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 inunderstanding 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 that 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 micro-nutrients 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. I have found the cannabis cuticle a marvel to work with. Cannabis cuticle will very greatly from leaf to leaf depending on the applications of some harsh toxic chemical like that contained on
Neem oil. Application of this type of chemical on cannabis will increase the cuticle thickness in what I call a (quick and thick cutical), hense leaf shine. Foliar feeding harsh sprays on cannabis fan leaves does not harm the smoke quality as long and those leaves are trimmed from the finished product.
Poor soil 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 micro-nutrients 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 eficiency. 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.
• Cannabis leaf wax (cuticle) is very reactive to foliar sprays.
• 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.
• Cannabis stomatal resistance is quick. In Colorado cannabis stoma's are usually closed tight during the Dry days. In the humid climate of Northern California the stomas will open/partial open more frequent. I have found a sudden increase in humidity and low intensity lighting will increase foliar application effectiveness.
And with this paper comes my cannabis conclusion on Foliar Feeding plants. Compost tea with Bt bacteria in balance and you'll experience dark green thick leaves and very healthy plants that will resist bugs.
NOTE: Whatever you sprayed on a cannabis plant will reside in cuticle (leaf wax) and will be absorbed in the top layers of leaf materiel that has been sprayed also. Try to keep chemicals off the bud material. The trick is to watch As your plant buds, it loosed it immunization to bugs, so you'll need to be in control with Prior Foliar food and Bt bacteria application to soil and leaves so your buds can finish without a massive Bad Bug invasion at the end toward harvest. If done correctly and perfected your bud can remain organic and taste better than most herb you smoke today.
Old research paper by Lino with lots of copy and paste from PHD professors research.
Now the real question begins, When should a farmer stop foliar feeding plants, ? I try to stop at a minimum of 4 weeks but 6 week if I feel brave.
