Microbiological health of soil has emerged during the last decade as a critical part of the knowledge base for successful crop and pasture production. A key component of biology is the profound plant/mycorrhizal fungi relationship which has enormous potential for improved management of contemporary farming systems. In the first of a five part series on the role mycorrhizae play in achieving successful crop and pasture production, soil microbiologist Dr Michael Amaranthus and Larry Simpson explain these fungi are not ‘new’.

The fossil evidence indicates that the specialized “mycorrhiza” literally meaning “fungus-root”) plant relationship dates back more than 460 million years and actually played a key role in allowing plants to utilize terrestrial habitats. Without mycorrhizal fungi, crop plants might not ole in allowing plants to utilize terrestrial – exist unless you are farming seaweed.

For the first 75 million years that plants colonised dry land, they did not have differentiated root tissue at all and depended entirely on this symbiotic relationship with mycorrhizal fungi to access nutrients and moisture from the various and often harsh terrestrial environments. The root structures of plants actually evolved specifically as specialized attachment sites to better accommodate these fungi and the efficiencies available through the symbiotic “trading” of water and nutrients for sugars produced by photosynthesis.

In this symbiosis, the plant is provided better access and uptake of nutrients and water from the soil. In return, the fungus, which cannot synthesize its own nourishment, receives its sole sustenance in the form of carbohydrates donated by the plant. This highly successful system continues in most plant species today.

Agricultural science has only in the last decade begun to recognize the importance of mycorrhizae fungi in farming ecosystems. Since World War II, scientific and technological advances in agronomy have 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 tillage and herbicides, and plant diseases controlled using an array of chemical pesticides.

More recently, modern science has begun to understand that in natural habitats, plant roots are a complex mixture of both fungi and plant organisms that is fundamental to life on the planet.

Approximately nine out of every 10 terrestrial plant species in the world, including most crop plants, form an association with these specialized mycorrhizal soil fungi in order to thrive. Among the few but notable exceptions are members of the Brassica plant family (cabbage, broccoli, cauliflower, radish, turnips, canola, etc., the Amaranthaceae plant family (beets, spinach, chard, etc.), and the Polygonaceae plant family (rhubarb, buckwheat.) Virtually all other crop plants worldwide are meant to host a mycorrhizal association. The body of the mycorrhizal fungus consists of microscopic filaments called hyphae. Individual hyphae are approximate 1/25th the diameter of a human hair and typically grow up to 40-60 centimeters in length. These strands grow from within the root cells of the host plant, spreading out into the surrounding soil, greatly increasing the surface area of the root system. The most widespread type of mycorrhizal relationship is known as arbuscular mycorrhizae (also commonly referred to as “AM,” “VAM“ or “endo mycorrhizae.”). Most agricultural plants, including grains, vegetables, fruit and nut trees, vines, and turf grasses evolved with, and are naturally “designed” to achieve optimum growth and vigor by forming these fungal relationships.

What they do

This plant-fungus association can deliver significant benefits in agricultural operations. The effect on the root system of a mycorrhizal-colonized plant is dramatic. Under this relationship, most of the absorbing area of the root system is actually fungal hyphae. Hyphae are far thinner than roots or root hairs and are able to penetrate the tiniest pores and fissures in the soil. The numbers of hyphae on a root system can be prodigious. Just a teaspoon of healthy soil can contain up to several miles of fungal hyphae. The resulting nutrient and water uptake efficiency of crop plants is increased considerably. Agricultural soil often contains abundant nutrients but the availability of these nutrients to the crops themselves may be limited. Research confirms that mycorrhizae are particularly important in mobilizing phosphorus, nitrogen, zinc, iron, calcium, magnesium, manganese, sulfur, and other tightly bound soil nutrients by enzymatic release from recalcitrant chemical bonds and transporting them back to the plant. Crop plant uptake and utilization of fertilizer inputs likewise become far more efficient, often leading to significant savings in fertilizer costs.

The Dreaded “D” Word

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 and wild systems without the benefit of irrigation. How do natural areas provide for such luxuriant plant growth without irrigation?

One key factor is the mycorrhizal threads attached to plant roots which so thoroughly scour the soil for available resources. They absorb water during periods of adequate soil moisture, 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 which act like a giant sponge to protect the plant communities from extreme soil moisture deficits. The mycorrhizal filaments can penetrate into the smallest of soil pores and fissures to access microscopic sources of water that are unavailable to the thicker roots. An extensive body of research documents the importance of the mycorrhizal relationship for efficient water use and drought protection among a wide array of important crop species. The declining availability of water and its ever-increasing cost are formidable issues facing today’s farmers and mycorrhizal fungi can be a powerful tool to enhance water-use efficiencies.

Does my farmland have mycorrhiza?

Certain modern agricultural practices are known to suppress the biological activity in soils. Fungicides, chemical fertilizers, cultivation, compaction, soil erosion, and periods of fallow are all factors that can contribute adversely to populations of beneficial mycorrhizal fungi. Soil testing worldwide indicates that many intensively managed crop and pasture paddocks lack adequate populations of mycorrhizal fungi.

We will explore how long-term agricultural activities on expansive acreage can impact the mycorrhizal relationship in more detail in subsequent articles, but basically, loss of naturally occurring mycorrhizal fungi comes about in two fundamental ways.

Fallow soil is first and foremost among the causes for the demise of mycorrhizae. The fungi are completely dependent on their host plants for sustenance and cannot survive for any extended duration without the presence of living roots.

Of course, tilling, though not necessarily lethal to mycorrhizae by itself, generally leads to a fallow condition which, in turn, eliminates the fungi. Therefore, even no-till practices may not necessarily preserve mycorrhizae either. When annual crops are harvested, the roots soon die and any mycorrhizal fungi die with them unless new living roots are introduced in the form of another crop within several weeks – a relatively rare sequence in the world of production agriculture.

One might think that the spores left in the soil would regenerate the mycorrhizal population in new crops planted even after an absence of living roots and hyphae. However, after repeated cycles that include regular intervals of fallow, the spores eventually expire and are not sufficiently replaced by the gradually diminishing mycorrhizal populations until they too have essentially disappeared.

The other critical situation that eventually eliminates mycorrhizal populations from many farm soils is the lack of proximity to natural populations from which new colonization usually spreads.

The crop plants become isolated from the beneficial mycorrhizal fungi that would, in natural ecosystems, be abundantly available to spread colonization to their roots. Without adjacency to natural areas acting as a source of mycorrhizal hyphae and spores to re-populate depleted lands, arbuscular mycorrhizal populations are very slow to re-establish.

Since arbuscular mycorrhizal fungi do not disperse spores via wind or water, but rather grow from root to root, re-colonization across long distances back into farm soil from undisturbed natural sites becomes slow and difficult. Unfortunately, growing crops immediately adjacent to undisturbed natural ecosystems is not always an option in modern agriculture.

Mycorrhiza Is a Symbiotic Relationship

Mycorrhiza nomenclature can be a bit bewildering. The main thing to remember is that the word mycorrhiza refers not to a specific fungus or group of fungal organisms, but to a defined, symbiotic relationship between fungi and plant roots. This relationship is defined as a specific, complex fungus /root cell mutualism.

