture, soil organic matter increased to a level of 3% over 10 years, from a starting point of 0.87%.
The Portuguese soil carbon offsets project aims to sequester 0.91 million tonnes of CO2 from 2010 to 2012 (Watson 2010). This equates to the sequestration of 10.85t CO2/ha/yr.
In addition to the carbon payments they receive, participating Portuguese farmers are reported as “enjoying the environmental spin-offs of greater biodiversity, higher soil fertility, higher water infiltration rates, less erosion, less desertification, fewer fires, fewer floods, improvement in water quality, less dependence on concentrated feed for their herds in protracted dry periods and better milk and meat quality” (Watson 2010).
US study on soil carbon sequestration rates under perennial grassland
Recent research by the United States Department of Agriculture (Liebig et al. 2008) investigated soil carbon sequestration under a perennial native grass, switchgrass (Panicum virgatum) grown for the production of cellulosic ethanol.
Despite the annual removal of aboveground biomass, low to medium rainfall, and relatively short growing season, the USDA-ARS research, averaged across 10 sites recorded average soil carbon sequestration rates of 4t CO2/ha/yr in the 0-30 cm soil profile and 10.6t CO2/ha/yr in the 0-120 cm profile (Liebig et al 2008).
The best performing site was at Bristol, where soil carbon levels increased by 21.67 tonnes in the 0-30 cm soil profile over a 5 year period. A soil carbon increase of 21.67t C/ha equates to the sequestration of 80t CO2/ha.
At the three sites where carbon was measured to 120 cm, the USDA research found relatively high sequestration rates below 30 cm. The sequestration rate was higher for the 30-60 cm increment than for the 0-30 cm increment (18.2t CO2/ha vs 16.5t CO2/ha, respectively). A possible interpretation is that the deeper the sequestration, the greater the likelihood that the carbon is protected from oxidative and/or microbial decomposition.
There were virtually no ‘biomass inputs’ to the soil in these trials, as all aboveground material was removed for ethanol production. This suggests the liquid carbon pathway (Jones 2008) as the primary mechanism for soil building.
Carbon trading in the real world
The recent demise of the Federal Government’s proposed Carbon Pollution Reduction Scheme provides an opportunity to reflect on the true meaning of a carbon-based economy.
For some time, analysts have tipped carbon to become the world’s most traded commodity. The reality is that it has been the world’s most traded commodity for millennia.
A great variety of life forms require liquid carbon – referred to in the scientific literature as ‘dissolved organic carbon (DOC) – for their growth and reproduction. The growth of trees, crops, and pastures, for example, requires the transport of dissolved carbon via sap within the plant; animal growth is dependant on the digestion of carbon-containing foods and the transport of dissolved carbon to cells via the blood; the formation of topsoil is dependent on photosynthesis and the transport of dissolved carbon, via a microbial bridge, from plants to soil.
Carbon is the currency for most transactions within and between living things. Nowhere is this more evident than in the soil. Here, carbon is king. Mycorrhizal fungi, which are totally dependant on liquid carbon from green plants, trade this carbon with colonies of bacteria located at their hyphal tips in exchange for macronutrients such as phosphorus, organic nitrogen, and calcium, trace elements such as zinc, boron, and copper, and plant growth stimulating substances (Killham 1994, Leake et al. 2004).
By means of an extraordinary physiological process known as ‘bidirectional flow’ nutrients are transported to roots at the same time as liquid carbon moves through fungal hyphae in the opposite direction (Killham 1994, Leake et al. 2004). Indeed, mycorrhizal roots are significant sinks for carbon, transferring as much as 15 times more carbon to soil as adjacent nonmycorrhizal roots (Killham 1994).
The impoverishment of agricultural soils
Mycorrhizal fungi and associative bacteria are very strongly inhibited by excessive soil disturbance and the high levels of water-soluble phosphorus and nitrogen commonly used in modern agriculture (Killham 1994, Leake et al. 2004). Where soils have been subjected to cultivation and/or the application of MAP, DAP, superphosphate, urea, or anhydrous ammonia, the suppressed mycorrhizal colonization of plant roots significantly reduces carbon flow. The structural degradation of agricultural soils, accompanied by mineral depletion in food, has largely been the result of the inhibition of this natural carbon pathway.
When carbon supply is limited by the loss of the primary pathway for sequestration, the physical, chemical, and biological functions normally performed by healthy soil are markedly reduced.
Historical levels of soil carbon
Noted Polish explorer and geologist, Sir Paul Edmund [Count] Strzelecki, traveled widely through the colonies of south-eastern Australia during the period 1839 to 1843, collecting minerals, visiting farms, and analyzing soils. One of the questions Strzelecki posed was, what factors determine soil productivity? He collected 41 soil samples from farmed paddocks of either high or low productivity. The analyses revealed that the most important determinant of soil productivity was the level of soil carbon (measured as organic matter in Strzelecki’s day).
Of the 41 samples analyzed, Strzelecki (1845) found …
The top 10 soils in the high productivity group had organic matter levels ranging from 11% to 37.75% (average 20%). The lowest ranking 10 soils in the low productivity group had organic matter levels ranging from 2.2% to 5.0% (average 3.72%)
The soils with the highest organic matter levels also had the highest moisture-holding capacity, with an 18-fold difference incapacity to hold moisture between the lowest and the highest (Strzelecki 1845).
Strzelecki’s data indicate that organic matter levels in the early settlement period were around five to ten times higher than in many soils today. The soil test data from Strzelecki is consistent with the writings of first settlers, who described soils in the early settlement period as soft, spongy, and absorbent. The 1840s journal of George Augustus Robinson, for example, contains numerous references to the extremely fertile and productive soils encountered by pastoralists in the mid-1800s (Presland 1977).
Soil carbon and soil moisture
In addition to enhancing nutrient availability, carbon performs many other functions in soil, including the maintenance of soil porosity, aeration, and water-holding capacity.
Glenn Morris (Morris 2004) extensively researched the water holding capacity of humus (an extremely stable form of soil carbon) and concluded that within the soil matrix, one part of soil humus could, on average, retain a minimum of four parts of soil water.
From this relationship, it can be calculated that an increase of 16.8 liters (almost two buckets) of extra plant available water could be stored per square meter in the top 30 cm (12”) of soil with a bulk density of 1.4 g/cm3, for every 1% increase (in absolute terms) in the level of soil organic carbon. This equates to 168,000 liters of water that could be stored per hectare, in addition to the water-holding capacity of the soil itself (Jones 2006).
The flip side is that the same amount of water-holding capacity will be lost when soil carbon levels fall. Low soil moisture and low levels of soil organic carbon go hand in hand.
Soil organic carbon levels in many areas have fallen by at least 3% (in absolute terms) since the time of European settlement, This reduction in soil carbon content represents the LOSS of the ability of soil to store around 504,000 liters of water per hectare.
Mycorrhizas and water
It is well known that mycorrhizal fungi access and transport nutrients in exchange for carbon from the host plant (Killham 1994, Leake et al. 2004). What is less well known is that in seasonally dry, variable, or unpredictable environments (that is, most of Australia), mycorrhizal fungi play an extremely important role in plant-water dynamics.
Mycorrhizal fungi can supply moisture to plants in dry environments by exploring micropores not accessible to plant roots. They can also improve hydraulic conductivity by bridging macropores in dry soils of low water-holding capacity (such as sands). In these situations, external wicking along the hyphae is of greater importance than cytoplasmic flow (Allen 2007). Mycorrhizal fungi can also increase drought resistance by stimulating an increase in the number and depth of plant roots.
Soil carbon and soil nitrogen
Aside from water, nitrogen is frequently the most limiting factor to crop and pasture production. It is one of the great ironies of agriculture that the atmosphere is around 78% nitrogen, but not one single molecule is directly available to plants. There are approximately 78,000 tonnes of nitrogen gas sitting above every hectare of land. Apart from small accessions via lightning, this nitrogen cannot be accessed without a microbial bridge.
Most nitrogen-fixing bacteria – be they free-living in the rhizosphere, confined to nodules on plant roots, contained in aggregates bound by the hyphae of mycorrhizal fungi – or existing as endophytes in plant leaves or stems – derive their energy from liquid carbon fixed during photosynthesis.
Adding water-soluble nitrogen in the form of urea, anhydrous ammonia or nitrate, destabilizes the plant-soil ecosystem by reducing the activity of mycorrhizal fungi and free-living N-fixing bacteria (Killham 1994). The presence of high levels of water-soluble nitrogen in soil sends a signal to plants to reduce the supply of liquid carbon to microbial symbionts, effectively inhibiting the microbial associations that would otherwise supply atmospheric nitrogen for free.
This contradicts the widely promoted belief that nitrogenous fertilizer must be added in order for stable soil carbon to form. Indeed, the opposite is true (Khan et al. 2007, Larson 2007, Mulvaney et al. 2009).
Soil test data show that as soil carbon levels increase in microbially active soils, availabilities of P, K, S, Ca, Zn and B commonly increase, while levels of nitrate-nitrogen are often reduced.
If plants are mycorrhizal, they don’t require nitrogen in a mineralized form, that is, in the form of nitrate or ammonium. In order to transport mineralized nitrogen, mycorrhizal fungi must first convert it to glutamate, a process that represents an energy cost. For this reason, nitrogen is preferentially transported in an organic form, generally as amino acids such as glycine and glutamine (Leake et al. 2004).
Utilization of organic nitrogen by mycorrhizal fungi closes the nitrogen loop and prevents soil acidity, as well as prevents volatilization of nitrogen to the atmosphere and leaching to aquifers, rivers, and streams. Changes to soil chemistry and nitrogen dynamics in microbially balanced soils also reduce the abundance of ‘weedy’ species such as annual ryegrass, capeweed, mustard weed, and thistles. The germination of these species is stimulated by the ready availability of nitrate nitrogen.
Soil as a methane sink
Wetlands, rivers, oceans, lakes, plants, decaying vegetation (especially in moist environments such as rainforests) – and a wide variety of creatures great and small – from termites to whales, have been producing methane for millions of years. The rumen, for example, evolved as an efficient way of digesting plant material around 90 million years ago.
Ruminants including buffalo, goats, wild sheep, camels, giraffes, reindeer, caribou, antelopes, and bison existed in greater numbers prior to the Industrial Revolution than are present today. There would have been an overwhelming accumulation of methane in the atmosphere had no sources and sinks been able to cancel each other over the past millennia.
Although most methane is inactivated by the hydroxyl (OH) free radical in the atmosphere (Quirk 2010), another source of inactivation is oxidization in biologically active soils. Aerobic soils are net sinks for methane, due to the presence of methanotrophic bacteria, which utilize methane as their sole energy source (Dunfield 2007). Methanotrophs have the opposite function to methanogens, which bind free hydrogen atoms to carbon to reduce acidosis in the rumen. Recent research undertaken by Professor Mark Adams, Dean of the Faculty of Agriculture at Sydney University, has found that biologically active soils can oxidize the methane emitted by cattle carried at low stocking rates (Cawood 2010). The highest methane oxidation rate recorded in the soil to date has been 13.7mg/m2/day (Dunfield 2007) which, over one hectare, equates to the absorption of the methane produced by approximately one livestock unit (LSU).
In Australia, it has been widely promoted that livestock is a significant contributor to atmospheric methane and that global methane levels are rising. However, there is no evidence to suggest that methane emissions from ruminant sources are increasing. Indeed, it would seem there has been no clear trend to changes in global methane levels, from any source, over recent decades.
The increase in global methane levels from 1930 to 1970 was due to emissions from the production, transmission, and distribution of natural gas (Quirk 2010). There was a tenfold increase in the use of natural gas through the 1960s and 1970s. The source of many of the natural gas emissions, such as leakages from the Trans-Siberian pipeline, have since been rectified (Quirk 2010). Measurements over the last 25 years show concentrations of atmospheric methane are merely exhibiting natural variation, with no significant trends in any direction (Fig.3).
There is therefore no scientific basis for selectively targeting ruminants for a ‘methane tax’, or worse, interfering with this natural process. Farming in ways that enhance, rather than inhibit, soil biological activity, would improve the capacity of agricultural soil to act as a methane sink, helping balance the greenhouse equation. The issue with today’s industrialized approach to agriculture is that methanotrophic bacteria are chemically sensitive. Their activities are reduced by nitrogenous fertilizers, herbicides, pesticides, acidification, and excessive soil disturbance (Dunfield 2007).
Soil carbon and human health
The nutritional status of soils, plants, animals, and people has fallen dramatically in the last 50 years, due to losses in soil carbon, the key driver for soil nutrient cycles. Soil health and human health are more deeply connected than many people realize. Food is often viewed in terms of quantity available, hence ‘food scarcity’ is not seen as an issue in Australia. However, food produced from depleted soils does not contain the essential trace minerals required for the effective functioning of our immune systems.
Routine premature deaths from degenerative conditions such as cardiovascular disease and cancer have become prominent when they were once relatively uncommon. The cancer rate, for example, has increased from approximately 1 in 100, fifty years ago, to almost 1 in 2 today. The effectiveness of the human immune system has been compromised by increased exposure to more and more chemicals coupled with insufficient mineral density in food.
