“Fertilizers sprayed on the leaves of trees and shrubs are more effective than soil applications?”
By Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University
Recently, I received an email from a professional colleague whose clients often ask about foliar feeding
as a method of fertilizing plants. As he says, “All the water soluble fertilizer companies advertise the
practice all the time.” What, he wondered, was my opinion of the practice?
Foliar feeding involves spraying the foliage of target plants with water-based fertilizers. The logic for the
practice is based on scientific research from the 1950’s, which demonstrated that leaves can take up
minerals through their stomata, and in some cases through their cuticles. This research is consistently
cited in the argument that foliar feeding is 8, 10, or even 20 times more effective than traditional soil
application.
In assessing the advertised claims for foliar feeding of shrubs and trees, I had particular questions that are
answered in bulleted lists below (all bullets are directly from marketing media):
(1) What are the advantages of foliar feeding over soil application?
• Immediate results
• Prolong bloom
• Increase crop yields
• Increase storage life of food crops
• Boost growth during dry spells
• Increase cold and heat tolerance
• Increase pest and disease resistance
• Maximize plant health and quality
• Help the internal circulation of the plant
(2) When should one use foliar nutrients sprays?
• When the soil is too cold for conversion of nutrient elements into usable forms
• When it is at least 72°F
• Any time except when it is too hot or too cold
• Transplant time
• Bloom time
• When a quick growth response is desired
• After fruit set
• Every 2-3 weeks
• Any time of stress
• As long as the plant has leaves that aren’t dormant
• When the soil is deficient in nutrients
(3) What time of the day, and in what quantity, should you apply foliar fertilizers?
• Early morning
• Until it drips from the leaves
• There is no improper way
(4) How long will material last on the leaves?
• 24 hours
• 1-2 days
• Four weeks
(5) What nutrients are critical components of foliar feed fertilizers?
• Nitrogen
• Phosphorus
• Micronutrients
(6) Apart from commercial formulas, what should homemade mixtures contain?
• Seaweed
• Compost tea
• Natural apple cider vinegar
• Blackstrap molasses
• Fish emulsion
• Baking soda
As one company states, “In our opinion, foliar feeding is by far the best approach to use to insure
maximum growth, yields, and quality by overcoming limitations of the soil and its ability to transfer
nutrients into the plant.”
The Reality
If these laundry lists look more like a multiple choice test rather than solid information, it’s not surprising.
Foliar feeding is yet another agricultural practice best suited to intensive crop production under specific
soil limitations rather than as a landscape management tool. Thus, advertisers take great liberties with the
facts, often resulting in contradictory messages (note especially the recommended temperature
conditions!). Rather than individually refute the numerous errors in the claims, I’ll explain when foliar
feeding might actually be beneficial.
The original 1950’s research came from Michigan State University and was particularly useful in
understanding how nutrients move within plant tissues. As explained by Dr. Tukey in his testimony to
the Joint Committee on Atomic Energy, use of radiolabelled nutrients allowed his team to discover
“…that a leaf is a very efficient organ of absorption. The amounts may at first seem relatively small, but
to offset this handicap, the efficiency is high.” From this advertisers claim that foliar feeding is 8, 10 or
20 times more effective than soil application. This is not accurate for several reasons.
Obviously, materials applied directly to a leaf are more likely to enter the leaf in large quantity than the
same materials applied to the soil. Leaching, chemical reactions, microbial activity, etc. can decrease
what actually reaches the roots and is taken up into the plant. But materials applied to the leaf do not
necessarily travel throughout the entire plant as effectively as they do through root uptake. They often
remain in the same or adjoining tissues but travel no further. This is especially true of those elements
recognized as “immobile” within plant tissues (apart from root uptake and xylem transport).
Research over many decades has explored the mineral uptake and transport of many species of fruit trees,
conifers including pine and spruce species, and some hardwoods of ornamental or commercial value.
Results have been mixed in many cases, with some species responding well to treatment and others
remaining unaffected. Generally, the results suggest that foliar application of particular nutrients can be
useful in crop production situations where soil conditions limit nutrient availability.
For instance, alkaline soils do not readily release many metallic nutrients, especially iron and manganese. Zinc,
copper, magnesium, molybdenum, boron, and calcium are other micronutrients required in small
quantities that have been applied to foliage in an effort to relieve deficiencies and combat fruit disorders.
Fruit, as adjacent tissue, can benefit from foliar spray. But this is a localized application that does not
affect the trunk or roots – and therefore is not a solution to soil imbalances. In fact, researchers
consistently state that foliar treatments are a specialized, temporary solution to leaf and fruit deficiencies
in tree fruit production but will not solve larger soil management issues.
On the other hand, macronutrients, such as nitrogen, phosphorus and potassium, are needed in larger
quantities. While many of these are mobile in the plant, it is pointless to apply them to foliage as leaves
cannot take up enough material to supply the entire plant’s demands. Furthermore, foliar application of
high concentrations of such nutrients often results in leaf burn as water evaporates and the fertilizer salts
remain behind. Substituting numerous, lower concentration applications would not be cost effective.
