You know how it is: you hear something enough, you believe it's true. Look on the side of any box of water soluble fertilizer, or any organic gardening guide, and there will be instructions on foliar feeding: spraying a water soluble fertilizer onto the leaves of a plant, as an alternative source of nutrition for the plant.
A few weeks ago on the radio show, a tempest in a teapot developed when Milo Shammas, the President of the Dr. Earth line of organic products, mentioned that the best way to apply a foliar fertilizer, which he endorses, is in as fine a spray as possible. In his corner, Rodale's Encyclopedia of Organic Gardening, which states: "Plants can absorb liquid fertilizers through both their roots and through their leaf pores. Foliar feeding can supply nutrients when they are lacking or unavailable in the soil, or when roots are stressed. It is especially effective for giving fast growing plants like vegetables an extra boost during the growing season...Any sprayer or mister will work, from hand trigger units to knapsack sprayers. Set your sprayer to emit as fine a spray as possible."
Disagreeing with the "fine spray" approach is another organic advocate and frequent guest of the radio shows, Steve Zien of the Sacramento area-based organic consulting service, Living Resources Company.
Zien says, "Some time ago I read a few studies that indicated that most of that (spraying with a fine spray) was not necessary. The studies used radio isotopes to follow the nutrients in the foliar fertilizer. They found that it got into the plant even when the water droplets were large. Another study indicated that even with the best spray equipment making the smallest water droplets possible with today's technology, the water droplets were still too large to physically enter the plant. They concluded that water droplet size is not important when foliar feeding. Other studies have shown that foliar fertilizer can even be absorbed by branches and tree trunks. These two facts indicate that where you spray is also not critical. Numerous studies have shown that foliar feeding is much more efficient at getting the nutrients absorbed and to the entire plant and more rapidly as well.
I think all the studies emphasize that even with all the benefits of foliar feeding, it cannot be considered a substitute for proper soil nutrition, and I fully agree with that. You need to feed the soil foodweb for healthy, pest resistant plants.I no longer worry about where I apply the foliar fertilizer. I try to apply it to as many plant surfaces as possible but do not worry about paying attention to the undersides of the leaves."
Throwing cold water on both those practices are a couple of college educators, Deborah Flower of the Horticulture Department of American River College in Sacramento; and, Linda Chalker-Scott of the Horticulture Department at Washington State University and author of the award-winning book, "The Informed Gardener", who says this about foliar feeding:
"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 a 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."
Chiming in is Deborah Flower of American River College: "I have been reading 'Plant Physiology, 3rd edition' by Taiz and Zeiger, and 'Mineral Nutrition of Higher Plants, 2nd edition' by Horst Marschner. The latter discusses foliar feeding in chapter 4. It says: there are small pores in the cuticle through which minerals can enter the plant. These pores are in highest density near guard cells around stomata and at base of trichomes (hairs, scales, etc.). They are tiny and lined with negative charges. So, only very small (less than one nanometer in diameter) cations and uncharged molecules will enter these openings.
It says leaves do absorb ions, but (pages 123-125):
Rate of uptake is VERY low.
Rate of uptake varies between species and growing conditions. Plants with thicker cuticle (due to species or growing conditions) absorb less.
Older leaves have lowest rate of uptake due to leaky plant cells that fill intercellular spaces, which is where ions travel in the leaf.
A very high concentration of ions is needed outside the leaf to get any into the leaf.
The supply of nutrients in the leaf from foliar feeding is temporary.
There is limited movement of nutrients from leaves to other plant parts.
Urea can enter leaves through these openings (ammonia and nitrate cannot), because it is an uncharged particle, but can cause damage in the leaf, due to nutrient imbalance in the leaf once it is absorbed.
Surfactants should be used with all foliar feeding to increase surface spread of spray.
So, my opinion is that yes, plants do absorb nutrients through their leaves (neither book mentioned absorbtion through branches or trunks) but the amount is very small, nutrients do not travel far from point of entry, and there is lots of nutrient run-off during the process, which can lead to pollution. Therefore, foliar feeding is not effective as the primary source of nutrients for plants. I disagree that foliar feeding gets nutrients to all parts of the plant. There is lots of evidence that fertilizer that gets into the leaf migrates little to other parts of the plant. It stays in the leaf or travels to a strong sink like a fruit. Foliar feeding can correct micro-nutrient deficiencies in leaves and some fruit, but until the nutrition is balanced in the root zone, the symptom will continue to appear in new plant parts. Many of my students seem to believe foliar feeding is better for the plant than nutrient absorption by roots, and that concerns me. Foliar feeding can be used to correct some nutritional problems, primarily in production situations, but should not be relied on as the primary source of nutrients for the plant. If people are foliar feeding I believe most of the nutrients being absorbed by the plant are entering the roots, probably after running off the plant onto the soil."
Milo Shammas, of Dr. Earth, responds: "Fred, all very true and I agree with her, I do not recommend foliar feeding as the primary source of nutrients. Whatever runs off the foliage will ultimately be absorbed by the root system. Nothing can replace ion absorption through the root system.
Foliar feeding as a supplement? Yes
Is it effective? Yes
Would I depend on it solely? No
Is there harm in using it? No
Do younger leaves absorb it better? Yes
I own and manage 45 acres of organic walnuts and I personally spray my ranch with Dr. Earth liquid solution twice a year, I do spend the money on it, I have conducted the efficacy and I know it works, I do believe in it, I do endorse it, I do not depend on it."
After standing back, listening to all this, I have come to the conclusion: although foliar feeding may have minimal value, it does have a bigger, positive effect: washing off bad bugs from the leaves. Of course, a spray of water can accomplish the same thing.