The low nutritional status of many basic food items is highlighted in data from the UK. Depletion in the level of minerals in vegetables for the period 1940-1991, for example, shows copper levels reduced by 76%, calcium by 46%, iron by 27%, magnesium by 24%, and potassium by 16% (Thomas 2003). Deficiencies in plants translate through to deficiencies in animals. A piece of steak now contains only half the amount of iron that it would have contained 50 years ago (Thomas 2007).
Vitamin and mineral deficiencies in food indicate that the symbiotic relationship between plants and soil microbes, whereby minerals are exchanged for liquid carbon, has been disrupted.
The best national health policy would be a national soils policy. But we don’t have one.
Our hospitals are over-filled and our health system is struggling to cope with illnesses that are highly correlated to the lack of essential vitamins, minerals, and trace elements in our diet. The availability of these nutrients is determined to a large extent by the integrity of the soil food-web and the effective functioning of the microbe bridge, which in turn are dependent on active soil sequestration of liquid carbon.
Food labeling and a ‘Soil Integrity Index’
Food choices can have very significant effects on the kind of food produced and how it is produced. Currently, it is not possible for consumers to choose foods high in minerals, grown on healthy soils, as there is no labeling for food quality.
It is proposed that a ‘Soil Integrity Index’ with index parameters of
i) level of microbial diversity
ii) changes to soil carbon content and
iii) soil water holding capacity
be used as the basis for a food labeling system.
The labels would need to be simple, with perhaps a star system (as in one, two, or three stars). If a food labeling mechanism was in place, Australia’s largely city-based population could use food choices to improve not only the health of their families but also the function and resilience of agricultural soils, thereby actively participating and supporting biology-friendly farming.
The future landscape
The challenge for the future prosperity of Australian agriculture is to convert soil from its current status as a net source of carbon to a revitalized state as a net carbon sink. Agricultural research tends to focus on conventionally managed crop and pasture lands where intensive use of agrochemicals has dramatically reduced the number and diversity of soil flora and fauna, including beneficial microbes such as mycorrhizal fungi. As a result, the potential contribution of microbial symbionts to agricultural productivity has been greatly underestimated (Allen 2007).
Building soil carbon does not require adding biomass to the soil. While crop stubbles and mulch are important for protecting soil from wind and water erosion and buffering temperature extremes, their contribution to soil carbon is limited by eventual decomposition to CO2.
The first step to restoring soil function is ‘do no harm. A simple change from high-analysis N and/or P fertilizers to biological products such as compost extract, worm leachate (vermiliquid), milk, seaweed extract, and/or fish emulsion, applied as a seed dressing and/or a post-emergent foliar spray, will support microbial diversity, increase plant photosynthetic rate, increase the flow of liquid carbon to soil and enhance humification.
As the soil chemistry adjusts and nitrogen is converted to an organic form (freely available to mycorrhizal fungi but not to annual weeds) the incidence of pests, weeds, and diseases that are stimulated by low levels of microbial diversity and high rates of water-soluble nitrogen, will decline. As a result, there will be less reliance on the use of pesticides and herbicides that reduce the ability of soil to act as a sink for carbon, nitrogen, methane, and moisture.
Changing the face of agriculture
Since 1960, global food production has doubled. At the same time, the soil resource on which food production is based has become seriously degraded. The impoverishment of agricultural soils through depleted levels of biological activity and reduced carbon flow poses a greater threat to human existence than climate change. In many regions of Australia, the effects of lower than average rainfall over the past decade have been compounded by the loss of soil resilience and reduced moisture-holding capacity (Fig.4).
It has been calculated that in the next 50 years, the planet will need to produce as much food as it has in the entire history of humankind. The way we produce that food will require a radical departure from business as usual.
At the beginning of this paper, it was noted that the level of agricultural debt in Australia had increased almost 6-fold over the last 15 years. The amount of money invested by the farming community on non-biological inputs increases every year. Many of these products inhibit microbial diversity, preventing natural carbon flow to soils. Cessation of carbon flow reduces soil integrity, the mineral density in food, and human health. It also prevents the processes of humification and topsoil formation from operating to any significant extent. The end result is even greater expenditure on agrochemicals in attempts to control the pest, weed, disease, and fertility ‘problems’ that ensue.
The statement that small farmers need to ‘get big or get out’ overlooks the fact that profit is the difference between expenditure and income. In years to come, we will perhaps wonder why it took so long to realize the futility of trying to grow crops in dysfunctional soils, relying solely on increasingly expensive synthetic inputs.
Economic development is only sustainable if it strengthens, rather than depletes, natural resources.
The soil’s ability to produce nutrient-dense, high vitality food – which after all, is agriculture’s real purpose – depends on appropriate management. Enhancing the natural flow of carbon to soils will result in increased microbial diversity, improved nutrient cycles, enhanced soil water-holding capacity, greater resilience, improved catchment health – and a more satisfying, profitable future for farmers.
The longer we delay undertaking regenerative changes to land management based on biology-friendly farming practices that rebuild carbon-rich soils, the more soil carbon and soil water will be lost, exposing an increasingly fragile agricultural sector to escalating production risks, rising input costs, and vulnerability to climatic extremes.
It’s time to move away from depletion-style, high emission, chemically-based industrial agriculture and get serious about grass-roots biologically based alternatives.
The future of Australia depends on the future of our soil – and our willingness to look after it.
Rebuilding soil productivity via the restoration of natural carbon flow and the sequestration of stable soil carbon is the only means of saving agriculture’s bacon – and ensuring a future for human society as we know it.
Invisible to the naked eye, soil fungi bring nutrients and water to roots, too.
IF YOU ASKED 1,000 people what part of the plant is responsible for uptake of moisture and nutrients, 999 would say it’s the roots, says Larry Simpson, director of education and training for Mycorrhizal Applications in Grants Pass, Ore. “It’s actually the mycorrhizae — or fungi — that live on the roots and in the soil that are the primary means for bringing nutrients and water into the plant through its roots,” Simpson says. “The root does anchor a plant, but it’s the mycorrhizae that become the main system to absorb water and nutrients from the soil. “The roots function as a backup system in case the fungi are lost.”
Mycorrhizal fungi are living organisms that naturally grow on the roots of about 95% of all terrestrial plant species, says Simpson, adding that most land plants around the world, including crop species, will form a symbiotic association with these beneficial fungi. Dr. Mike Amaranthus, Mycorrhizal Applications’ chief scientist, isolated and propagated spores of these naturally occurring fungi to create effective inoculants, Simpson says. The company offers granular, powdered, and liquid products containing concentrated mycorrhizae spores. Simpson says it’s important to differentiate between mycorrhizal inoculants and other inoculants used in agriculture. “Mycorrhizae are beneficial, symbiotic fungi,” he says. “Rhizobium bacteria are the active organism in an inoculant specifically used on legume crops. They enable legume plants to produce plant-available soil nitrogen from atmospheric nitrogen.
“Mycorrhizae perform functions that are altogether different from those of rhizobium bacteria. The most important functions that mycorrhizae perform are improving the host plant’s ability to absorb nutrients and water.” No-tillers typically inoculate corn, soybean, and wheat seed with the company’s powder formulation or apply the liquid in the furrow, Simpson says. The powder can be applied with the seed or banded with dry fertilizer.
The powder containing the mycorrhizae spores clings to the seed, especially those of small grains like wheat that have microscopic hairs. Some no-tillers apply the liquid to seed using a cement mixer or by spraying it on seed as it passes over a conveyor belt, Simpson explains. Amherst, Wis., no-tiller Matt Hintz and Bickleton, Wash., direct-seeder Steve Matsen have both tried the mycorrhizae inoculant.
In 2009, Hintz ran a trial, splitting his corn planter with untreated seed and corn treated with the granular mycorrhizae inoculant. The treated corn looked healthier last summer than the untreated corn, says Hintz, who no-tills corn, soybeans, and winter wheat.
Hintz was under the gun last fall to finish harvesting before snow made fields impassable, so he didn’t weigh corn treated with the inoculant and compare it with the untreated corn.
But Hintz says other no-tillers told him the inoculant increased soybean yields 10% to 15%, or about 3 to 5 bushels per acre. That’s good, he says, considering soybean yields range from 20 to 50 bushels per acre on the light ground in his area. Hintz says no-tillers who used the inoculant on corn reported yield increases of up to 40 bushels per acre.
“It sounds like the inoculant is working,” he says. “It sounds like it’s paying for itself.” Treating corn with the inoculant cost about $7.50 per acre, Hintz says.
Hintz worried last spring that the granules would hurt his vacuum planter. “It seemed like it was a little bit abrasive,” he says. “It felt gritty, like a really fine sand.” While his no-till planter didn’t have any problems with the dry formulation, Hintz switched to liquid this year to avoid any potential problems. There are two ways no-tillers can treat seed with the liquid inoculant, he says. They can use a planter box treatment or ask a distributor to treat the seed. “I think either way will work, but you will have more even covering if someone treats the seed for you,” Hintz says. “I plan on using the inoculant again this year. “I definitely want to do some yield trials with it. I wished I had weighed it last fall.” Washington direct-seeder Steve Matsen tried the inoculant the last 2 years on hard red spring wheat. He belongs to a group of no-tillers in Washington, Oregon and Idaho that sells its Shepherd’s Grain wheat flour to artisan bakeries and university food services that value the environmental benefits of no-till. Matsen used the granular formulation of the inoculant because it was the least expensive and easy to use. “I just dropped it in with the seed,” he says. “It doesn’t activate until the seed is sprouted. “I thought it did a great job and I think I got a yield bump. But I think I had healthier plants, especially in areas with problem soils. It was an educational process for me. I learned how much mycorrhizal activity we had.”
Matsen says that tests on a field he planted with inoculant-treated wheat indicated the fungi had colonized 67% of the roots. “That is a fantastic rate,” he says. “Some fields indicated a presence of unknown mycorrhizae suppressing the applied inoculant. Those fields have recovered on their own.” Matsen is not sure he will need to use the inoculant every year. He may target problem areas in fields, like ridge tops that are lower in organic matter and higher in clay content or have a little higher pH measurement. “Direct-seeders have become aware of how vital mycorrhizae are,” Matsen says. “Dr. Jill Clapperton showed us how crucial they are to a wheat producer. “No-till enhances the mycorrhizae base in the soil.” Other no-tillers in Shepherd’s Grain are considering using the mycorrhizal inoculants, Matsen says. “We feel good that no-till allows mycorrhizae to repopulate. It’s crucial for us to assess the biological health in the soil,” he says, adding that no-tillers need to be more sophisticated in analyzing their soil samples and need to know what organism are active in the soil.
Matsen and other no-tillers in Shepherd’s Grain are becoming more interested in how soil biology and health can affect the quality and nutritional characteristics of their wheat flour. “Can we push some nutrients through the crop from the soil?” Matsen asks. “Mycorrhizae fits right in there. “We know it makes more phosphorus available to the plant from the soil. We believe there’s a nitrogen benefit, too.”
Simpson says he understands why many no-tillers may be skeptical about the benefits of using the inoculant containing the concentrated mycorrhizae spores. “If I knew nothing of this, I would think it’s too good to be true,” he says. “Farmers tend to be a ‘show me’ group. “We encourage farmers to try this on a limited scale — whatever is affordable and measurable. If they are planting a 100-acre field, we like them to try it on 10 acres and compare the results.” Mycorrhizae do much more than pull water and nutrients into a plant’s roots.
As you may have noticed, some producers and manufacturers of mycorrhizal inoculant products are currently giving mixed messages on how they represent the mycorrhizae content of their finished goods. This seems to be causing some confusion, and this confusion is not good for the horticultural industry. Mycorrhizal technology is already complex to understand, and the confusion between “spore” and “propagule” in mycorrhizal inoculant labeling is making it difficult for some consumers to understand the differences between products and truly compare apples to apples when assessing mycorrhizal products. With these mixed messages, buyers are forced to sort through the confusion, and in some cases, this causes growers, landscapers, and turf managers to give up on the idea of utilizing mycorrhizae altogether.
The purpose of this article is to attempt to clarify this issue and allow the industry to fairly judge the mycorrhizal inoculant products available to the industry based on their merits in a true side-by-side comparison, based on industry-approved labeling guidelines and definitions.
Clarity in Labeling
Probably the best place to start sorting this out is by discussing what is listed on the label. If a mycorrhizal manufacturer wants to be able to sell their product in the United States and Canada, they need to get the label approved and inspected by 53 regulatory agencies. These agencies represent all fifty states, the District of Columbia, Puerto Rico, and the country of Canada. Since its start in 1946 the Association of American Plant Food Control Officials (AAPFCO) has been the industry organization that fostered cooperation between all of these North American regulators. AAPFCO works to develop labeling guidelines so manufacturers know what to include on their labels. Originally started as a fertilizer-focused group, AAPFCO now includes soil manufacturers and non-EPA or Environment Canada biological products.
With the increased interest in biological products, an AAPFCO working group was formed in 2018 to create definitions for a variety of commonly referenced terms used for both bacteria and mycorrhizae. In 2020, AAPFCO approved the definitions. Here are three of the AAPFCO-approved definitions that help to clarify mycorrhizal product labeling, and impact the content of this article.