We tend to loosely refer to mycorrhiza as a fungal organism, but technically this is not correct. The fungus is one entity, the plant is another, and when the two combine to form this symbiotic mutualism, the resulting relationship is a mycorrhiza. Several of these relationships or a group of plants and fungi all having the relationship is expressed in the plural as mycorrhizae.

A reference to any aspect of this relationship, such as one or more of the partner organisms, may be described by the adjective mycorrhizal. Therefore we have mycorrhizal fungi, mycorrhizal plants, mycorrhizal colonization, mycorrhizal populations, etc. – Larry Simpson.

A critical turning point in the health of North American farm soils can be traced to a single day in 1947. On this day the gigantic munitions plant in Muscle Shoals, Alabama switched from bomb making to manufacturing chemical fertilizers. When World War II ended, the government was left with an extremely large surplus of ammonium nitrate that had been used in the manufacturing of bombs. This same chemical is also a powerful source of nitrogen that is essential for plant growth. Initially, this surplus was targeted for the timber industry as a cheap source of nitrogen to spray on the nation’s forests. However agronomists at the US Department of Agriculture had another idea in mind. Ammonium nitrate could be spread on agricultural land as fertilizer. Indeed, nerve gases developed for wartime purposes were already being used, with minor modifications, for insect control on the nation’s farmland. While fertilizer craving crops benefited from cheap sources of chemical “N” (Nitrogen) fertilizer, life in the soil suffered.

Bye Bye Biology

With the advent of synthetic fertilizers farmers could short-circuit many of the “biological” practices that kept soils alive and healthy (Figure 1). The negative effects have also had an impact on the environment. Soil organisms are critical to capturing and storing fertility in the ground (Read et al. 1992). In their absence, fertility, especially nitrogen in the form of nitrates is readily leached into surface waters and aquifers. This has a significant and detrimental impact on drinking water and aquatic life (Runge 2002). Because only a fraction of the synthetic fertilizers farmers put on their fields are utilized by crops, much of its application results in “mainlining” inorganic nitrogen into soil. In an Iowa State University study, corn fertilized with anhydrous ammonia utilized only 29% to 45% of the added nitrogen. The balance of it ends up damaging the surrounding environment. Some is volatilized into the air contributing to acid rain and global warming, but much of it travels off farms, washing down through the soil profile into groundwater and neighboring streams. In the same Iowa State study 49% to 64% of the added nitrogen was volatilized or leached.

The environmental costs of the use of chemical fertilizers have been high. In some places, such as Des Moines, Iowa, “blue baby alert” is issued when the nitrogen runoff from surrounding farmland is heavy (Ward et al. 1998). It is a warning to parents not to give tap water to children because nitrates in water bind to hemoglobin in blood and block the distribution of oxygen to the brain. Nitrates move downstream from Midwest farmlands traveling down the Mississippi and into the Gulf of Mexico. There they form a “hypoxic” or dead zone as big as the state of New Jersey (Goolsby et al. 1999). Nitrates poison the marine world, stimulate the wild growth of algae, and consume oxygen, which smothers fish. In essence we have been “bombing” non-target organisms with excess nitrates leaching from farmlands.

Nitrogen fertilizers and fossil fuels

In 1909, Fritz Haber, a German chemist, discovered how to artificially take atmospheric nitrogen and fix it into a chemical form that could be used by plants. Until that time, the vast majority of usable nitrogen on the planet was converted to plant-available forms biologically by soil organisms such as Rhizobia (Figure 2). These beneficial soil organisms were supported by the sun’s energy in a symbiotic relationship with certain plants. With the advent of cheap synthetic fertilizers these biological processes were no longer necessary, at least in the short run, to maintain fertility on the farm.

When humans discovered how to fix atmospheric nitrogen, the massive conversion of farms to synthetic nitrogen fertilizer resulted in an agricultural system heavily dependent on fossil fuels. On average, these fertilizers account for about 30% of a conventional farm’s energy consumption (Manning, 2001). University of Kentucky researchers attributed 42% of the energy cost of corn production to nitrogen fertilizer compared to 29% for drying the grain and 7% for plowing and disking the field. As a result, it tasks 50 gallons of oil to produce an acre of industrial corn (Pollan 2006). In fact, it takes more than a calorie of fossil fuel energy to produce one calorie of food.

Chemical fertilizers and the living soil

Rod Arkley, a soil science professor at the University of California, Berkeley has stated that,“ Dirt is what you get under your fingernails, soil is what gives the earth life.” When soil is healthy it contains an abundance of biological activity. One heaping tablespoon of healthy soil may contain billions of soil organisms, which is equal to the whole human population of the earth! An acre of healthy topsoil can contain a web of life that includes 900 pounds of earthworms, 2,500 pounds of fungi, 1,500 pounds of bacteria, 130 pounds of protozoa, 900 pounds of arthropods and algae, and in most cases, it even contains small mammals (Amaranthus et. al 1989). (Figure 3).

The farmers share of farm income has declined because tasks such as controlling pests. High levels of chemical fertilizers can have a devastating impact on the soil food web (Lowenfel and Lewis 2006). This is obvious even in what table salt does to a worm. Chemical fertilizers are essentially salts that suck the
water out of beneficial bacteria, fungi, protozoa and a wide array of other organisms in the soil. It is these organisms that form the basis of the food web which conserve and process nutrient capital in the soil. By destroying large segments of living soils, farmers are stuck with an agricultural regime that requires the continued and on-going use of synthetic chemicals.

Consequently, tasks such as controlling pests, pathogens and releasing stored soil fertility once accomplished largely by beneficial soil organisms must then be done by farmers. Input costs have skyrocketed because of this reliance on increasingly complicated chemical approaches to managing the farm (Figure 4).

Bottom line: Declining food quality

Recent studies show an alarming decline in the quality of food over the last 50 years. Davis and others in 2004 analyzed USDA Food composition data for 43 crops from 1950-1999 and found statistically reliable declines for 6 nutrients. In another study, Dr. Mayer, 1999, found significant reduction for 7 minerals in 20 fruits and 20 vegetables between the 1930s and 1980s in the United Kingdom. Experts have wondered if the reliance on chemical fertilizers and large-scale destruction of life in the soil may have had an effect on the nutritional value of food. Thousands of research studies have shown that beneficial organisms play key roles in the conservation, mobilization and transportation of nutrients from soils into plants (Read et al. 1994). The relationship between the quality of soil and the quality of food is undeniable (Figure 5).

Prior to WWII, farms were biologically based, and most had been successfully managed in a sustainable manner for generations. It has only been since WWII that the synthetic approach has been utilized and within those 50 years serious problems have become clearly identifiable. There are biological tools though that can decrease the need for chemical fertilizers on farms. Two examples of beneficial organisms are nitrogen fixing bacteria and mycorrhizal fungi. Farmers once knew how important these organisms were to the farm.