Species differ widely in their ability to take up nutrients through their leaves. Differences in cuticle
thickness, stomatal resistance, and other genetic factors will influence uptake, as will environmental
conditions. Plants in a protected situation (like a greenhouse) have thinner and more porous cuticles than
plants in the field and take up foliar sprays much more readily. Likewise, plants adapted to arid
environments naturally have thicker, less penetrable cuticles than those from more moderate locations.
A better management solution to the problem of nutrient availability is to choose plants that can adapt to
the existing soil conditions. If you have alkaline or calcareous soils, for heaven’s sake don’t install acid
loving plants! Poor plant selection in terms of mineral nutrition will be a management problem for the
lifetime of the plant – which may be pretty short. Choose cultivars of species that are more resistant to
alkaline soils – they are able to acidify the root environment so that micronutrients are remobilized from
the soil and available for uptake.
The existing research does not justify foliar fertilization of landscape plants as a general method of
mineral nutrition. It can be useful for diagnosing deficiencies; for instance, spraying leaves with iron
chelate can help determine if interveinal chlorosis is from iron deficiency. It would obviously have
benefit for those landowners with landscape fruit trees that perpetually have flower or fruit disorders
associated with micronutrient deficiencies. Applying fertilizers to leaves (or the soil) without regard to
actual mineral needs wastes time and money, can injure plant roots and soil organisms, and contributes to
the increasing problem of environmental pollution.
The Bottom Line
• Tree and shrub species differ dramatically in their ability to absorb foliar fertilizers.
• Proper plant selection relative to soil type is crucial to appropriate mineral nutrition.
• Foliar spraying is best accomplished on overcast, cool days to reduce leaf burn.
• In landscape plants, foliar spraying can test for nutrient deficiencies, but not solve them.
• Micronutrients are the only minerals that are effectively applied through foliar application.
• Foliar application will not alleviate mineral deficiencies in roots or subsequent crown growth.
• Foliar spraying is only a temporary solution to the larger problem of soil nutrient availability.
• Minerals (especially micronutrients) applied in amounts that exceed a plant’s needs can injure or
kill the plant and contribute to environmental pollution.
• Any benefit from foliar spraying of landscape trees and shrubs is minor considering the cost and
labor required.
extensions. This allows for the plant to be able to reach more nutrients and minerals more efficiently than it could on its own. The plant feeds the mycorrhizae extra sugars it produces and in turn the mycorrhizae brings in water, nutrients and minerals the plant needs. It’s a mutually beneficial arrangement that has had hundreds of millions of years in the making. Many plants still grow without mycorrhizae but not as well or as quickly as they would with it. 
water comes, a steady source moisture is required to keep them active and thriving. Making sure your soil doesn’t bake in the sun or dry out for prolonged periods of time can help keep it healthy. A great way to do this is to never leave the soil bare, whether it is seeded with a cover crop or mulched heavily, shading the soil and preventing it from drying is essential to healthy soil life. 
Total coliforms levels check your water for bacteria that usually lives in soil and both animal and human waste. Though these bacteria themselves are not particularly harmful, their presence can act as an indicator that your water supply may be contaminated with run off water from a septic tank, or even the muckheap, paving the way for contamination with things you definitely do not want your horse drinking! Pesticides and extremely harmful substances such as blue-green algae can also be identified through having your horse’s water checked. Testing puts you in the know, and arms you with the information you need to make choices about exactly what your horse’s drinking water exposes him to.
Mycorrhizae are fungi that live in a symbiotic relationship with plants in which energy and nutrient exchange benefit both organisms. The relationship results in improved survival and growth, increased rooting amidst more efficient soil structure, and bio-balance pathogens or other plant species. It is thought that as many as 90% of vascular land plants participate in such a relationship; many could not survive on their own.
The benefits of the ultraviolet radiation in sunlight are more than most people think about. We all have heard that it is a good source of vitamin D, however have you heard all the good affects that vitamin D has to offer. Ultraviolet light and the vitamin D it passes on can help you by strengthening bones and inhibiting the development of some cancer. It also helps maintenance of circadian rhythms as well as well as helping to reduce the risk of seasonal affective disorder which is becoming a more focused on disorder in recent years.
Yet that side of the argument saying that until proof comes out it should be business as usual is not just the cell phone companies. Consider the whole power grid in the United States is an EMF generating network that reaches most people in America and 75% of the world’s population. Not only is that not something that can just be switched off, it is something with powerful backing. In the United States there are leaders who say that there is no danger because the FCC says so, despite any indications to the contrary. Big corporations pay their scientist to find answers they want, not answers that are actually real answers to the question, and it is becoming more and more clear that that is who the FCC listens to in many cases.
Having a beautiful garden can seem complicated and expensive but if you follow these steps, you will see that is something you can do with little money. Make the project design and creation of your garden. Divide the project into phases of your garden for you to go by gradually creating the garden of your dreams. Consider consulting a landscaper and get their opinion, although we can give you some tips on how to make a garden with little money.