Mycorrhizal fungi – are fungi that are capable of forming mutually beneficial symbiotic associations between the fungal mycelium and the roots of vascular plants. These fungi include endomycorrhizal fungi and ectomycorrhizal fungi.
Endomycorrhizal fungal propagule – are the structures of endomycorrhizal fungi that are capable of forming a symbiotic association with plant roots. These structures are endomycorrhizal spores and root fragments colonized by endomycorrhizal fungi.
Ectomycorrhizal fungi propagule – is a structure of ectomycorrhizal fungi that is capable of forming a symbiotic association with plant roots. These structures are spores of ectomycorrhizal fungi.
So, what do these definitions mean? The symbiotic relationship between mycorrhizae and plants is defined in the first definition. The third definition clarifies spores as the only form of propagules for ectomycorrhizae and plants.
The quest for clarity in this article begins with the second definition. Endomycorrhizal propagules are structures capable of forming symbiotic relationships with plant roots. These structures are defined as both spores and colonized root fragments.
Why Choose Propagule Products vs. Spore-Only Products?
There are three mycorrhizal structures capable of forming symbiotic relationships with plants. Hyphae are the first capable of forming that symbiotic relationship. A good example of this is in the landscape, where you treat one landscape plant through the application of mycorrhizae, and as the initial plant’s hyphal network expands, it comes in proximity with the roots of a neighboring plant. The neighbor plant can then initiate the relationship with the mycorrhizae and the symbiotic relationship will begin again. Hyphae play an important part in the spread of mycorrhizae in a multi-plant container, with turf, and in the landscape. However, since they are short-lived when they become hyphal fragments, they are not included, listed, or labeled as propagules by AAPFCO. This short shelf life would prevent their use in commercial products with a 2-year shelf life.
Colonized root fragments and spores are the next two types of mycorrhizal structures capable of forming symbiotic relationships with plants. Colonized root fragments and spores both play an important role in the inoculation of plants by mycorrhizae. Plants, and the mycorrhizae which they form symbiotic relationships with, dictate the balance between the production of colonized roots and actual spores. Different plant types, different mycorrhizal species, and different environmental conditions influence the breakdown between these two important types of propagules in commercial production. Root fragments represent an important part of the mycorrhizal propagules present in inoculant products because of the way the inoculum (or “active ingredient”) is produced at a commercial scale, i.e., on the roots of a host plant. These tiny root pieces contain various fungal structures including hyphae, spores, and vesicles that can be an effective means of fungal propagation and help re-establish the symbiosis with plant roots. Often, one root fragment propagule can contain many spores within it, so even though it is counted and labeled as one propagule, it can contain many spores.
Endomycorrhizal spores can be thought of as the most dormant stage of mycorrhizal propagules. Colonized root fragments can be thought of as a long-term ready-for-immediate-action version of endomycorrhizal propagules. In horticulture, we are often wanting the symbiosis period to happen ASAP. This could be during plug or liner production, or if the mycorrhizae are introduced during the growing stage of production. But as growers, of course, we want results as early as possible in the production process.
The response rate of endo mycorrhizae is also influenced by the genus and species offered in any particular mycorrhizal product. Some endomycorrhizal genera and species are quicker to respond to the plant’s root hormones or “exudates” and initiate the symbiosis than others. The response rate is also influenced by the amount of colonized root fragments ready for immediate action once exudates are released by the inoculated plant’s roots. Conditions in the rhizosphere (root-zone ecosystem) including phosphorus concentration, soil pH, soil temperature, and soil air capacity also influence all propagule germination rates and speeds.
So, if you opt for an endomycorrhizal product with fewer, or only one mycorrhizal species and only spores, you have the potential to have the slowest response rate by the mycorrhizal inoculant because the symbiosis is dependent on fewer species of mycorrhizal fungi, combined with the slower response of the spores as propagules. If you opt for a product like the endomycorrhizal inoculants, which have multiple endomycorrhizal species and contain both colonized root fragments and spores, you will have the fastest mycorrhizal symbiosis response rate based on the diversity of mycorrhizal species and propagule types offered.
The formation of the symbiosis between a plant and mycorrhizae can take up to two weeks after the application of the product. Often, the plant can start to benefit from the relationship sooner, but we generally estimate two weeks to be safe. Growers will then start to see the difference approximately two weeks after the symbiosis is completed.
Greenhouse production of plants is often at a rapid pace. Whether it is the number of weeks during propagation or the entire crop cycle, time is of the essence. If that is the case in your greenhouse, nursery, landscape, or turf planting, I suggest you opt for a product that contains multiple species and propagules that contain both colonized root fragments and spores, in order to have the most fast-acting mycorrhizal inoculant formulation, and most rapid and thorough root colonization.
Tiny Soil Organisms Provide Big Soil Health Benefits
Here’s a shocker: The next big thing in agriculture might not be big at all. According to biologists a single tablespoon of healthy farm soil may contain up to a billion assorted microbes, a mile or more of fungal filaments plus scores of various macrofauna creatures such as nematodes and arthropods. What do crop plants think of all these “little bugs” near their roots? They love ’em! Most plants convert 40 percent or more of their energy produced by photosynthesis into root exudates that actually feed and stimulate soil microbes (See Figure 1). In return, the numerous activities of these tiny soil organisms help keep the plant world running. Natural ecosystems have teemed with soil microbes for millions of years yet they remain productive and healthy despite the fact that no one irrigates, fertilizes or applies pesticides to these areas. Natural ecosystems utilize a simple and elegant system to maintain soil fertility. Plants fuel the process by providing carbohydrates (sugars), the major food source for these various microbes. The sun energizes the plants, which feed the microbes that then fertilize the soil and promote plant growth. This neverending cycle employs billions of tiny organisms to do the “heavy lifting” such as decomposing organic matter, promoting soil health, improving soil structure and storing, gathering and processing nutrients and water from the surrounding soil for plant use. This ensures that soil fertility and optimum plant-growing conditions are continued year after year. Of the incredible plethora of microorganisms contained in healthy soil, most are very short-lived so that they function as tiny time-release fertilizer units. Just 1,000 square feet of root zone in healthy soil can contain about 70 pounds of dead microorganisms, which represents nearly 7 pounds of nitrogen, 3½ pounds of phosphate, 1.4 pounds of calcium oxide, 1.4 pounds of magnesium oxide, and .28 pounds of sulfate. Obviously, the continuous release from the remains of these tiny creatures can go a long way toward improving crop production.
As world populations continue to grow and global food demands surge, a realization and shift toward sustainable and biological farming methods is underway. In recent years, worldwide fertilizer and other agricultural chemical costs have increased dramatically and savvy growers are starting to look for alternative ways to extend inputs and support crops.
One of the most important things they are finding is that these tiny soil creatures deliver very big results — enhanced nutrient release, elevated nutrient uptake, improved moisture efficiencies, augmented soil tilth — these are just some of the many benefits of living soil. By increasing yields and/or reducing costs, these “small things” offer natural and powerful solutions to some of the farmers’ most persistent and vexing challenges.
In this article, we will explore this interesting and under-appreciated aspect of plant growth and food production.
FERTILIZER FROM THIN AIR
For decades farmers have added specialized Rhizobia bacteria to the seeds of legume crops such as soybeans, field beans, peas and alfalfa that capture atmospheric nitrogen for their crops in root nodules. Scientists have researched the activities of these fertility-enhancing microbes in detail. These bacteria have the unique ability to convert nitrogen gas from the air into proteins in a process called nitrogen fixation. Eventually, these proteins biodegrade, adding essential available nitrogen fertilizers to the soil. Rhizobia and Frankia bacteria are important types of symbiotic nitrogen fixers (See Figure 2). They can produce a lot of plant-useable nitrogen and can also be used in conjunction with cover crops. The nitrogen-producing root nodules are filled with these organisms that capture nitrogen from the air and trade it to the legume plant for some of the sugars it produces. The pinkish color of the nodules comes from a compound similar to hemoglobin found in animal red blood cells. The use of Rhizobia in growing legume crops and producing soil nitrogen is a well-established practice and can be a great way to grow your own nitrogen if you have the right kind of climate and crop.
Azotobacters and Azospirillum are other types of nitrogen-fixing bacteria that are “free-living” and produce soil nitrogen without entering into a symbiotic plant relationship such as a root nodule. Much research is underway to enhance the practical role of Azotobacter and Azospirrillum in crop production. Other types of bacteria specialize in decomposing dead plant and animal proteins, releasing even more nitrogen compounds. There are still other organisms such as mycorrhizal fungi that have the unique ability to uptake nitrogen in an organic form before it is converted to an inorganic form or leached away (See Figure 3). Without these processes, nitrogen, the most important plant nutrient, would be rapidly depleted from soils, thereby diminishing both plant growth and crop yields.
Another critical nutrient needed in abundance is phosphate. While most soils contain significant amounts of phosphate, most of this is present in forms of insoluble rock phosphate (See Figure 4). Plants cannot absorb minerals unless they are dissolved in water. Therefore, rock phosphate, which is insoluble, is unavailable and useless to plants. Fortunately, numerous bacteria and fungi can convert insoluble phosphate into water-soluble forms that plants can use. In the presence of these phosphorus solubilizing bacteria and fungi, the phosphate reserves present in otherwise insoluble phosphate rocks become much more accessible to plant roots.
Researchers are working to develop mixtures of phosphate-solubilizing bacteria and fungi for farm use. Certain types of non- symbiotic bacteria (Bacillus megaterium var. phosphaticum, Bacillus subtilis Bacillus circulans, Pseudomonas striata) can free up insoluble phosphorus and either make it directly available to plants or put it into the “diffusion” stream in which nutrient ions (including P) flow through the soil toward roots. Similarly, certain fungi (Penicillium sp, Aspergillus sp) also free up phosphorus. They act indirectly on insoluble phosphorus by producing organic acids as they go about their business. These acids break the chemical bonds that tie up phosphorus, releasing some for plants.
SCAVENGING TRACE MINERALS
Some bacteria and fungi can efficiently remove or “sequester” from soils other important minerals that plants require such as iron, zinc, magnesium, copper and manganese. While bacteria and fungi are busy “gathering” these minerals for themselves, the process also makes them available to nearby plants. These organisms excrete special enzymes that unlock the chemical bonds that tightly bind these valuable micronutients in soil particles. Needless to say, the presence of such talented bacteria and fungi in the root zone is a significant benefit to plants.
While the fertility-enhancing activities of soil bacteria are very impressive, the wealth of benefits provided to plants by mycorrhizal fungi is keystone to a healthy soil. Pronounced “Mike-O-Ri-Zal,” these fungi form a close partnership with plant roots. The plant makes sugar and shares it with the fungi. In exchange, the fungi provide water and minerals to the plant. The mycorrhizal fungi grow through the soil as fine hair-like strands called hyphae, which are similar in appearance to spider webs. These strands form a network of canals that absorb water and minerals from the soil and then transport them back to the plant root. Mycorrhizal fungi are unique because they cannot feed themselves. In order to survive and grow, the strands of mycorrhizal fungi must first enter the living root tissue of a plant. Once inside the root, the fungus is allowed to absorb sugar and other compounds from the plant. The mycorrhizal fungus uses this food to build an elaborate network of absorbing strands that reach out into the soil. This network of fungal strands act effectively as ultrathin, absorbent roots. Water and minerals absorbed by this fungal network are delivered to the plant roots which is the “home base” for the fungus. Amply supplied with much-needed water and minerals, the plant grows vigorously, producing more sugar, which it continues to share with its fungal partner. Both the fungus and the plant benefit from this partnership, called a “symbiosis.” (See Figure 5a, Figure 5b). This plant-fungal symbiosis is so successful that the vast majority of the world’s terrestrial plants, over 90 percent, form a mycorrhizal relationship in their natural habitats. In fact, the fossil record shows that mycorrhizal fungi were present some 460 million years ago, around the time when plants first began to colonize dry land. Today, it is common for many farmers to use mycorrhizal fungi to increase a crops’ utilization of soil nutrients and water. These “little” things work to make a big difference in crop production in the dark, microscopic confines of a healthy soil. The individual mycorrhizal filaments, or hyphae, are approximately 1/25th the diameter of a human hair and can grow up to 18-24 inches in length (See Figure 6). These strands originate from within the root cells of the host plant, spreading and branching into the surrounding soil, greatly increasing the surface area of the root system. The most widespread type of mycorrhizal relationships are known as arbuscular mycorrhizae (also commonly referred to as “AM,” “VAM“ or “endo mycorrhizae.”). Most agricultural crops are naturally disposed to achieve optimum growth and vigor by forming this fungal relationship.
Agricultural soils often contain abundant nutrients, but the availability of these nutrients to the crop is sometimes limited. Research confirms that mycorrhizae are especially important in mobilizing phosphorus, nitrogen, zinc, iron, calcium, magnesium, manganese and other tightly bound soil nutrients. The hyphae produce enzymes that can release these nutrients from their recalcitrant chemical bonds and then transport them in soluble forms back to the crop roots. The crop plant’s uptake and utilization of fertilizers thus becomes far more efficient, often leading to significant savings in fertilizer and irrigation costs.