Beans and Nitrogen-fixing Rhizobia bacteria

Before the advent of chemical fertilizers, farmers could not maintain high levels of production with the same crops on the same ground year after year. Such practices would produce soils devoid of fertility. In the old days, farmers were careful about rotating crops and incorporating nitrogen-fixing legumes into management practices, which added fertility and organic matter into soils. An excellent example of these practices is the use of Rhizobia inoculant when growing beans and other nitrogen-fixing legume crops on farmland. Eighty percent of the atmosphere is nitrogen, but in spite of it being so plentiful, plants aren’t able to utilize it as a gas form. It is important to note that symbiotic nitrogen-fixing bacteria associated with the roots of legumes are capable of taking substantial quantities of the vast pool of atmospheric nitrogen and convert it to an organic form usable by plants. A good cover crop can add 200- 300 pounds of nitrogen per acre into the soil From ancient times until recent decades, these soil organisms were essential partners in building soil productivity. Until recently, these organisms were among the most important tools in maintaining the productivity of the farm. The expense and environmental costs of chemical forms of nitrogen fertilizer are increasingly making biological or “bean” approaches more profitable to farmers.

Mycorrhizal fungi

Most plants, including more that 90% of all agricultural crops, form a root association with specialized fungi called mycorrhizae. Mycorrhizae literally means “fungus roots.” In this association, fungal filaments extend into the soil and help the plant by gathering water and nutrients and transporting these materials back to the roots. Miles of fungal filaments can be present in a small amount of healthy soil. The plant’s association with mycorrhizal fungi increases the effective surface absorbing area of roots several hundred to several thousand times (Harley and Smith 1983). In return, the plant helps the fungus by giving it sugar produced by photosynthesis. This symbiosis is a win-win association (Figure 6).

Recent research published in the Journal Nature (Govindarajulu et al. 2005) has emphasized the important role mycorrhizal fungi play in delivering nitrogen to crop plants, thus lowering the need for synthetic fertilizers. With this in mind, farmers may benefit from promoting the proliferation of mycorrhizal fungi through diminished fertilizer input, thereby making more efficient use of the nitrogen stores in agricultural soils. The authors found that beneficial mycorrhizal fungi transfer substantial amounts of nitrogen to their plant hosts. The researchers also discovered a novel metabolic pathway. An ammonium form of nitrogen, not subject to leaching losses compared to nitrates, is taken up by the mycorrhizal fungus in soil and incorporated into plant tissue. The fungus facilitates conservation of nitrogen through the uptake of ammonium and the minimizing of a plant’s need for conversion to leachable nitrates in the soil.

1+1=3
Well-documented research trials are also available that document the important role of mycorrhizal fungi with most legume crops. The Rhizobia bacteria that form with important legume crops have a high phosphorus requirement, which help optimize their level of nitrogen-fixation. Two examples of biological tools are nitrogen fixing bacteria and mycorrhizal fungi. Mycorrhizal fungi produce specific enzymes to extract phosphorus out of the soil and make it available to nitrogen-fixing bacteria. The synergetic effect of a combined treatment with nitrogen-fixers and mycorrhiza can increase yield (Linderman, 1991). In soybeans, inoculation with mycorrhizal fungi increased the amount of biological-fixed nitrogen and stimulated phosphorous uptake, soybean growth and yield (Shabayev et al. 1996). Other studies have shown mycorrhizal inoculation improve rates of nitrogen fixation for other species (Tian 2003). Yield increases of 30% or greater have been realized for corn and soybeans with savings of 160 and 213 lbs/acre phosphorus respectively (Plenchette and Morel 1996).

Biological nitrogen management aims to provide crops with enough nitrogen at the appropriate time while avoiding resource depletion and nitrogen pollution. Strategies include growing cover crops such as legumes that utilize nitrogen fixing Rhizobia bacteria to replenish nitrogen exported at harvest. Another approach is to keep the ground covered with live vegetation that supports a mycorrhizal web of filaments below the soil surface so as to capture and transport nitrogen directly to the plant itself. The millions of tiny mycorrhizal filaments prevent leaching losses and supports the closed nitrogen cycle found in nature.

Putting the good critters back in soil
How do you re-establish beneficial soil organisms when they have been lost from a farm? The beneficial bacteria, Rhizobia, have long been available as inoculants for legumes and is usually applied as a liquid or peat-based form to the seed. More recently, advancements in our understanding of mycorrhizal fungi and their requirements has led to the production of high-quality, mycorrhizal inoculum at affordable prices. Mycorrhizal inoculum is currently available in granular, powder, seed coat, and liquid forms (Figure 7).

The most important factor for re-integrating mycorrhizae is to place mycorrhizal propagules near the root systems of target crops. There are a variety of ways to achieve this. Inoculum can be incorporated into the planting hole at the time of transplanting, watered into porous soils, mixed into soil mixes or directly dipped on bare-root systems using gels. For agricultural purposes it is best banded or applied with seed at sowing. The form and application of the mycorrhizal inoculum depends upon the needs of the farmer and the equipment used on the farm. (Figure 8). It is clear that on farms where mycorrhizal fungi have been lost, inoculation can cut input costs and increase yields.

Bombs and dirt vs. the living soil

Over the last few decades many farms have developed a growing reliance on pesticides and synthetic fertilizers. This approach to managing the land relies on chemical inputs rather than biological approaches to solving land management issues. While this practice has allowed farmers to control pests and nutrient supply in the short term, the ”bombs and dirt” approach has caused more problems and expense in the longterm. The costs include deterioration of many aquatic systems and a decline in food quality. For example, in Great Britain the estimated cost of removing nitrates from drinking water is 2 billion dollars. Such costs fail to include the impact on human health such as the fact that childhood-onset diabetes has been linked to increased nitrates in drinking water (Parslow et al 2004). The biological or “ beans” approaches to managing nitrogen are especially needed and have been successfully practiced for hundreds of years before the development of cheap synthetic nitrogen sources powered by fossil fuels. It is time to get back to nature’s solutions.

Biological management of nitrogen is a key ingredient in many organic approaches to managing farmland. The low nitrogen availability associated with organic production systems is considered a chief obstacle to organic farming competing with conventional agriculture. The Rodale Institute, in collaboration with USDA Agriculture Research Service, designed a well-replicated and randomized field trial to respond to performance gaps between the two methods (Hepperly et al. 2006). This trial is the longest-running comparison of organic and conventional maize and soybean cropping systems in the world and is presently in its 25th season.

Over the years, this experiment has demonstrated the following:

1. Increased soil carbon and nitrogen levels in the organic vs. conventionally farmed plots.
2. Crop yields are similar for organic vs. conventional in years of average precipitation, and greater in organic during drought years due to higher moisture availability,
3. Fossil energy inputs for organic crop production were over 30% lower than for conventionally produced maize and soybeans,
4. Labor inputs averaged about 15% higher in organic farming systems than in conventional,
5. The net economic return per hectare for organic is often equal or higher than conventionally produced crops because organic foods frequently bring higher prices in the marketplace.