Water has become a highly precious resource. There are some places on earth where a barrel of water costs more than a barrel of oil. No one understands better than farmers that agriculture’s need for freshwater is not always in sync with nature’s propensity to provide it. We often see abundant, verdant vegetation in natural ecosystems without the benefit of irrigation. How do natural areas provide for such luxuriant plant growth without irrigation? One key factor is the extensive network of mycorrhizal threads attached to plant roots that thoroughly scour the soil for water. Like a sponge, they absorb water during moist periods then retain and slowly release it to the plant during periods of drought. Plant systems in natural areas generally achieve levels of drought tolerance far exceeding those found in agriculture partly due to the enormous web of mycorrhizal hyphae and specialized storage cells which protect the plant communities from extreme soil moisture deficits. Mycorrhizal filaments are so thin that they can penetrate into the tiniest of soil openings to access microscopic sources of water that are unavailable to the much thicker roots. Research confirms the importance of the mycorrhizal relationship for efficient water use and drought protection among a wide array of important crop species (See Figure 6). The declining availability of water and its ever-increasing cost are formidable issues facing today’s farmer and mycorrhizal fungi can be a powerful tool to enhance water-use efficiencies.
NOT HOME ALONE
Roots and mycorrhizal fungi produce a variety of organic compounds which fuel the activities of the other little things in the soil. Healthy soil hosts a whole complex of microscopic life-forms engaged in living, dining, reproducing, working, building, moving, policing, fighting, and dying; all these activities help the crop plants that feed them. The microbes excrete an array of important and beneficial exudates which include amino acids, organic acids, carbohydrates, enzymes, and others. Soil life such as bacteria, fungi, algae, protozoa, earthworms, and beneficial nematodes feed on and utilize these exudates. To show their appreciation for all these goodies, these tiny, beneficial soil organisms help the plants in BIG ways as noted earlier in this article. Plant growth-promoting rhizobacteria (PGPR) such as Pseudomonas fluorescens are good examples of a whole host of bacteria that aid in the synthesis of nutrients, promote root growth and thus contribute to plant nutrition. PGPR are known to directly and indirectly enhance plant growth by a variety of mechanisms: fixation of atmospheric nitrogen that is transferred to the plant; production of chemicals that chelate iron making it available to the plant root; solubilization of nutrient minerals such as phosphorus and synthesis of root enhancing compounds. Further studies of PGPR are underway and will help increase our understanding of the role these organisms play in crop production.
DOES MY FARM ALREADY HAVE BENEFICIAL SOIL ORGANISMS?
Since World War II, agronomy technology has focused primarily on the development of chemical and mechanical approaches to improving crop plant performance. Nutrient needs have been addressed using synthetic fertilizers; weed suppression accomplished by herbicides and sophisticated mechanized tillage and plant diseases controlled using an array of chemical pesticides. However, scientists now recognize that bacteria and mycorrhizal fungi are key components to optimum plant root efficiency and that plant roots in natural habitats are actually just one component in a complex “rhizosphere” of made up of many soil organisms. Certain modern agricultural practices, including some common management methods, are known to suppress the biological activity in soils. Populations of soil microbes are lost when the land is cleared and turned over during tillage. Soil fumigation, fungicide use, cultivation, compaction, soil erosion and periods of fallow are all factors that can adversely affect populations of beneficial soil organisms.
Most research on soil disturbance effects has focused on mycorrhizal fungi. Soil testing worldwide indicates that many intensively managed croplands lack adequate populations of mycorrhizal fungi. Fallow soil is first and foremost among the causes for the demise of the mycorrhizal relationship. Remember, these fungi are dependent on their host plants for sustenance and cannot survive for any extended duration without the partnership of living roots. Field preparation prior to planting usually involves thorough tilling and sometimes fumigation and generally leads to a fallow condition that, in turn, eliminates the fungi. Large-scale agricultural areas also become isolated from the beneficial mycorrhizal fungi that would, in natural ecosystems, be abundantly available to spread colonization to crop roots. Arbuscular mycorrhizal fungi, the type usually most important to agricultural plants, do not readily disperse spores via wind or water, but primarily reproduce by growing from root to root. The re-colonization of farm soil across long distances from undisturbed natural sites becomes slow and difficult. Plants cultivated in containers are also isolated from natural sources of mycorrhizal colonization. Inoculation with mycorrhizal inoculants has proven to be highly beneficial when growing container crops. A lab experienced in identifying mycorrhizal fungi can analyze feeder root samples to determine mycorrhizal population levels in farm soil or container media.
CONSIDER THE NEXT BIG THING
Products are now available that contain many species of beneficial soil bacteria and fungi. Some will pull nitrogen out of the air making it available to plants. Others contain microorganisms that solubilize phosphorus, decompose organic matter and break down toxic soil residues. And still other products contain beneficial mycorrhizal fungi that grow on the roots of plants, increasing the roots’ ability to pull nutrients and water from the soil (See Figure 7). These biological products can be applied at planting or to established plants, but they must get into the soil and, in some cases, need direct contact with the roots to work. It is also possible to inoculate ornamental, vegetable, fruit and nut crops that begin their life cycles in nurseries with beneficial soil organisms. For a variety of reasons, most crop plants propagated in greenhouses and nurseries are propagated in sterile “synthetic soil” media where they receive intensive fertilization, optimum irrigation and any necessary pesticides. While such artificial conditions often result in high plant production, these plants grown without beneficial soil organisms can be poorly adapted to the eventual harsher out-planted conditions of the open-field environment. However, nursery-grown plants already inoculated and colonized with beneficial organisms tend to establish faster and more successfully in the field. Why? Because their tiny biological allies enable them to better handle environmental stresses and take full advantage of limited soil resources. Here is one more tiny fact that is BIG: The application of a quality mycorrhizal inoculum is required only once for the life of the crop. The relatively recent development of liquid and concentrated powder inoculants makes it easier than ever to inoculate seed or in furrow. Note that the inoculum must contact the roots to activate colonization, so foliar applications will not work. Understanding the role of these tiny beneficial soil organisms and supporting the soil conditions that promote their populations are BIG steps toward achieving healthier crops, increasing yields and reducing costs. Another BIG step is the addition of these special fungi and bacteria to the root zone when planting, transplanting or restoring distressed soils. Mother Nature and her microorganism allies invested millions of years to develop that precious resource we know as good soil. Today’s successful farmer wisely rec-ognizes these helpers as the small things that are the next big thing when it comes to successful crop management.
Imagine there was a process that could remove carbon dioxide (CO2) from the atmosphere, replace it with life-giving oxygen, support a robust soil microbiome, regenerate topsoil, enhance the nutrient density of food, restore water balance to the landscape and increase the profitability of agriculture?
Fortunately, there is. It’s called photosynthesis.
The power of photosynthesis
In the miracle of photosynthesis, a process that takes place in the chloroplasts of green leaves, carbon dioxide (CO2) from the air and water (H2O) from the soil, are combined to capture light energy and transform it to biochemical energy in the form of simple sugars.
These simple sugars – commonly referred to as ‘photosynthate’ – are the building blocks for life in and on the earth. Plants transform sugar to a great diversity of other carbon compounds, including starches, proteins, organic acids, cellulose, lignin, waxes and oils.
Fruits, vegetables, nuts, seeds and grains are ‘packaged sunlight’ derived from photosynthesis. The oxygen our cells and the cells of other living things utilise during aerobic respiration is also derived from photosynthesis.
We have a lot to thank green plants for!!
Significantly, many of the carbon compounds derived from the simple sugars formed during photosynthesis are also essential to the creation of well-structured topsoil from the lifeless mineral soil produced by the weathering of rocks.
Without photosynthesis there would be no soil.
Weathered rock minerals, yes … but fertile topsoil, no.
The plant-microbe bridge
It comes as a surprise to many to learn that over 95% of life on land resides in soil – and that most of the energy for this amazing world beneath our feet is derived from plant carbon.
Exudates from living roots are the most energy-rich of these carbon sources. In exchange for ‘liquid carbon’, microbes in the vicinity of plant roots – and microbes linked to plants via networks of beneficial fungi – increase the availability of the minerals and trace elements required to maintain the health and vitality of their hosts (1, 2). Microbial activity also drives the process of aggregation, enhancing soil structural stability, aeration, infiltration and water-holding capacity. All living things – above and below ground – benefit when the plant-microbe bridge is functioning effectively.
Sadly, many of today’s farming methods have severely compromised soil microbial communities, significantly reducing the amount of liquid carbon transferred to and stabilised in soil. This creates negative feedbacks all along the line.
Over the last 150 years, many of the world’s prime agricultural soils have lost between 30% and 75% of their carbon, adding billions of tonnes of CO2 to the atmosphere (3). Losses of soil carbon significantly reduce the productive potential of the land and the profitability of farming. Soil degradation has intensified in recent decades, with around 30% of the world’s cropland abandoned in the last 40 years due to soil decline (4). With the global population predicted to peak close to 10 billion by 2050, the need for soil restoration has never been more pressing.
Soil dysfunction also impacts on human and animal health. It is sobering to reflect that over the last seventy years, the level of every nutrient in almost every kind of food has fallen between 10 and 100%. An individual today would need to consume twice as much meat, three times as much fruit and four to five times as many vegetables to obtain the same amount of minerals and trace elements as available in those same foods in 1940.
Dr David Thomas (5, 6) provided a comprehensive analysis of historical changes in food composition from tables published by the Medical Research Council, Ministry of Agriculture, Fisheries and Foods and the Food Standards Agency. By comparing data available in 1940 with that in 1991, Thomas demonstrated a substantial loss in mineral and trace element content in every group of foods investigated.
The nutrient depletion summarised in Thomas’s review represents a weighted average of mineral and trace element changes in 27 kinds of vegetables and 10 kinds of meat. Significant mineral and trace element depletion was also recorded in the 17 varieties of fruit and two dairy products tested over the same period (5).
The mineral depletion in meat and dairy reflects the fact that animals are consuming plants and/or grains that are themselves minerally depleted.
In addition to the overall decline in nutrient density, Thomas found significant changes in the ratios of minerals to one another. Given that there are critical ratios of minerals and trace elements for optimum physiological function, it is highly likely that these distorted ratios impact animal and human health and well-being (5).
Restoring nutrient density to food
It is commonly believed that the significant reduction in the nutrient density of today’s chemically produced food is due to the ‘dilution effect’. That is, as yield increases, mineral content falls. However, compromised nutrient levels are not observed in high-yielding vegetables, crops, and pastures grown in healthy, biologically active soils. Indeed, the opposite applies.
Only in rare instances are minerals and trace elements completely absent from soil. Most of the ‘deficiencies’ observed in today’s plants, animals, and people are due to soil conditions not being conducive to nutrient uptake. The minerals are present, but simply not plant available. Adding inorganic elements to correct these so-called deficiencies is an inefficient practice. Rather, we need to address the biological causes of dysfunction.
The soil’s ability to support nutrient-dense, high vitality crops, pastures, fruit, and vegetables require the presence of a diverse array of soil microbes from a range of functional groups. The majority of microbes involved in nutrient acquisition are plant-dependent. That is, they respond to carbon compounds exuded by the roots of actively growing green plants.
Most plant-dependent microbes are negatively impacted by the use of ‘cides’ – herbicides, pesticides, insecticides, and fungicides. The use of these chemicals reduces nutrient uptake, compromising the plant’s immune response and often requiring even further use of chemicals.
In short, the functioning of the soil ecosystem is determined by the presence, diversity, and photosynthetic rate of actively growing green plants – as well as the presence or absence of chemical toxins.
But who manages the plants – and the chemicals?
You guessed it … we do.
Fortunately, consumers are becoming increasingly aware that food is more than a commodity. Indeed, increased global awareness of the links between food quality, soil function, and planetary health may well prove to be a significant driver for much-needed social and environmental change (7). It is up to us to restore soil integrity, fertility, structure, and water-holding capacity – not by applying ‘bandaids’ to the symptoms, but by the way we manage our food production systems.
The soil carbon sink
Soil can function as a carbon ‘source’ – adding carbon to the atmosphere – or a carbon ‘sink’ – removing CO2 from the atmosphere. The dynamics of the source-sink equation are largely determined by land management.
Over millennia a highly effective carbon cycle has evolved, in which the capture, storage, transfer, release, and recapture of biochemical energy in the form of carbon compounds repeats over and over. The health of the soil – and the vitality of plants, animals, and people – depends on the effective functioning of this cycle.
Technological developments since the Industrial Revolution have produced machinery capable of extracting vast quantities of fossil fuels from beneath the Earth’s surface – as well as machinery capable of laying bare large tracts of grasslands and forests. Taken together, these factors have resulted in the release of increasing quantities of CO2 to the atmosphere while simultaneously destroying the largest natural sink over which we have control. The decline in natural sink capacity has amplified the effects of anthropogenic emissions.