In addition to yield and economic benefits, environmental benefits of organic agriculture include enhanced sequestration of carbon in the soil, in addition to less nutrient leaching into groundwater than in conventional agriculture. Clearly biological management of nitrogen is a key element in maintaining yields and profitability in organic farming.

Today, farmers have many biological tools to improve the health of the land and the people that live there by putting the living soil back to work. A “beans” approach works by incorporating important soil organisms that build and maintain soil fertility into the management of crops. This change will happen only when we begin to appreciate the fact that healthy, productive soils are dynamic ecosystems composed of a mixture of minerals, air, water, organic materials and a healthy population of beneficial microorganisms. The next step then will be to recognize the inoculation opportunities available to reestablish healthy living soils on farms. Lastly, it will be important to manage and maintain management of the soil environment where beneficial soil organisms can survive.

Nursery And Landscape Professionals Today Are Faced With A Bewildering Array Of Conditions And Treatments. Propagate, Aerate, Irrigate, Fertilize, Mulch, And Transplant Are But A Sampling Of The Activities Utilized By Today’s Practicing Industry Professional. One Of The Activities That Fuels The Greatest Debate In The Industry Is Using Mycorrhizae. What are they and are they really useful? The answer to these questions depends on what you, the professional, are dealing with. For years the nursery and landscape industry has responded to plant problems with conventional solutions fertilizing, pruning, spraying, and other cultural practices. Some practices are successful, many are not. In most cases, the choice of practices failed to consider “the root” of the problem that lies hidden from view beneath the soil surface.

Below the soil surface some 400 million years ago, long before plants had “help” to survive man-made environments, plant communities were faced with many natural stresses. Infertile soils, diseases, drought, extreme temperatures, competition, and wind are not new. To survive, plant species adapted strategies to persist in the physical, chemical and biological stresses that surrounded them. Perhaps the most fundamental and successful strategy, the mycorrhizal relationship, has allowed plants to adapt to the harsh conditions of life on land. Radiating out from the roots of plants are miles of tiny filaments that occupy great expanses of soil volumes and trap mineral nutrients and water essential to support plant growth needs.

These tiny filaments (mycorrhizal fungi) actually attach and penetrate between and within the outer cells of the root cortex of plants and effectively become extensions of the root system itself.

The association between roots and fungi has been known or suspected since classical times. Theophrastus, a Greek naturalist of some 2000 years ago, traced the mycelium of certain mushroom species back to oak trees. The word Mycorrhizae is of Greek origin (fungus-roots) and defines the mutually beneficial relationship between an estimated 90% of the world’s land plants and this specialized group of root colonizing soil fungi. The mycorrhizal relationships with plants have been found on every continent except Antarctica. It is likely that there is no woody plant on the face of the earth that does not form a “fungus-root” in some part of its range. Mycorrhizal relationship with plants have been found in the earliest fossil records. It is quite possible that the fungus-root relationship spawned the evolutionary leap that allowed plants to colonize the harsh terrestrial land surface.

What Are The Benefits?

Mycorrhizal fungi function through a network of threads. At one end the threads attach to and enter the root tissue. It is here that the plant and fungus exchange essential materials. The plants receive mineral nutrients, water, and a variety of other growth promoting substances. In exchange, the fungus receives essential sugars and compounds to fuel its own growth. On the other end, fungal threads as individuals (hyphae) or in clusters (mycelium) fan out into the soil and exponentially expand the amount of soil which the roots may explore for raw materials. Estimates of amounts of mycorrhizal filaments present in soil associated with plants are astonishing. Several miles of filaments can be present in less than a thimbleful of soil. Mycorrhizal fungal filaments in the soil are truly extensions of root systems and more effective in nutrient and water absorption than the roots themselves.

Many other exchanges occur between plants and their mycorrhizal symbionts. Mycorrhizal fungi produce soil compounds which stimulate the plant to produce additional roots on which the fungus can grow. Conversely, roots in turn secrete substance upon which stimulate the growth of the fungus. Mycorrhizal fungi release powerful chemicals into the soil that dissolve hard to capture elements such as phosphorous, iron and other “tightly bound” soil nutrients. Other chemicals produced by mycorrhizal fungi include enzymes to degrade organic carbon and nitrogen sources. These extraction processes are particularly important in plant  nutrition and explains why non mycorrhizal plants require high levels of fertility to maintain their health.

Many other exchanges occur between plants and their mycorrhizal symbionts. Mycorrhizal fungi produce soil compounds which stimulate the plant to produce additional roots on which the fungus can grow. Conversely, roots in turn secrete substance upon which stimulate the growth of the fungus. Mycorrhizal fungi release powerful chemicals into the soil that dissolve hard to capture elements such as phosphorous, iron and other “tightly bound” soil nutrients. Other chemicals produced by mycorrhizal fungi include enzymes to degrade organic carbon and nitrogen sources. These extraction processes are particularly important in plant  nutrition and explains why non mycorrhizal plants require high levels of fertility to maintain their health.

Mycorrhizal fungi form an intricate web that captures and assimilates nutrients and water, conserving the ability of soils to remain productive during periods of stress. In non irrigated conditions, mycorrhizal plants are under far less drought stress compared to non mycorrhizal plants. Results from numerous studies strongly indicate that mycorrhizal fungi can help plants to tolerate and recover from soil water deficits. The mycelial network produced by mycorrhizal fungi play an important role in water uptake storage and movement back into plants demonstrated the enhanced tolerance to drought stress.

Mycorrhizal fungi also improves soil structure. Mycorrhizal filaments produce humic compounds and organic “glues” (extra cellular polysaccharides) that bind soils into aggregates and improves soil porosity. Soil porosity and soil structure positively influence the growth of plants by promoting aeration, water movement into soil, root growth, and distribution. In sandy or compacted soils the ability of mycorrhizal fungi to promote soil structure may be the most important factor improving plant performance.

Mycorrhizal Diversity Is Important

Natural areas generally contain an array of mycorrhizal fungal species. Not all mycorrhizal fungi have the same capacities and tolerances. Some are better at imparting drought resistance while others may be more tolerance to soil pH or temperature extremes. Because of the wide variety of soil, climatic, and biotic conditions characterizing man-made environments, it is improbable that a single mycorrhizal fungus could benefit all host species and adapt to all conditions. For example, the types and activities of mycorrhizal fungi associated with young plants may be quite different from those associated with mature may be quite different from those associated with mature plants. Likewise, mycorrhizal fungi needed to help seedlings establish themselves on difficult sites may differ from those which sustain productivity at the nursery. The diversity of mycorrhizal fungi formed by a given plant may increase its ability to occupy diverse below ground niches and survive a range of chemical and physical conditions.

Modern Practices And Man-Made Environments

The below-ground landscape of native habitats is teeming with thousands of organisms, including mycorrhizal fungi that provide many of the necessary components that all plants need to survive. Mycorrhizal fungi, like plants, are affected by soil conditions. Research indicates many common practices can degrade the mycorrhizae-forming potential of soil (figure of degradation environment). Tillage, fertilization, removal of topsoil, erosion, site preparation, road and home construction, fumigation, invasion of non native plants, and leaving soils bare are some of the activities that can reduce or eliminate these beneficial soil fungi. In many man-made landscapes we have reduced or eliminated the soil organisms necessary for plants to function without high levels of maintenance.

Reintroducing mycorrhizal fungi in areas where they have been depleted can dramatically improve plant establishment and growth. In fact, in their natural environments it is likely that the distribution of mycorrhizal fungi is what determines the distribution of a particular plant species. For example, when an oak tree grows on a particular site, it may do so because the soil conditions there are hospitable for it. In fact, the tree is possibly growing where the soil conditions are best for the fungus! Utilizing this important partnership on an operational basis can assist the practicing nursery and landscape professional in getting high value plants established and maintaining plant health over time.

Plant Production

Plants raised in most nurseries receive intensive care and feeding. The artificial conditions, high levels of water and nutrients and sterile soils at the nursery keep certain soil born diseases to a minimum and produce vast quantities of plants for sale. Unfortunately, the high levels of water and nutrients and the lack of mycorrhizae discourage the plant to produce the extensive root system it will need for successful transplantation. The result are plants poorly adapted to the eventual out planted condition that must be weaned from intensive care systems and begin to fend for themselves. Application of mycorrhizal inoculum during the nursery growth cycle or transplanting can encourage plant establishment and set the plant on track to feed for itself. Research studies document the need of plants to generate a mycorrhizal roots system in order to become established. There are practical solutions to some of the mycorrhizal deficiencies in man-made environments and reintroducing mycorrhizal fungi in areas where they have been depleted can dramatically improve plant establishment and growth.

To reduce input costs of water/fertilizers inputs, many growers have looked at increasing the plants ability to uptake these vital components. But if the soil and its vital components are in disrepair this process will be limited. Thus the plant will have slower growth rates, higher management maintenance cost and overall poor health. Improving the soil and its beneficial organisms can reduce many problems common in nursery and agricultural production. Until recently, this would have been economically unfeasible. The cost of mycorrhizal inoculant has declined significantly with greater usage. When one considers the benefits and low cost of application, mycorrhizal inoculation has become very cost effective for nursery and landscape use.

New Landscape And Plant Installation

Current construction practices can destroy 20,000 to 40,000 years of soil development with just one pass of heavy equipment. The challenge for the landscape professional is to try to establish quality plant material on a severely degraded site. The new construction landscape business has truly become a land reclamation business. The objective is to re-create thousands of years of soil improvements in a short period of time, with a limited budget and with a public that wishes to have more and more unusual, nonnative plant material. In many cases the industry is asked create a healthy plant ecosystem in a biological desert of poor soil conditions and the altered environments of concrete and asphalt.

How does the landscape professional improve soil conditions in new landscape installations at highly disturbed sites? Most new construction sites are a formidable foe for plant establishment without intervention of good soil reclamation practices. Green industry professionals must plan and develop a plan for soil reclamation as they would for site design. Creating a favorable below ground environment is as important to the overall success of the planting as the plant selections themselves. Mycorrhizae can be a key to the success of a planting on a highly disturbed site where plants must quickly access the water and nutrients necessary to become established. In the field, as in the nursery, introducing mycorrhizae can be accomplished in a variety of ways. 

What Types Of Mycorrhizal Products Are Available?

A nursery manager and landscape contractor can enhance plant root growth and transplant success and ameliorate many problems that result from current intensive care practices. Plants grew and thrived on this planet for millions of years without intensive care. Nature provides the template. A more sustainable approach to plant establishment and growth includes using mycorrhizal fungi. Certain mycorrhizal spores or “seeds” of the fungus have been selected for their establishment and growth-enhancing abilities. The goal is to create physical contact between the mycorrhizal inoculant and the plant root. Generally, mycorrhizal application is inexpensive and requires no special equipment. Typically for small plants the cost ranges from less than a penny to a few cents per seedling. For larger plants more inoculum is needed and costs are higher.

Nursery And Landscape Professionals Have At Least Four Options To Inoculate With
Mycorrhizal Fungi.

The first method is an incorporation of a powdered mycorrhizae at the time of planting or out planting plant stock into the field site. The powdered material can be incorporated into potting soil or soilless mixes. It can also be poured beneath the plant before placing into the planting hole or distributed around the root ball after placement.

The second option is a soil drench. This method can be accomplished through existing spray devices in the nursery or field or by soil injection . It is essential that the mycorrhizal fungi reach the vicinity of roots themselves. Porous or artificial soils are not a problem for a drench. In clayey or compacted soils the drench must be added to the planting back fill or sprayed to the surface of the root ball.

The third option is to purchase plant stock that have been preinoculated and have mycorrhizae present. The difference in performance of pre-inoculated liners vs. uninoculated plant stock can be dramatic. Pre inoculated stock can more rapidly establish and grow in the field.

The fourth option is inoculation with a mycorrhizal root dip gel during out planting in the field. Root dip gels are used by many landscapers as a means of reducing losses and increasing speed of establishment especially in non irrigated conditions. The grower dips small potted liners or bare root liners into a slurry. This slurry contains a blend of mycorrhizal spores/biostimulants/water holding gel. The mycorrhizal spores attach to the roots, and rapidly form mycorrhizae associations. A root gel treatment is often a very economical treatment and can reduce transplant losses significantly.

Mycorrhizal products often contain other ingredients designed to increase the effectiveness of the mycorrhizal spores. For example, organic matter is often added to encourage microbial activity, soil structure and root growth. Stress vitamins improve nutrient uptake and builds root biomass. Water absorbing gels help “plaster” beneficial mycorrhizal spores in close proximity to feeder roots and encourage favorable soil moisture conditions for mycorrhizae to form and grow. Organic biostimulants, in general are effective ingredients in mycorrhizal products. By promoting field competitiveness, stress resistance and nutrient efficiency biostimulants reduce barriers for rapid mycorrhizal formation especially during the critical period following root initiation or transplanting. The synergistic relationship between mycorrhizal fungi and biostimulants can mean 1+1=3 for the successful establishment of plants.

So Are Mycorrhizae For Me?

The answer to this questions begins with a simple assessment of the below ground landscape or nursery environment. If your soils are in a natural state or produce plants of model health and growth without large levels of human intervention, then mycorrhizae are probably not for you. But if you grow plants in below ground environments that are sterile or severely disturbed and the plant community suffers from drought, soil compaction, infertility, transplanting shock, and simply poor outward appearance, then mycorrhizae is for you! The landscape industry is asked to create ecosystems of plant communities with limited budgets and with less than acceptable soil conditions in a very short period of time. Professionals now have an assortment of low-cost and effective mycorrhizal inoculants that can build a sustainable plant mycorrhizae system as nature has done for millions of years.

Today’s nursery and landscape professionals integrate complex and changing economic and environmental values. This requires understanding of how plants function, how the soil and plants work together, and how they are linked and interdependent. Plants and mycorrhizal fungi form a dynamic and coherent partnership whose relationship has co-evolved over millennia. We can no longer consider plants in isolation from the belowground symbionts that promote plant health. Mycorrhizae, are they right for you?