Carbon, nitrogen, and water
When areas of intact vegetation are first cropped, good yields of high protein grain can usually be obtained without the addition of fertilizer. Over time, the replacement of a diverse ecosystem with single-species crops, the use of excessive cultivation, and the practice of maintaining a ‘bare fallow’ between cash crops result in losses of soil carbon and the deterioration of soil health. In an effort to maintain yield, more and more fertilizers, particularly inorganic forms of nitrogen, are often applied.
Rather than applying ‘more fertilizer’ the solution to deteriorating soil function lies in the adoption of management practices that increase levels of stable soil carbon. Organic carbon, organic nitrogen, and moisture-holding capacity always move together. When levels of soil carbon increase, so too do levels of organic nitrogen and the ability of the soil to infiltrate and store water.
Organic carbon holds between four and twenty times its own weight in water. In many environments, moisture availability (rather than nutrient availability) is the most limiting factor for production. Over time, improvements to soil carbon levels eliminate the need for inorganic fertilizers.
Increasing the level of stable soil carbon also has a positive effect on landscape function. Carbon is essential to the formation of water-stable aggregates that enhance soil structure, which in turn reduces run-off and minimizes erosion.
Carbon Conversion Efficiency (CCE)
Carbon Conversion Efficiency (CCE) is the percentage of carbon inputs (plant litter, animal manure, root exudates, etc) biologically converted to stable soil carbon. For a range of physical, biological, and chemical reasons, the conversion of carbon inputs into stable carbon is higher for root-derived materials than for above-ground biomass (8, 9).
An analysis of 10 stable isotope experiments undertaken in the field with roots grown in situ (i.e. no soil disturbance) found the stabilization of root-derived carbon ranged from 18 to 91%, with an average of 46%, while the stabilization of carbon derived from above-ground biomass ranged from 3 to 17% with an average of 8.3% (9). Overall, the stabilization of root-derived carbon was five times higher than that of above-ground biomass (9). The authors suggested that the stabilization of root-derived carbon could be even higher in perennial-based ecosystems than in the annual systems studied (9).
One reason root-derived materials make such a positive contribution to stable soil carbon pools is that in addition to providing a carbon source, the rhizosphere supports the free-living nitrogen-fixing bacteria and beneficial fungi essential to stabilization. Stable soil carbon is around 60% carbon and 6-8% nitrogen. Ideally, this nitrogen should come from the atmosphere. Well-aggregated soil is porous and has high rates of gaseous exchange. As soil carbon levels improve, soil structure improves and the conditions for associative biological N-fixation are enhanced, creating a positive feedback loop. When soil carbon is declining, the opposite applies. Populations of beneficial fungi fall, aggregate stability declines and the resulting poor structure limits gaseous exchange. This in turn reduces biological nitrogen fixation by free-living bacteria and hence the stabilization of carbon.
Currently, many agricultural, horticultural, forestry and garden soils are a net carbon source. That is, these soils are losing more carbon than they are sequestering.
The potential for reversing the net movement of CO2 to the atmosphere through the improved plant and soil management is immense. Indeed, managing vegetative cover in ways that enhance the capacity of soil to sequester and store large volumes of atmospheric carbon in a stable form offers a practical and almost immediate solution to some of the most challenging issues currently facing humankind.
The key to successful sequestration is to get the basics right.
Five Principles for Soil Restoration
Green is good – and yearlong green is even better Every year, photosynthesis draws down hundreds of billions of tonnes of CO2 from the atmosphere. The impact of this drawdown was dramatically illustrated in a stunning visualisation released by NASA in 2014 (10). The movement of carbon from the atmosphere to soil – via green plants – represents the most powerful tool we have at our disposal for the restoration of soil function and reduction in atmospheric levels of CO2.
While every green plant is a solar-powered carbon pump, it is the photosynthetic capacity and photosynthetic rate of living plants (rather than their biomass) that drive the biosequestration of stable soil carbon.
Photosynthetic capacity: the amount of light intercepted by green leaves in a given area. Determined by percentage canopy cover, plant height, leaf area, leaf shape and seasonal growth patterns. On agricultural land, photosynthetic capacity can be improved through the use of multi-species covers, companion cropping, multi-species pastures and strategic grazing. In parks and gardens plant diversity and mowing height are important factors. Bare soil has zero photosynthetic capacity. Bare soil is not only a net carbon source but is also vulnerable to erosion by wind and water.
Photosynthetic rate: the rate at which plants are able to convert light energy to sugars. Determined by many factors including light intensity, moisture, temperature, nutrient availability, plant species richness and the demand placed on hosts by microbial symbionts. Colonisation by mycorrhizal fungi and/or trichoderma can significantly increase photosynthetic rate. Plants photosynthesising at an elevated rate have a high sugar and mineral content, and contribute to improved weight gains in livestock. Photosynthetic rate can be assessed by measuring Brix levels with a refractometer.
An increase of around 5% in global photosynthetic capacity and/or photosynthetic rate would be sufficient to counter the CO2 flux from the burning of fossil fuels, provided the extra carbon was sequestered in soil in a stable form. This is do-able. On average, global cropland is bare for around half of every year (11). If you can see the soil it is losing carbon – and nitrogen!!
Both photosynthetic capacity and photosynthetic rate are strongly impacted by management. Leading-edge ‘light farmers’ are developing innovative and highly productive ways to keep soil covered and alive, while producing nutrient dense food and high quality fibre.
One of the most significant findings to emerge in recent years has been the improvements to infiltration, water-holding capacity and drought resilience when bare fallows have been replaced with multi-species covers. This improvement has been particularly evident in lower rainfall regions and in dry years (12).
A healthy agricultural system is one that supports all forms of life. All too often, many of the life forms in soil have been considered dispensable. Or more correctly, have not been considered at all.
2. Microbes matter!!
The significance of the plant-microbe bridge in transferring and stabilising carbon in soil is becoming increasingly recognised, with the soil microbiome heralded as the next frontier in soils research.
One of the most important groups of plant-dependent soil-building microbes are mycorrhizal fungi. These extraordinary ecosystem engineers access water, protect their hosts from pests and diseases – and transport nutrients such as organic nitrogen, phosphorus, sulphur, potassium, calcium, magnesium, iron and trace elements including copper, cobalt, zinc, molybdenum, manganese and boron – in exchange for liquid carbon. Many of these elements are essential for resilience to climatic extremes such as drought, waterlogging and frost.
When the mycorrhizal symbiosis is functioning effectively, 20-60% of the carbon fixed in green leaves can be channelled directly to soil mycelial networks, where a portion is combined with biologically fixed nitrogen and converted to stable humic compounds. The deeper in the soil profile this occurs, the better. Humic polymers formed by soil biota within the soil matrix improve soil structure, porosity, cation exchange capacity and plant growth.
Soil function is also strongly influenced by its structure. In order for soil to be well structured, it must be living. Life in the soil provides the glues and gums that enable soil particles to stick together into pea-sized lumps called aggregates. The spaces between the aggregates allow moisture to infiltrate more easily. Moisture absorbed into soil aggregates is protected from evaporation, so that soil remains moister for longer after rain or irrigation. This improves farm productivity and profit.
Well-structured soils are also less prone to erosion and compaction and function more effectively as bio-filters.
Sadly, many of the microbes important for soil function have gone missing in action. Can we get them back? Some producers have achieved large improvements in soil health in a relatively short time. What are these farmers doing differently?
3. Diversity is not dispensable!!!
Every plant exudes its own unique blend of sugars, enzymes, phenols, amino acids, nucleic acids, auxins, gibberellins and other biological compounds, many of which act as signals to soil microbes. Root exudates vary continuously over time, depending on the plant’s immediate requirements. The greater the diversity of plants, the greater the diversity of microbes and the more robust the soil ecosystem.
The belief that monocultures and intensively managed systems are more profitable than diverse biologically-based systems does not hold up in practice. Monocultures need to be supported by high and often increasing levels of fertiliser, fungicide, insecticide and other chemicals that inhibit soil biological activity. The result is even greater expenditure on agrochemicals in an attempt to control the pest, weed, disease and fertility ‘problems’ that ensue.
The natural grasslands that once covered vast tracts of the Australian, North American, South American and sub-Saharan African continents – plus the ‘meadows’ of Europe – contained several hundred different kinds of grasses and forbs. These diverse grasslands and meadows were extremely productive prior to simplification through overgrazing and/or cultivation. The variation in plant root architecture in a range of North American prairie plants is illustrated in Fig. 2.
Innovative farmers are experimenting with up to 60 to 70 different plant species to see which combinations perform best for soil restoration. Some grain and vegetable producers are setting aside up to 50% of their cash crop area for multi-species ‘soil primers’. They believe the benefits far outweigh the costs. It has been reported that two full seasons of a multi-species cover can perform miracles in terms of soil health.
However, it doesn’t need to be complicated. Something as simple as including one or two companions with a cash crop can make a world of difference. Indeed, it is becoming increasingly common to see peas with canola; clover or lentils with wheat; soybean, pigeon pea, faba beans, mungbeans or vetch with corn; flax with chickpeas; buckwheat and/or peas with potatoes … and so on. Monoculture will hopefully soon be a thing of the past.
In addition to improving soil function, companion plants provide habitat and food for insect predators. Recent research (14) has shown that as the diversity of insects in crops and pastures increases, the incidence of insect pests declines, hence avoiding the need for insecticides.
Plant production is often higher in diverse communities (15,16,17,18) with diversity reported to influence yield to a greater extent than fertilizer (15). A study on the effects of nitrogen applied at rates of 0, 100, and 200 kg N/ha/yr to 78 experimental grassland communities of increasing plant species richness (1, 2, 4, 8, or 16 species) found high plant diversity with zero fertilizer produced better yields than low diversity with 200 kg N/ha/yr (15).
Increased plant production in diverse communities is closely linked to the sequestration of both carbon and organic nitrogen (17,18,19, 20). It has been suggested that the high carbon storage capacity of remnant native prairie with ’11+’ plant species indicates there would be economic, ecological, and environmental advantages to increasing the diversity of CRP plantings, from the current 5 or 6 species, despite the higher upfront costs (20).
An aspect of plant community structure that is gaining increased research attention is the presence of ‘common mycorrhizal networks’ (CMNs) in diverse pastures, crops, and vegetable gardens. It has been found that plants in communities assist each other by linking together in vast underground super-highways through which they can exchange carbon, water, and nutrients (21, 22). Common mycorrhizal networks enhance plant vigour and improve soil health.
Beneficial saprotrophic fungi are also stimulated by plant diversity. The increased quantity and diversity of root-derived organic inputs, particularly exudates, enhances fungal biomass resulting in a significant shift in fungal-to-bacterial biomass ratios (24).
In my travels, I’ve seen many examples of monocultures suffering severe water stress while diverse multi-species crops beside them remained green (Fig. 3).
For a humorous insight into how diverse mixes of plants collaborate with soil life to rejuvenate the soil and enhance drought tolerance, see reference 25 at the end of this document.
In mixed-species plantings, warm-season grasses (such as sorghum and corn) are the most generous ‘givers’ to soil carbon pools, while broadleaf plants benefit the most from the increased availability of nutrients.
4. Limit chemical uses
The mineral cycle improves significantly when soils are alive. It has been shown, for example, that mycorrhizal fungi can supply up to 90% of plants N and P requirements (26). In addition to including companions and multi-species covers in crop rotations, maintaining a living soil often requires that rates of high-analysis synthetic fertiliser and other chemicals be reduced, to enable microbes to do what microbes do best.
Profit is the difference between expenditure and income. In years to come, we will perhaps wonder why it took so long to realize the futility of attempting to grow crops in dysfunctional soils, relying solely on increasingly expensive synthetic inputs.
No amount of NPK fertilizer can compensate for compacted, lifeless soil with low wettability and low water-holding capacity. Indeed, adding more chemical fertilizer often makes things worse. This is particularly so for inorganic nitrogen (N) and inorganic phosphorus (P). An often-overlooked consequence of the application of high rates of N and P is that plants no longer need to channel liquid carbon to soil microbial communities in order to obtain these essential elements. Reduced carbon flow has a negative impact on soil aggregation – as well as limiting the energy available to the microbes involved in the acquisition of important minerals and trace elements.
Inorganic N: The use of high-analysis N fertilizer poses a significant cost to both farmers and the environment. Only 10 to 40% of applied N is taken up by plants, the remaining 60 to 90% being lost through a combination of volatilization and leaching (27).
It is often assumed that nitrogen comes only from fertilizer or legumes. However, all green plants are capable of growing in association with nitrogen-fixing microbes. Even when N fertilizer is applied, plants obtain much of their N from microbial associations.
Farmers experimenting with ‘yearlong green’ farming techniques that incorporate high diversity are discovering that their soils develop the innate capacity to fix atmospheric nitrogen. However, if high rates of N fertilizer have been used for some time, it is important to wean off N slowly (27), as free-living nitrogen-fixing bacteria require time to re-establish.
One of the many unintended consequences of the use of nitrogen fertilizer is the production of nitrous oxide in water-logged and/or compacted soils. Nitrous oxide is a greenhouse gas with almost 300 times the global warming potential of carbon dioxide.