Background

Mycorrhizal fungi alter plant-water relations in several ways, but the potential role of the fungal hyphae as regulators of plant water uptake remains a controversial issue. Many mycorrhizal inoculants have been introduced into the market that claim the ability to improve water uptake, thereby reducing drought stress.

Since irrigation water is becoming increasingly scarce and global climate changes are creating weather pattern fluctuations, these products are receiving considerable attention; however, the effect of soil type, specific crop, and weather patterns on the ability of the mycorrhizae to affect plant-water relations is largely unknown.

Objectives

• Determine the soil water content percentage at then time of plant death for both mycorrhizal inoculated and un-inoculated corn plants.
• Determine whether mycorrhizal inoculation is a viable agronomic tool for growers.

Methods

Conventional corn was planted into 2-gallon pots and grown in a greenhouse setting. Two treatments were replicated ten times. The pots were filled with a 1:1 mix of pasteurized field soil and a peat moss/composted bark mix. One trial had two grams of mycorrhizal powder incorporated in the soil at the time of seeding. The experimental design was completely randomized.

The plants were allowed to grow in a regulated climate of 21°C during the day and 18°C at night, with a sustained relative humidity of 60 percent. All plants were watered regularly for five weeks. On the 36th day a drought was initiated.

The plants were allowed to wilt and die. When the sixth foliar leaf wilted below a 45° angle in relation to the stalk or the stalk lodged, the plant was considered dead, and an approximately 300 g soil sample was collected. Soil was collected 5 cm below the soil surface near the root mass using a small spade.

The soil sample was added to a beaker, weighed, and placed in an oven at 105°C until its mass stabilized. The moisture content (MC) of the soil as a percentage was calculated using the following equation:

A – Mass of beaker
B – Weight of moist soil and beaker
C – Weight of dried soil and beaker

Results

A slight average MC difference appeared between inoculated and un-inoculated plants at the time of death. On average, inoculated corn plants died with 0.526% less water in the soil, but after running one-way ANOVA data analyses, it was determined that there was no statistical difference between treatments. Inoculated plants did, however, survive 5.6 days, or 28%, longer during a drought than un-inoculated plants, on average, at the 95% level of significance.

Conclusion

While there was no statistical difference in soil moisture content percentages between treatments, there was a statistical difference between treatments in number of days a plant survived a drought. The research suggests that utilizing a mycorrhizal inoculant as a means of improving drought stress tolerance in corn plants may be a viable agronomic tool.

Further Research

There are two methods that could be applied to this particular study to enhance the results: first, the soil could be taken from much deeper in the soil profile to get a more realistic representation of the moisture content.

Secondly, the roots could be dyed and the mycorrhizal hyphae counted in order to ensure the effectiveness of the inoculation.

I hope you all have a great Christmas and New Year. If you are looking for a gift for a green thumb then maybe MycoGold or Seaweed Saver. It has been a pleasure and we value your support. 

Kind regards

Tim Lester and team.

Thank you to all who have submitted reviews over the last year. It is much appreciated. Chris from Endeavour Hills in Victoria was the random selected review winner for 2021.

Soil biology has emerged over the last decade as a critical part of the knowledge base for successful and sustainable agricultural production. A key component of biology is the profound plant/mycorrhizal fungi relationship, which has enormous potential for improved management of contemporary farming systems. Although using these fungi has the potential to revolutionize agriculture they are certainly not new in terms of the evolution of plants.

Where We’ve been

The fossil evidence indicates that the specialized “mycorrhiza” (meaning “fungus-root”) plant relationship dates back over 460 million years and actually played a key role in allowing plants to utilize terrestrial habitats. Without mycorrhizal fungi, today’s crop plants might not exist, unless you are farming seaweed! For the first 75 million years that plants colonized dry land, they did not have differentiated root tissue and depended entirely on this symbiotic relationship with mycorrhizal fungi to access nutrients and moisture from the various and often harsh terrestrial environments. The root structures of plants actually evolved specifically as specialized attachment sites to better accommodate these fungi and the efficiencies available through the symbiotic “trading” of water and nutrients for sugars produced by photosynthesis. In this symbiosis, the plant is provided better access to and uptake of nutrients and water from the soil. In return, the fungus, which cannot synthesize its own nourishment, receives its energy source in the form of carbohydrates donated by the plant. This highly successful system continues in 90 percent of plant species today. Agricultural science has only in the last decade begun to recognize the importance of mycorrhizal fungi in farming ecosystems.

Since before World War II, scientific and technological advances in agronomy have focused primarily on the  development of chemical and mechanical approaches to improving crop production yields. Nutrient needs have been addressed using synthetic fertilizers while weed suppression has been accomplished through tillage and herbicides and plant diseases controlled using an array of chemical pesticides. More recently, modern science has begun to understand that in natural habitat plant roots are a complex mixture of both fungi and plant that is fundamental to life on the planet. The vast majority of crops form an association with these specialized mycorrhizal soil fungi in order to maximize performance. Among the few but notable exceptions are members of the Brassicaceae plant family (cabbage, broccoli, cauliflower, radish, turnips, canola, etc.) the Amaranthaceae plant family (beets, spinach, chard, etc.) and the Polygonaceae plant family (rhubarb, buckwheat). Virtually all other crop plants worldwide are meant to host some form of mycorrhizal association.

What Are Mycorrhizal Fungi?

The body of the mycorrhizal fungus consists of microscopic filaments called hyphae. Individual hyphae are approximately 1/25th the diameter of a human hair and can grow up to 15 to 25 inches in length. These strands grow from within and around the root cells of the host plant, spreading out 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”). As stated above, most agricultural plants, including most grains, vegetables, orchard trees, vines and turfgrasses evolved with, and are naturally “designed” to achieve optimum growth and vigor by forming these fungal relationships. There are approximately 150 arbuscular mycorrhizal (AM) fungal species on the whole planet forming with perhaps 300,000 plant species.

Nearly all of the AM fungal species are generalists which will associate with a wide variety of plants, in a broad assortment of soil types, geologies, topographies and climates. The numbers of hyphae on a root system can be prodigious. Just a teaspoon of healthy soil can contain up to several miles of fungal hyphae. The resulting nutrient and water uptake efficiency of crop plants is increased considerably. Agricultural soil often contains abundant “pools” of nutrients but the availability of these nutrients to the crops themselves may be limited.

What they do

This plant-fungus association can deliver considerable benefits in agricultural operations. The effect on the root system of a mycorrhizal-colonized plant is extensive. As part of this relationship, most of the absorbing area of the root system is actually fungal hyphae. Hyphae are far thinner than roots or root hairs and are able to penetrate the tiniest pores and fissures in the soil.

Research confirms that mycorrhizae are particularly important in mobilizing phosphorus, nitrogen, zinc, iron, calcium, magnesium, manganese, sulfur and other important soil nutrients by enzymatic release from tightly held chemical bonds and transporting them back to the plant. Crop plant uptake and utilization of fertilizer inputs likewise becomes far more efficient, often leading to significant savings in fertilizer costs. 