Inorganic P: The application of large quantities of water-soluble P, such as found in MAP, DAP, or superphosphate, inhibits the production of strigolactone, an important plant hormone. Strigolactone increases root growth, root hair development, and colonization by mycorrhizal fungi, enabling plants to better access soil P (28). The long-term consequences of the inhibition of strigolactone include destabilization of soil aggregates, increased soil compaction, and mineral-deficient (eg low selenium) plants and animals.
In addition to having adverse effects on soil structure and the nutrient density of food, the application of inorganic water-soluble phosphorus is highly inefficient. At least 80% of applied P rapidly adsorbs to aluminum and iron oxides and/or forms calcium, aluminum, manganese, or iron phosphates. In the absence of microbial activity, these forms of P are not planted available (28).
It is widely recognized that only 10-15% of fertilizer P is taken up by crops and pastures in the year of application. If P fertilizer has been applied for the previous 10 years, there will be sufficient for the next 100 years, irrespective of how much was in the soil to begin. Rather than continually adding P, it may prove more economical to incorporate P-scavenging plant species in cover and intercrop mixes as well as managing land in ways that support the soil microbes able to access ‘locked up’ soil P.
Mycorrhizal fungi are extremely important for increasing the availability of soil P. Their abundance can be significantly improved by the presence of perennial plants, the use of diverse cover crops, the inclusion of companions in cash crops, and appropriate grazing management.
5. Animal integration
A multitude of animal species was in contact with soils prior to agricultural intensification. There is no doubt that soil function is improved by their presence. The re-integration of animals into cropland can be extremely beneficial – for both the soils and the animals. Grazing multi-species covers with domestic livestock, for example, helps recover seed costs and improves both soil and animal health.
The way livestock is managed has a significant impact on soil function. In actively growing perennial pastures, it is vitally important that less than 50% of the available green leaf be grazed at any one time (Fig.1). Retaining adequate leaf area reduces the impact of grazing on photosynthetic capacity and enables the rapid restoration of biomass to pre-grazed levels. Significantly more forage will be produced during the growing season – and more carbon sequestered in soil – if pastures are grazed ‘tall’ rather than ‘short’. In addition to maintaining photosynthetic capacity through the management of leaf area, the height of pasture has a significant effect on moisture retention, nutrient cycling, and water quality.
Maintaining photosynthetic rate is also important. Higher Brix levels in pastures translate to improved feed conversion efficiency, higher average daily gain, and enhanced milk production. It is quite possible that higher Brix levels also result in higher Carbon Conversion Efficiency (CCE), given that plant roots and their exudates represent the primary pathway for stable soil carbon sequestration (8, 9, 24).
Relationship between leaf area removed and impact on roots (30):
Up to 40% leaf area removed = no effect on root growth
50% leaf area removed = 2-4% root growth inhibition
60% leaf area removed = 50% root growth inhibition
70% leaf area removed = 78% root growth inhibition
80% leaf area removed = 100% root growth inhibition
90% leaf area removed = 100% root growth inhibition
Regenerative grazing can be extremely effective in restoring soil carbon levels at depth, particularly in perennial pastures. The deeper the carbon the more it is protected from oxidative and microbial decomposition. The ‘sequestration of significance’ is that which occurs below 30cm (31).
All food and fiber producers – whether of grain, beef, milk, lamb, wool, cotton, sugar, nuts, fruit, vegetables, flowers, hay, silage, or timber – are first and foremost ‘light farmers’.
Sadly, the intensification of agricultural activity since the Industrial Revolution has resulted in significantly less photosynthetic capacity – that is, green groundcover – on the earth’s surface, while also impacting the photosynthetic rate of the groundcover that remains.
Our role, in the community of living things of which we are part, is to ensure that the way we manage green plants results in as much light energy as possible being transferred to, and maintained in, the soil battery – as stable soil carbon. Increasing the level of soil carbon improves farm productivity, restores landscape function, reduces the impact of anthropogenic emissions, and increases resilience to climatic variability.
It is not so much a matter of ‘how much’ carbon can be sequestered by any particular method in any particular place, but rather, ‘how many’ soils are sequestering carbon. If all agricultural, garden and public lands were a net sink for the carbon we could easily draw down sufficient CO2 to counter emissions from the burning of fossil fuels.
Everyone benefits when soils are a net carbon sink. Through our food choices and farming and gardening practices, we all have the opportunity to influence how soil is managed. Profitable agriculture, nutrient-dense food, clean water, and vibrant communities can be ours … if that is what we choose.
For our futures and the futures of our children and grandchildren, why not begin today to rewrite the story of soil??
Hidden from view beneath the soil surface in the farmer’s field there is a relationship between fungi and plants that is fundamental to life on the planet. Fungi can’t make their own food, they have to absorb their nourishment from living or dead organic matter. Organisms like fungi help assure the earth’s resources recycle as they should. There is one particular group of fungi that works in cooperation with important crop species. This article will shed some light on this special “farmers’ fungus” that pays big dividends. We have come to understand that in natural habitats, plant roots are a complex mixture of both fungi and plant. This relationship is called a “mycor-rhiza” which literally means ‘fungus-root’. Approximately nine out of every 10 species of plants form an association with these specialized mycorrhizal soil fungi in order to thrive. The plant needs the fungus and the fungus needs the plant. The fungus is responsible for getting the nutrients and water from the soil, and in return, it gets carbohydrates from the plant (figure 1). This is what is called a “symbiotic” relationship; one in which both plant and fungus benefit. The fossil evidence indicates that this plant/fungus relationship dates back over 460 million years.
What are they?
The body of the fungus consists of very thin strands called hyphae (figure 2). In healthy soils, these strands grow from within the root cells of the crop and spread out into the soil, greatly increasing the surface area of the root system. The most widespread type of mycorrhizal relationship are known as arbuscular mycorrhizae (also known as “endo” mycorrhizae) and are formed by most agricultural plants. These plants include most grains, vegetables, fruit and nut trees, vines and turf grasses.
What they do
The mycorrhizal relationship effect on the root system is dramatic. Most of the absorbing area of the root system is actually fungal hyphae. Hyphae are much thinner than roots or root hairs and are able to penetrate the tiniest pores in the soil. A thimbleful of healthy soil can contain miles of fungal hyphae! As a result, the efficiency of the plants’ nutrient and water uptake is increased enormously. Agricultural soil often contains abundant nutrients but availability to the crops themselves can be limited. Research demonstrates that mycorrhizae are particularly important in mobilizing phosphorus, nitrogen, zinc, iron, calcium, magnesium, manganese, sulfur and other tightly bound soil nutrients, transporting them back to the plant. This plant-fungus relationship can pay of big on the farm. Crop plants become able to absorb soil nutrients previously unavailable and utilize fertilizer inputs much more efficiently. The result is often significant savings in fertilizer costs (figure 3).
Water, water everywhere?
Agriculture’s need for fresh water is growing faster than nature can provide. It’s quickly becoming one of the key resource issues of the 21st century. How do natural areas provide for such luxuriant plant growth without irrigation? One key factor are the mycorrhizal threads attached to plant roots scouring the soil for available resources. They absorb water during periods of adequate soil moisture, then retain and slowly release them to the plant during periods of drought. Natural areas have achieved a level of drought tolerance that far exceeds agricultural areas partially because an enormous web of mycorrhizal threads act as a sponge, protecting plant communities from extreme moisture deficits. The mycorrhizal threads can penetrate into the small soil pores to access pools of water that are unavailable to the thicker roots. An extensive body of research has documented the importance of the mycorrhizal relationship for efficient water use and drought protection for a wide array of important crop species. The ever-increasing cost and declining quality of water are formidable issues facing farmers today. Today, mycorrhizal fungi can be a powerful tool for farmers seeking to improve water-use efficiency and lower irrigation costs.
Does my farm have mycorrhizal fungi?
Some modern agricultural practices reduce the biological activity in soil. Fungicides, chemical fertilizers, cultivation, compaction, soil erosion and periods of fallow can all adversely affect beneficial mycorrhizal fungi. Extensive testing of agricultural soils indicates that many intensively managed lands such as agricultural fields lack adequate populations of mycorrhizal fungi. Farming extensive acreage affects the mycorrhizal relationship in two fundamental ways. First, it isolates the crop plant from the beneficial mycorrhizal fungi available from natural settings. Secondly, it increases the need for water, nutrients, and soil structure required to sustain a healthy crop. Once lost from a farm, arbuscular mycorrhizal populations are very slow to re-colonize, unless there is close access to natural areas that can act as a source of mycorrhizal spores and hyphae to re-populate the affected area. Arbuscular mycorrhizal fungi do not disperse their spores in the wind, but rather grow from root to root. The spores do not easily move long distances back to the farm soil from undisturbed natural sites. Unfortunately, growing crops immediately adjacent to undisturbed natural ecosystems is not always an option on the modern farm.
How do I use mycorrhizal inoculants on my farm?
A farmer can enhance crop root growth, nutrition and yield, reduce irrigation and ameliorate many problems resulting from intensive agriculture by inoculating with mycorrhizal fungi. A more sustainable approach to crop establishment and growth includes using mycorrhizal fungi as an inoculant before, during, or following planting. The goal is to create physical contact between the mycorrhizal inoculant and the crop roots. They can be sprinkled onto roots during transplanting, banded with or beneath seed, used as a seed coating or watered in via existing irrigation systems. Treating seed either before or during sowing produces excellent results. Just one pound of a mycorrhizal inoculant concentrated powder can easily treat enough seed to plant one acre. The type of inoculum product and application method depends upon the conditions and needs of the crop and farmer. Generally, mycorrhizal application is easy, inexpensive, and requires no special equipment. Liquid forms of mycorrhizal inoculants are becoming very popular due to the ease of handling, mixing, storage, and their effectiveness in penetrating many soil types and treating existing plants. It is also now possible to have vegetables, fruit and nut crops which begin their life cycle in a nursery inoculated with mycorrhizal fungi. Unfortunately, most crop plants raised in nurseries are started in sterile soils and receive intensive fertilization, water, and pesticides. Although these artificial conditions can produce vast volumes of plants, they also result in non-mycorrhizal plants that are often poorly adapted to the eventual out-planted conditions on the farm where they will be subject to the harsher environment of the open field. Conversely, nursery-grown plants that have already been colonized with mycorrhizal fungi are better equipped to take advantage of soil resources and can establish rapidly and successfully in the field.
What about Fungicides?
Of course, mycorrhizae are fungi so it stands to reason that some fungicides will reduce or eliminate them from the soil and roots. Fortunately, research and experience indicates that certain types of fungicides do not adversely affect mycorrhizae. A list of common agricultural fungicides and their effects on mycorrhizae can be accessed at BioStim. Sometimes it helps to apply fungicides four to six weeks prior to the mycorrhizal treatment. Mycorrhizal inoculums may also be applied after the use of a fungicide. Follow manufacturers’ guidelines for the time required for the fungicide to “clear” the soil media.
Farm fungi pay dividends
Many mainstream agricultural markets are already benefiting from the use of mycorrhizal inoculums, and use continues to increase dramatically. Recent advancements in mycorrhizal research and application technology have made farm use of mycorrhizae easier and more cost effective than ever. The economic return for mycorrhizal inoculation can exceed its cost several-fold, not only from increased yields, but also by reduced fertilizer, and water costs. Using a mycorrhizal inoculant, Del Gates of North Dakota increased flax yields by 27%. Ron Miller’s wheat farm in Nebraska increased its yield of organic wheat by 42% by treating the seed with a mycorrhizal inoculant powder. Agronomists in California’s San Joaquin Valley documented a 20% yield increase of sorgum sudan grass at four different seeding rates following mycorrhizal inoculant treatment. Other studies have shown similar success with onions, alfalfa, melons, garlic, carrots, rice, strawberries, tomatoes, potatoes, almonds and a host of other crops where yield increases have ranged from 10 – 40%, often with reduced inputs and cost. Learning about the role of mycorrhizal fungi and the conditions that inhibit or promote their presence in the soil is the first step toward healthier crops, increased yields and lower costs. The next step is to add the fungi to the root zone when planting or transplanting and when restoring distressed soils. Good soil is a precious resource containing millions of years worth of nutrients and microorganism development. However, to be successful the farmer requires an appreciation of the “friendly fungus” that can pay big dividends.
The process whereby gaseous carbon dioxide is converted to soil humus has been occurring for millions of years. Indeed, it is the only mechanism by which deep topsoil can form.
Not only is rebuilding carbon-rich topsoil a practical and beneficial option for productively removing billions of tonnes of excess carbon dioxide from the atmosphere, but when soils gain in carbon, they also improve in structure, water-holding capacity and nutrient availability.
Understanding the soil building process is therefore of fundamental importance to the future viability of agriculture.
Building topsoil is a biological process
‘Biological carbon capture and storage’ begins with photosynthesis, a natural process during which green leaves transform sunlight energy, carbon dioxide and water into biochemical energy. For plants, animals and people, carbon is not a pollutant, but the stuff of life. All living things are based on carbon.
In addition to providing food for life, some of the carbon fixed during photosynthesis can be stored in a more a permanent form, such as wood (in trees or shrubs), or as humus (in soil). These processes have many similarities.
i) Turning air into wood. The formation of wood requires photosynthesis to capture carbon dioxide in green leaves, followed by lignification, a biological process within the plant whereby simple carbon compounds are joined together into more complex and stable molecules to form the structure of the tree.
ii) Turning air into soil. The formation of topsoil requires photosynthesis to capture carbon dioxide in green leaves, followed by exudation of simple sugars from plant roots and humification within biologically active soil aggregates. Humification is a process whereby simple carbon compounds are joined together into more complex and stable molecules. The formation of humus requires a vast array of soil microbes, including mycorrhizal fungi, nitrogen fixing bacteria and phosphorus solubilising bacteria, all of which obtain their energy from plant sugars (liquid carbon).
How can it be that trees are still turning carbon dioxide into wood, but soils are no longer turning carbon dioxide into humus?
The answer is quite simple. In order for trees to produce new wood from soluble carbon, they must be living and covered with green leaves. In order for soil to produce new humus from soluble carbon, it must be living and covered with green, actively growing plants.
Building stable soil carbon is a four-step process that begins with photosynthesis and ends with humification. Many broadacre agricultural production systems fail to build stable soil carbon at depth due to lack of sufficient photosynthetic capacity and/or the use of high rates of synthetic fertilisers or other chemicals that inhibit the plant-microbe bridge.
These factors have been overlooked in most models of soil carbon sequestration.
The ‘biomass model’
Models designed to mathematically predict the movement of carbon in and out of soils are generally based on the assumption that carbon enters soil as ‘biomass inputs’, that is, from the decomposition of leaves, roots and crop stubbles. These models provide useful estimations of soil carbon fluxes in conventionally managed agricultural soils, but fail to account for the significant levels of carbon sequestration observed in soils actively fuelled by soluble carbon.
When carbon enters the soil ecosystem as plant material (such as crop stubble), it decomposes and returns to the atmosphere as carbon dioxide. Hence the lamentation “my soil eats mulch”, familiar to home gardeners and broadacre croppers alike. While plant residues are important for soil food-web function, reduced evaporative demand and the buffering of soil temperatures, they do not necessarily lead to increased levels of stable soil carbon.
Conversely, soluble carbon channelled into soil aggregates via the hyphae of mycorrhizal fungi can be rapidly stabilised by humification, provided appropriate land management systems are in place.
The types of fungi that survive in conventionally managed agricultural soils are mostly decomposers, that is, they obtain energy from decaying organic matter such as crop residues. As a general rule these kinds of fungi have relatively small hyphal networks. They are important for soil fertility and soil structure, but play only a minor role in carbon storage.
Mycorrhizal fungi differ quite significantly from decomposer fungi in that they acquire their energy in a liquid form, as soluble carbon directly from actively growing plants. There are many different types of mycorrhizal fungi. The species important to agriculture are often referred to as arbuscular mycorrhiza (AM), [previously known as vesicular arbuscular mycorrhiza (VAM)]. The term VAM is no longer used as not all AM fungi have vesicles.
It is well known that mycorrhizal fungi access and transport water – plus nutrients such as phosphorus, nitrogen and zinc – in exchange for carbon from their living host. They also have the capacity to connect individual plants below ground and can facilitate the transfer of nutrients between species. This is one reason why above-ground diversity is important. Plant growth is usually higher in the presence of mycorrhizal fungi than in their absence.
What is less well known is that mycorrhizal fungi can play an extremely important role in humification and soil building processes.
Under appropriate conditions, a large proportion of the soluble carbon channelled into aggregates via the hyphae of mycorrhizal fungi undergoes humification, a process in which simple sugars are resynthesised into highly complex carbon polymers. Humus polymers are made up of carbon and nitrogen from the atmosphere, combined with a range of minerals from the soil. These organo-mineral complexes form a stable and inseparable part of the soil matrix that can remain intact for hundreds of years.
Humified carbon differs physically, chemically and biologically from the labile pool of organic carbon that typically forms near the soil surface. Labile carbon arises principally from biomass inputs (such as crop residues) which are readily decomposed.
Conversely, most humified carbon derives from direct exudation or transfer of soluble carbon from plant roots to mycorrhizal fungi and other symbiotic or associative microflora. It is ‘microbial carbon’ as opposed to ‘plant carbon’.
Humus can form relatively deep in the soil profile, provided plants are managed in ways that encourage vigorous roots. Once atmospheric carbon dioxide is sequestered as humus it has high resistance to microbial and oxidative decomposition.
The soil conditions required for humification are diminished in the presence of herbicides, fungicides, pesticides, phosphatic and nitrogenous fertilisers – and enhanced in the presence of root exudates and humic substances such as those derived from compost.
The biological soil environment required for humus formation is supported by farm practices that promote diverse green cover for as much of the year as climate allows. Yearlong Green Farming practices include adaptive high density short duration grazing, pasture cropping and multi-species cover crops.
Remember, photosynthesis and the ‘liquid carbon pathway’ are the most important drivers for soil building. Living hosts (green plants) provide soluble carbon and the necessary habitat for colonisation by mycorrhizal fungi.
Under appropriate conditions, 30-40% of the carbon fixed in green leaves can be transferred to soil and rapidly humified, resulting in rates of soil carbon sequestration in the order of 5-20 tonnes of CO2 per hectare per year.
In some instances, high soil carbon sequestration rates have been recorded where there were virtually no ‘biomass inputs’, suggesting that the liquid carbon pathway was the primary mechanism for soil building.
Every 27 tonnes of carbon sequestered biologically in soil represents 100 tonnes of carbon dioxide removed from the atmosphere. As a bonus, it also enables more reliable and profitable production of nutritious food.
Currently, most agricultural land is a net carbon source. That is, the soil is losing more carbon than it is sequestering. A biology-friendly approach to crop production – and carefully planned grazing of pastures and rangeland – would enable agricultural land to become a net carbon sink (that is, soil sequestering more carbon than it was losing).
If all farmland was a net sink rather than a net source for CO2, atmospheric CO2 levels would fall at the same time as farm productivity and watershed function improved. This would solve the vast majority of our food production, environmental and human health ‘problems’.
Solar isn’t just for rooftops. It builds soil too!
It may come as a surprise to many to find that in healthy soil there is a poor relationship between plant productivity and the amount of applied nitrogen (N) or phosphorus (P). Recent research undertaken by Dr David Johnson and his team at New Mexico State University (NMSU) found there are other factors of much greater importance. What are these factors? And what can farmers do to optimise them?
The NMSU researchers discovered that plant growth is highly correlated with how much life—and what kind of life—is in the soil. In fact, microbial community structure, particularly the ratio of fungi to bacteria, had significantly more influence on yield than the concentration of inorganic N or P.
Given that flourishing communities of beneficial soil microbes are the ‘key’ to plant production, what is the secret to ensuring the right microbes are present in the right amounts?
Plants. That’s right. The most important factor for promoting abundant plant growth is to have green plants growing in the soil all year round.
The plant-microbe-soil connection
You may have heard that ‘plants take from the soil‘. Nothing could be further from the truth. Observe what happens in bare soil. It dies. Then it blows or washes away. If you could ’see’ what happens around the roots of actively growing plants you would want to have as many green plants in your soil for as much of the year as possible. The NMSU researchers found that planting diverse cover crops between cash crops resulted in better yields than the use of synthetic fertilisers and that wasn‘t all. Soil tests showed that the availability of essential minerals and trace elements increased. How does it work? Carbon inputs from living plants support the microbial activity required to improve soil structure, increase macro- and micronutrient availabilities and enhance soil water-holding capacity. In turn, these factors improve plant productivity. It’s a positive feedback loop.
The NMSU research team found that as cover drop density increased, the effect became quadratic, due to the synergies between living plants and soil microbial communities. That is, 1 + 1 = 4.
It all starts with photosynthesis
The energy needed to maintain flourishing soil ecosystems begins as light. This energy must cross two bridges in order to recharge the soil battery. First, the photosynthetic bridge. In the miracle of photosynthesis, light and CO2, are transformed to biochemical energy (carbon compounds) in the leaves of green plants.
Second, the microbial bridge. In the presence of beneficial bacteria and fungi photosynthetic rate increases and carbon ‘flows’ from plant roots into soil microbial intermediaries.
If one of these bridges has been blown (e.g. no green plants or compromised microbial communities), soil health declines.
Every summer, around 22 million hectares of Wheatbelt soils lie bare across eastern, southern, and western Australia. Herbicides are commonly used to maintain the soil in a plant-free state. Bare ground and low levels of biological activity result in declining structure, reduced infiltration, poor moisture retention, inadequately buffered pH, and an open invitation to weeds.
Take a step back in time…
Most of the temperate regions currently used for crop and pasture production supported vigorous, diverse groundcover at the time of European settlement. Summers in the southern half of the Australian continent have been hot and dry for thousands of years, yet there were more summer-active than winter-active plants in the original vegetation. This is an important point. It is not ‘natural’ for the soil to be bare over summer (or winter, for that matter).
Despite successive months of summer temperatures above 100° Fahrenheit (37 °C) and little or no rain, observers of the original groundcover reported it to remain remarkably green (Presland 1977). Active growth was possible during hot dry periods because the soil had a high water-holding capacity.
After many decades of the bare ground over summer—every summer—the water-holding capacity of our agricultural soils has significantly declined. The original groundcover contained more broadleaved plants (forbs) than grasses (Lunt et al 1998). Nutritious summer-active native legumes within genera such as Lotus, Hardenbergia, Kennedia, Cullen (formerly Psoralea), Glycine, and Desmodium were once abundant in their respective endemic areas, as were many food plants used by indigenous people, including yarn daisies (Microseris). As a general rule, broadleaved plants are more important than grasses for microbial diversity and nutrient cycling.
Not surprisingly, the most palatable and mineral-dense summer-active plants quickly disappeared from the original groundcover due to unmanaged grazing.
Restoring soil function
The more closely we can mimic the structure and function of year-round species-rich groundcover, the more productive and ‘problem-free’ our agricultural enterprises will be.
If there is sufficient moisture to support summer weeds there is sufficient moisture to support a summer cover crop. Furthermore, it is generally cheaper to sow a summer cocktail than to spray weeds. The purpose of a multi-species cover crop is to restore below-ground diversity which will, in turn, restore biological soil function (natural N-fixation and P-solubilisation) and plant productivity.
The nutrient sourcing and moisture retention benefits of diverse cover crops will continue to build in successive years as soil health improves.
Examples of broad-leaved plants that can be used in multi-species summer cover crops (cocktail crops) include sunflowers, buckwheat, chickpea, sunn hemp, amaranth, cowpeas, soybean, safflower, camelina, sugar beet, squash, and lab-lab. These can be combined with a range of plants from the grass family, including pearl and proso millet, sudangrass, forage sorghum, maize, etc. Aim for at least I0 species or varieties in your mix, with more broad-leaved plants than grasses.
Summer cocktail of sunflower, maize, soybean, cowpea, camelina, sugar beet, sudangrass, pearl millet, proso millet, pasja turnip, tillage radish, sweet clover, and squash on Menoken Farm. Cover crops can be either grazed or rolled while green, prior to the sowing of the follow-on crop.
Will there be a yield penalty?
Yield penalties may be observed In crops following summer groundcover If:
i) the summer groundcover did not include a diversity of broadleaved plants (aim for more non-grasses than grasses);
ii) high rates of inorganic N (e.g. urea) or P (e.g. MAP, DAP) were applied to either the cover crop or the follow-on crop, damaging the microbial bridge. Note: Inorganic N has been applied previously, for several years in succession, N use must be reduced slowly, as populations of free-living N-fixing bacteria will initially be very low.
What’s N got to do with it?
Aside from water, nitrogen is frequently the most limiting factor to crop and pasture production.
Nitrogen is nitrogen, irrespective of the source, but the same nitrogen compounds can have opposite effects, depending on the way they enter the soil and the form in which they exist in plants.
This paradox has created much confusion.
It is neither natural nor healthy for crop and pasture plants to contain high levels of inorganic nitrogen (nitrite, nitrate, etc). Nitrogen is much safer and more productive when in an organic form.
Closing The Nitrogen Loop
The efficiency of the use of applied N is generally less than 50% due to losses from leaching, volatilization, and denitrification (Kennedy et al 2004). These inefficiencies cost farmers a great deal of money as well as contribute to environmental pollution.
Fortunately, biological N fixation is a spontaneous process when adequate carbon is available under actively growing plants, provided large amounts of synthetic N have not been applied. In biologically active soils, sugars and other carbon compounds exuded by plant roots support vast colonies of beneficial fungi and bacteria, which in turn produce sticky substances that glue soil parties together and enhance soil structure.
Once aggregates (small lumps) start to form, free-living nitrogen-fixing bacteria, which require a low partial pressure of oxygen, can begin their work of fixing atmospheric nitrogen. These bacteria are called associative diazotroph, ‘associative’ because they are only found inside aggregates attached to living plant roots or connected to plants via the hyphae of mycorrhizal fungi-and ‘diazotrophs’ because of their ability to use nitrogenase enzymes to fix atmospheric nitrogen.
The nitrogen fixed by associative diazotrophs does much more than support plant growth. It also makes a significant contribution to the soil food web and is essential to the formation of stable forms of soil carbon, such as hummus.