Drought tolerance & yield

No one understands better than farmers that agriculture’s need for fresh water is not always in sync with nature’s inclination to provide it. We often see abundant, lush vegetation in natural and wild systems without the benefit of irrigation. How do natural areas provide for such luxuriant plant growth without irrigation?

One key factor is the mycorrhizal threads attached to plant roots, which so thoroughly scour the soil for available resources. They absorb water during periods of adequate soil moisture, 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 which act like a giant sponge to protect the plant communities from extreme soil moisture deficits.

Mycorrhizal filaments can penetrate into the smallest of soil pores and fissures to access microscopic sources of water that are unavailable to the thicker roots. An extensive body of research documents the importance of the mycorrhizal relationship for efficient water use and drought protection among a wide array of important crop species. The declining availability of water and its ever-increasing cost are formidable issues facing today’s farmer and mycorrhizal fungi can be powerful tools to enhance water-use efficiencies.

Monitoring various crops indicates that mycorrhizal inoculation is having a positive effect on yields. For example, data presented at the 2010 National Allium Research Conference in Sparks, Nevada showed onion seed treated with a powdered endo-mycorrhizal outperformed controls. Average yields on the mycorrhizal-treated onions were 62 percent greater compared to controls and significantly greater than any other treatments. At this rate of increased production, the return to the grower (at $10 per 50 pound bag and an extra 600 bags acre) would be $6,000 per acre on a $20 per acre investment in mycorrhizal inoculum. Recent harvest results of a rice trial in side by side 75-acre blocks in California’s Sacramento Valley yielded 8 percent more grain in the mycorrhizal inoculated seed area compared to the control area an extra $173 per acre and a 10-fold return over the cost of the mycorrhizal inoculum. In another 2010 California study, Barley seed inoculated with a powdered, endo-mycorrhizal resulted in average yields of 7,778 pounds per acre compared to 6,030 pounds per acre in control areas a 29 percent increase. The mycorrhizal inoculated barley was at least 6 inches taller than the untreated crop and netted an extra $145 per acre. In Wisconsin at Gagas Farms, Inc., soybeans were inoculated with a mycorrhizal inoculant. Yield was significantly increased by over 9 percent compared to side-by-side, nontreated areas.

Conserving nutrient Capital

Nutrient loss from agricultural ecosystems is among the top environmental threats to ecosystems worldwide, leading to reduced plant productivity on the farm and eutrophication of surface water near nutrient-rich ecosystems. Hence, it is of pivotal importance to understand AM fungi, whose absorbing filaments are widespread across nearly all natural ecosystems and reduce nutrient loss from irrigation or rain-induced leaching events. Corkidi, et al. 2010 at the University of California, Riverside and International Plant Propagators Society found that mycorrhizal plants grown in half and full rate of fertilizer were significantly larger than non-mycorrhizal plants. The authors also found that shoots from mycorrhizal plants averaged 30 percent higher nitrogen content and a whopping 300 percent higher phosphorus content than shoots of non-mycorrhizal plants. Perhaps more importantly, the authors documented a significant reduction in the nitrates, ammonium and phosphate found in leachates for the full rate of fertilization. Ammonium and phosphate losses were reduced 30 percent with mycorrhizal inoculation.

In other recent research (van der Heiden et al. 2010,) AM fungi-inoculated soils lost 60 percent less phosphorus and 7.5 percent less ammonium compared to control areas without AM fungi. Similar results were obtained for areas planted with each of three different plant species. In research by Rinaudo et al. 2010, the authors note that “previous work has emphasized that AM fungi are important for the sustainability of agricultural ecosystems by enhancing crop nutrition by providing protection against stress and by improving soil structure.”

Does My Farmland Have Any Mycorrhizae?

Certain modern agricultural practices are known to suppress biological activity in soils. Land clearing, fungicides, certain chemical fertilizers, cultivation, compaction, isolation from natural area, soil erosion, organic matter loss and periods of fallow are all factors that can contribute adversely to populations of beneficial mycorrhizal fungi. Soil testing worldwide indicates that many intensively managed crop and pastures lack adequate populations of mycorrhizal fungi. Basically, loss of naturally occurring mycorrhizal fungi comes about in three fundamental ways:

1. Fallow soil is among the biggest causes for the demise of mycorrhizal fungi. The fungi are dependent on their host plants for sustenance and cannot survive for any extended duration without the presence of living roots.
2. Tilling, which by itself may adversely affect mycorrhizae, generally leads to fallow conditions, which in turn, impacts the fungi. Therefore, even no-till practices may not necessarily preserve mycorrhizae either. When annual crops are harvested, the roots soon die and any mycorrhizal fungi die with them unless new living roots are introduced in the form of another crop within several weeks. One might think that the spores left in the soil would regenerate the mycorrhizal populations in the next crops planted even after an absence of living roots and
   hyphae. However, for annual crops, few if any spores develop in one year and after repeated cycles that include regular intervals of fallow, those few spores that are present eventually expire and are not sufficiently replaced by the gradually diminishing mycorrhizal populations. 
3. The other critical situation that eventually eliminates mycorrhizal populations from many agricultural lands is lack of proximity to natural populations from which new colonization usually spreads. The crop plants become isolated from the beneficial mycorrhizal fungi that would, in natural ecosystems, be abundantly available to spread colonization to their roots. Without adjacency  to natural areas acting as a source of mycorrhizal hyphae and spores to repopulate depleted lands, arbuscular mycorrhizal populations are very slow to re-establish. Arbuscular mycorrhizal fungi produce their spores below ground and do not move readily via wind or water, but rather grow from root to root. Therefore re-colonization across long distances back into farm soil from undisturbed natural sites becomes slow and difficult. Unfortunately, growing crops immediately adjacent to undisturbed natural ecosystems is not always an option in modern agriculture.
   
   

Living soil

From the food we eat, to the air we breathe, to the clothes we wear, humans depend upon the thin covering of the Earth’s surface we call soil. Arguably this thin and fragile layer of living topsoil is the Earth’s most critical natural resource. Lately there has been tremendous interest from farmers about using soil biology, the evolutionary revolution that brought plants to land 460 million years ago, to increase crop yields while protecting and improving our valuable soil resource.

The predicted increase in world population and the inevitable global surge in demand for food, feedstocks and biofuels will require a shift toward more sustainable, biological farming methods. Worldwide fertilizer and herbicide costs have already increased dramatically over the past five years and farmers may be finding themselves relying less on synthetic chemicals to grow crops. Mycorrhizal inoculation offers a great solution to a more efficient use of fertilizers. More and more growers are finding that they can reduce their fertilizer and water costs up to 30 percent while increasing yields by inoculating with mycorrhizal inoculum, an evolutionary success story that is starting a revolution in our thinking today.