In addition to associative diazotrophs, mycorrhizal fungi are indispensable for closing the nitrogen loop. Their ability to transfer organic N from the soil food web into plant roots circumvents the need for nitrogen to be present in an inorganic form (Leake et al 2004, Leigh et al 2009).
The activities of mycorrhizal fungi also contribute to the rapid sequestration of soil carbon.
But here’s the rub.
The applicant on large quantities of inorganic N-such as found in urea, MAP, DAP, etc inhibits the activities of both associative diazotrophs and mycorrhizal fungi. Long-term use of these products results in a decline in soil structure, the decline in soil carbon-and ironically, a decline in soil nitrogen (Khan et al 2007, Mulvaney et al 2009).
Reducing N dependence
Where diverse summer cover crops are being grown to support soil microbial communities, it is advisable to reduce N use, but this must be done slowly, to provide time for free-living N fixing bacteria to re-establish. There is no need for synthetic N in the cover crop provided a variety of broadleaved plants, including legumes, are present. Nitrogen inputs in follow-on crops can be reduced to 80% in the first year, 50% In the second year, and 20% In the third year. In the fourth and subsequent years, the application of a very small amount of N (around 1kg/ha) will help to prime the natural nitrogen-fixing processes in soil. Remember, associative diazotrophs (the most important of the free-living N-fixing bacteria) and mycorrhizal fungi (needed for N transfer to plants) have only one energy source liquid carbon from an actively growing green plant. At the same time as you are weaning your soil off synthetic N, you must also be maintaining as much diverse year-round living groundcover as possible.
Will I need to add P?
Plant roots produce hormones called strigolactones that control root extension, lateral root development, and the production of root hairs. The presence of strigolactones in the soil also stimulates root colonization by mycorrhizal fungi (Czarnecki et al 2013). Vigorous root systems and symbiotic relationships with mycorrhizal fungi are essential for maximizing the ability of crop plants to obtain water, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and a wide variety of trace elements such as zinc, copper, boron, manganese, and molybdenum.
Many of these elements are essential for resilience to climatic extremes such as drought and frost. The application of large quantities of water-soluble P such as those found in superphosphate, MAP, DAP, etc inhibits strigolactone production by plant roots. That is, the use of these products will reduce root extension, root hair development, and colonization by mycorrhizal fungi. The long-term results in destabilization of soil aggregates, loss of porosity, reduced aeration, increased soil compaction, and mineral-deficient plants.
In addition to having adverse effects on soil structure, the application of inorganic phosphorus is highly inefficient. Around 80% adsorbs to aluminum and iron oxides and/or forms calcium, aluminum, or Iron phosphates, which, in the absence of microbial activity, do not plant-available(Czarnecki et al 2013). Only 10-15% of fertilizer P is taken up by crops in the year of application.
In old and deeply weathered soils, biological processes are more important than chemical processes when it comes to making nutrients.
Your soil already contains sufficient P, but it will only be in a plant-available form when the right microbes are present. If levels of mycorrhizal colonization are high, there will be no need to add large quantities of inorganic P.
Cover crops (and follow-on crops) can be supported with biology-friendly products such as pelletized compost or liquids such as compost extract, worm leachate, or milk. Compost extract containing around 1kg/ha (no more) of each of N, P, and S, plus whatever trace elements are required (as determined by plant tissue test) should be sufficient in most situations.
Land can respond positive y to the presence of animals, but the way they are managed is extremely important. Strategic (high-density, short-duration) grazing of summer groundcover helps to stimulate biological activity and cycle nutrients tied up in plant material. Aim to graze no more than one-third to one-half of the biomass, using mob stocking or strip grazing techniques to ensure the soil surface is completely covered with trampled plant material (Jay Fuhrer, pers. comm.).
Where grazing is not an option, cover crops can be rolled. Menoken Farm.
Putting it all together
Changing fertilizer practice alone is not sufficient to improve soil health. Unless biology-friendly fertilizers are used in combination with diverse year-round living cover the essential microbes won’t be there to be supported. For the same reasons, the presence of summer groundcover alone is not sufficient-indeed it may prove detrimental. There will be a tie-up of N and a yield penalty in the follow-on crop unless key functional groups, particularly the associative diazotrophs and mycorrhizal fungi, are working together. This simply cannot happen if large amounts of inorganic N or water-soluble P are applied.
• Strategic grazing of summer groundcover helps cycle nutrients tied up in plant material. Aim to graze no more than 30-50% and trample the remainder onto the soil surface. If grazing is not an option, cover crops can be rolled while still green.
• There is no need for either synthetic N or P in your ‘summer cocktail’ provided a good range of broadleaved plants, including legumes, are present.
• Remember to wean off N slowly in the follow-on crop. Cut back to 80% in the first year, 50% in the second year, and 20% in the third year, then maintain levels at 1kg/ha/yr. If you feel you must, also apply 1kg/ha/yr of inorganic P and 1kg/ha/yr of S-but no more!
• Improved weed management is one of the many benefits of integrated land management. Most crop and pasture weeds are stimulated by nitrate. The current farming model is essentially creating the problem. Weeds become less of an issue under biological forms of cover cropping. This is partly to do with groundcover but more usually the result of closing the nitrogen loop.
• Above all, the capacity of the soil to absorb and hold water is critical for dryland crop and pasture production. Although it may seem counter-intuitive, the most effective method for improving soil structure and increasing water-holding capacity is to maintain active year-round plant cover, which increases soil carbon, supports microbial activity, and improves the ratio of fungi to bacteria.
From light to life
Diverse summer cover crops are sown with biology-friendly fertilizers are the fastest way to restore soil function in Wheatbelt soils. These principles also apply to dairy, beef, lamb, wool, and horticultural enterprises in the winter rainfall zone.
Sunlight intercepted by bare earth is converted to heat energy, driving evaporation and soil loss.
Sunlight intercepted by green leaves is converted to biochemical energy, fuelling soil life, enhancing soil structure, improving nutrient cycling, and increasing water-holding capacity.
Why not turn ‘light’ into ‘life’ on your farm?
Perhaps just try one paddock to begin? Your soil will love you-you will love your soil.
With big changes on the horizon for the Texas Olive Oil Industry it is quickly becoming important to discuss some of the new growing methods available to the olive producer. One of the most important new developments is the rise of organic methods of production. One of the most significant of these new developments is the ability of nursery owners and growers alike to be able to “inoculate” their trees to help protect them from harsh conditions in the soil. Research has proven that one such inoculation (Mycorrhizae) has the potential to dramatically alter olive production in our state.
Let me begin with the definition. The word “Mycorrhizae” is derived from the Greek – myco meaning fungi and rrhizae meaning roots. Therefore, Mycorrhizae is a group of fungi which inhabit the root systems of higher plants. Mycorrhizae fungi are not parasites. They are symbiots. They set up house in the root systems of higher plants and utilize the plants ability to withdraw nutrient matter from the soil. The fungi are able to aid the plant in the breakdown of the complex nutrient material. This system allows both fungi and plant to benefit. In the simplest terms, the fungi feed on the nutrients and take in plant by-products, mainly carbon and then aid in the breakdown of nutrients at the molecular level. The result is molecules and compounds, with phosphorous being the most prominent, that are more easily utilized by the plant. How all of this happens is an organic chemistry nightmare best left to those who derive some enjoyment from the analysis of such systems.
Mycorrhizae fungi are one of, if not the most historically significant group of organisms on planet earth. Many researchers believe that the tolerance of saline conditions (salt) in higher plants lies in the symbiotic relationship of plant and fungi. About 1 billion years ago simple organisms began to emerge from the seas to colonize the land masses. The fossil record indicates that even then they may have been closely aligned with fungi. Tests today indicate that higher plants “infected” with mycorrhizae fungi are significantly more tolerant of heavy saline conditions and higher pH soils than those plants that are not associated with the fungi. This is how, some researchers say, the higher plants were able to colonize the land and eventually become significant land based organisms. So, if I may extrapolate, the reason you are here on this planet is partially because of mycorrhizae fungi.
Mycorrhizae fungi are divided into two main groups – Ectomycorrhizae and Endomycorrhizae. Our discussion will be limited to the endomycorrhizae. The Ectomycorrhizae are chiefly inhabitants of the major forests and are associated with those species. We shall further limit our discussion to the subgroup of endomycorrhizae which form shrub-like penetrations of the root hairs. The name given to this group is Arbuscular Mycorrhizae Fungi (AMF). This is the economically important group and they are associated with almost 80% of all the higher plants on earth.
Now that we know what these fungi can do, it is time to discuss how they can benefit us directly as producers of olive oil/table olives. The European olive (Olea europaea L.) has typically been grown in the arid or semi-arid regions surrounding the Mediterranean Sea. Because of the geographical distribution of the European Olive most researchers believe that the association of AMF and the olive has probably been on a minimal basis. Research of wild populations though is indicating that where there has been a robust association with the fungi, olives have prospered. Arbuscular Mycorrhizae Fungi (AMF) colonize the roots of agricultural plants and promote the uptake of nutrients, and aid in the transport of water into the plant. In return, the fungi utilize carbon and various other elements produced by the olive tree. Therefore, the fungi have a pervasive effect upon the plant form and function.
Let us examine these claims. There is scientific evidence that the association with AMF promotes the uptake of nutrients. Most of the studies have indicated that AMF plays a significant role in the uptake of phosphorous. Phosphorous is an element that is very immobile in most soils. So, even with the addition of organic phosphorous, it quickly reacts with other elements in the soils to become calcium phosphate or other fixed forms. This benefit alone would make inoculation with the AMF worthwhile, but we don’t stop there. AMF act as extensions of the plant root structure. This attribute enables the plant to significantly increase its surface area in contact with the soil. This attribute in turn, greatly increases the amount of nutrients and micronutrients that become available to the plant.
In controlled experiments, using rooted cuttings of Olea europaea L. varieties Frantoio, Moriaolo and Leccino were inoculated with AMF and compared with non-inoculated control cuttings. After 6 months the control and inoculated cuttings were analyzed. The results indicated that the AMF cuttings had significantly more root development. Depending upon variety, 15%-20% greater root structure was observed. The larger root structure also translated into greater development of the aerial compartment. Unquestionably, the inoculated cuttings showed a significant increase in plant growth.
In addition to nutrient uptake, AMF have been proven to increase the plants root hydraulic conductivity. This enables the plant to use water more efficiently, minimizing the stress associated with drought conditions. Studies with olives and grapevines indicate that plants with roots that have been inoculated with AMF have the ability to take moderate to severe water stress during which time non-inoculated plants showed obvious signs of stress. The ability to effectively utilize soil moisture provides the olive producer with a far greater range of soils in which to plant trees within arid or semi-arid communities. For orchards with well developed irrigation systems, the ability of the plants to ‘handle” less watering, often means significant savings from pumping less water. The ecological benefits from using less water, even from modest drip irrigation systems, has some obvious secondary benefits.
Now that we know that inoculation promotes plant growth in the laboratory, is it possible to get the same results in the field? Studies in several olive nurseries have shown that inoculated olive plantlets in the nursery performed just as well as in the laboratory. One study showed that three sets of plantlets, one with fertilizer, one without fertilizer and one with AMF inoculation only were compared for growth characteristics. The olive trees inoculated with AMF showed consistently higher growth than even the plantlets with fertilizer. In addition, the inoculated plantlets showed higher levels of potassium and phosphorous deposited in the leaves and shoots. Many nurseries in Europe and Israel are now inoculating their plantlets early and have abandoned the inorganic fertilization of nursery stock.
In field grown trees, the Israelis and others have experimented with the injection of inoculum into the drip irrigation system. Early results have indicated surprising gains in both plant growth and in the olives ability to resist drought and saline soil conditions. Trials of AMF injection into field trees are underway in many olive producing countries. Many researchers involved in these studies are confident that the AMF in conjunction with beneficial bacteria will also provide some extra benefits.
Below are pictures sent from Israel by a grower participating in trials of field inoculation of AMF. The field trees were planted from 4” pots with trees (whips) approximately one meter in height. The trees were injected with inoculum and allowed to grow for 17 months prior to these photos.
The inoculated trees shown in the photo are now 2.5 meters (8’) tall with calipers at nearly 3”. Researchers and growers alike claim inoculated trees have some 15% to 25% more growth in both aerial and root compartments in a given period of time than with noninoculated trees. Those same growers are reporting the use of 20% less water during irrigation and significantly less problems during drought periods. Trials of flowering plants have also indicated a significant increase in bud and flower production that many attribute to the plants increased levels of potassium and phosphorous in the leaves and shoots.
Arbuscular Mycorrhizal Fungi are known to increase the root absorption zone and provide the foundation for a healthy soil structure. This results in increased uptake of nutrients, especially phosphorus and contributes to the plant’s overall resistance to environmental stresses, caused by soil salinity, and high pH. Our conclusions are that there exists sound scientific evidence for the inoculation of nursery stock and field grown olive trees. The use of Arbuscular Mycorrhizae Fungi inoculum in field grown trees is especially important in South Texas where relatively poor soils, and drought stress are a common factor that all olive growers are faced with on a daily basis.
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