WHAT ARE MYCORRHIZAE?
A mycorrhiza (plural mycorrhizae) is an anatomical structure that results from a symbiotic association between a soil fungus and plant roots. In exchange for a “home,” the fungus provides numerous benefits to the host plant which we’ll discuss in the next section. Mycorrhizal fungi produce an extensive network of microscopic hyphal threads that extend into the surrounding soil or growing medium (Fig. 1). Literally, thousands of research papers have been written on mycorrhizal fungi, but many growers are unsure whether their plants have mycorrhizae or how to identify them. Numerous brands of commercial mycorrhizal inoculums are available but, unfortunately, some have been marketed as a “silver bullet” that will cure all your propagation problems. Since you are all experienced propagators who already know how to grow plants, we’d like to share with you how to make them even better.

BENEFITS OF MYCORRHIZAE
The numerous benefits of mycorrhizae can be divided into those that help to grow plants in the nursery and those that improve sales or outplanting performance.

Nursery Benefits. The hyphal network of mycorrhizal roots (Fig. 1) greatly increases the access and uptake of water and mineral nutrients. However, because growers supply these normally limiting factors, the benefits of mycorrhizae are often much harder to see in nurseries. Progressive propagators are looking for ways to minimize potentially polluting nutrients such as nitrogen and phosphorus in runoff water and inoculating your plants with mycorrhizae can do just that (Sharma and Adholeya, 2004). Mycorrhizae greatly increase rooting volume physiology which translates to faster growth and a shorter production cycle (Gianinazzi et al., 1990). A dramatic example demonstrating the benefits of mycorrhizae can be seen in bareroot nurseries following soil sterilization by fumigation.

Fungal recolonization can be very slow and irregular producing a mosaic pattern of normal and stunted plants (Fig. 2).

Marketing mycorrhizal inoculated plants also increases their value and “green appeal.” Knowledgeable customers and consumers recognize the noticeably larger root plugs and root systems typical of mycorrhizal plants. They know that more vigorous and physiologically active roots translate into successful establishment and long-term plant health.

Figure 1. Several benefits of mycorrhizae are due primarily to the network of hyphal filaments which extend out into the soil or growing medium.

Figure 2. The slow and irregular recolonization of fumigated soils produces a mosaic growth pattern of healthy plants with mycorrhizae (note mushrooms) and stunted plants without mycorrhizae.

TYPES OF MYCORRHIZAL ASSOCIATIONS AND THEIR HOST PLANT SPECIES

The vast majority of plants form mycorrhizal relationships with one or more species of soil fungi. Mycorrhizae can be divided into three categories, which vary by types of inoculum application methods (Table 1):

Ectomycorrhizae. These are the most well-known type of mycorrhizae because the fungal mycelia are easily visible on the root systems and because the spores are airborne. Characteristic fruiting bodies usually develop above the soil or growing medium. Two types of ectomycorrhizal inoculums are available: vegetative or spores (Table 1). Vegetative inoculum consists of a carrier such as vermiculite which contains strands of fungal mycelia and is incorporated into the soil or growing medium prior to sowing seeds or sticking cuttings. Because spores can remain dormant for long periods, spore inoculum has an extended shelf-life and can be watered into the soil or growing medium after the crop plants have become established. Inoculation success can be visually confirmed by estimating percent coverage under a binocular microscope.

Arbuscular Mycorrhizae.   This is by far the largest and most diverse category of mycorrhizal fungi in terms of host plants. Arbuscular mycorrhizae are often un-recognized because the root structures are internal and therefore invisible to the naked eye and no fruiting bodies are produced (Fig. 3). The relatively large spores are soilborne or waterborne which greatly slows the re-colonization process in areas where they have been lost due to disturbance, soil erosion, and loss of vegetation (Perry and Amaranthus, 1997, Reeves et a1., 1979). Arbuscular mycorrhizal inoculum consists of spores or colonized root fragments (Table 1). Powders are effective because they may be applied directly to seeds and therefore are in immediate contact with developing roots. Inoculation success can be confirmed in the laboratory by clearing and staining the roots and measuring percent colonization through a microscope.

Ericoid Mycorrhizae. This category of mycorrhizae is the rarest and occurs primarily with plants from the order Ericales, such as azaleas and rhododendrons. Because spores are not produced reliably, there is no commercial source of inoculum. So, the only way to inoculate plants is to collect soil containing roots from beneath colonized host plants and mix it into the soil or growing medium (Table 1). Success of colonization must be confirmed by clearing and staining roots and examining them under a microscope.

ECONOMICS OF INOCULATION

The decision to inoculate your crop with mycorrhizae often comes down to cost, so there are many things to consider.

In general, it is most cost effective to inoculate very young plants where the average cost can be as low as a penny per plant. Larger  plants require  more inoculum so costs are proportionately greater. Another important consideration is to learn which type of mycorrhizal fungi occurs with your crop species. The next decision is what type of inoculum to buy and how best to apply it. Spore based inoculums have the most options. The least expensive inoculation method is to apply powdered inoculum to seeds. Incorporating mycorrhizal inoculums into soil or growing media also works well. Here is one important consideration regarding cost benefit ratios. When evaluating the use of mycorrhizal inoculums, it’s tempting to consider only the benefits of increased plant growth or reduced fertilizer cost in the nursery. Instead, try to account for all the cumulative mycorrhizal benefits in the nursery, including at sale, or after outplanting —if each incremental benefit contributes 5% to 10%, the combined benefits can total 20 to 40%.

LITERATURE CITED

Gianinazzi, S., A. Trouvelot, and V. Gianinazzi-Pearson. 1990. Role and use of mycorrhizas in horticultural crop production. Adv. Hort. Sci. 4:2W30.

Klironomos,   J.,    M.   Hart,  and  P.   Moutoglis.  2008.  The  effect  of  inoculation  with the arbuscular  mycorrhizal fungus,  Glomus  intraradices  on transplant  survival of perennial herbaceous plants. University of Guelph, Biological Sciences Dept. Ontario, Canada.

Figure 3. Arbuscules (A) are found inside the root cell of all arbuscular mycorrhizal plants, and arbuscular mycorrhizal inoculum contains spores and colonized root fragments (B).

Perry, D.A., and M. Amaranthus. 1997. Disturbance and recovery of microbial populations in ecosytems of the Pacific Northwest, pp. 31-56. In: K. Kohm and J. Franklin (eds.). Creating a forestry for the 21st Century: the science of ecosystem management. Island Press, Washington D.C.

Reeves,  F.B., D. Wagner,  T. Moorman,  and  J.   Kiel. 1979.  The role of endomycorrhizae in revegetation practices in the semi-arid west I. A comparison of incidence of mycorrhizae in severely disturbed vs. natural environments. Amer. J. Bot. 66:6—13.

Sharma, M., and A. Adholeya. 2004. Effect of arbuscular mycorrhizal fungi and fertilizer reduction on the yield of micropagated plants grown in sandy loam soils. Can J. Bot. 82: 322—328.

Steinfeld, D., M.P. Amaranthus, and E. Cazares. 2003. Survival of ponderosa pine seedlings outplanted with Rhizopogon mycorrhizae inoculated with spores at the nursery. J. Arboriculture 29(4):197-208.