Healthy soil and sustainable growing

["TradingPostPaul" <tradingpost@riseup.net>]

http://agcrops.osu.edu/story.php?setissueID=143&storyID=850

Foliar Fertilizers – How Much is Actually Taken Up and What is the
Agronomic
Value?
by Robert Mullen, Peter Thomison, Maurice Watson

Every year there are more and more foliar fertilizer materials hitting the
ag market, and the question always comes up – how efficiently does a
plant
take up foliar nutrients and should I be relying more on foliar
fertilizers?
(Especially when dry, soil applied fertilizers are increasing in price).

We will begin at a fundamental agronomic principle – if the soil is
supplying an adequate amount of a given nutrient then application of that
nutrient in a foliar form is unlikely to result in increased yield. This is

true for any nutrient. Another agronomic principle that has to be
considered
– application of a foliar nutrient that results in an increase of that
nutrient within the plant’s tissue will not necessarily lead to a yield
benefit. Often times the application of a foliar nutrient will alter plant
nutrient concentration, but at harvest no appreciable yield benefit is
evident. These two agronomic principles being considered, how much of a
foliar fertilizer is taken up through the leaf?

The amount of a nutrient taken up by plant leaves is highly variable and
strongly dependent upon environmental conditions. Some researchers report
nutrient uptake as high as 90% for foliar applied nutrients while others
report values as low as 30%. Leaf uptake of a foliar applied nutrient is
not
trivial when you consider the physiologic structure of a plant leaf. Leaves

have a waxy cuticle designed to act as a moisture retaining and
consequently
moisture retarding layer. Thus absorption of nutrients can only occur
through small cuticular cracks, stomata, or aqueous pores.

The agronomic value of foliar fertilizers is primarily related to the
nutrient being supplied and whether or not the soil is providing an
adequate
amount. Foliar application of nitrogen, phosphorus, and potassium is
unlikely to overcome severe nutrient deficiencies because you can not apply

enough to provide the crop adequate nutrition throughout the growing season

(unless multiple passes are made). Much work has been conducted throughout
Iowa and Minnesota with the results being less than promising
(http://www.ipm.iastate.edu/ipm/icm/2005/6-20/folfert.html;
http://www.extension.umn.edu/cropenews/2003/03MNCN21.htm). Maintaining a
good soil fertility regimen should decrease the need for foliar fertilizers

in a crop production system. Ohio State University (and other Land Grant
Universities) continues to evaluate new fertilizer materials to determine
their efficacy for crop production.

The application of micronutrients can be beneficial to crop production
because these nutrients are needed in much lower concentrations, but care
must be taken to ensure that phytotoxicity does not occur from the
application. Application of foliar zinc, manganese, boron, iron, etc. can
benefit grain yield if they are found to be deficient in the soil. Soil
deficiency is the key to determining whether or not additional nutrient is
required. Soil testing can be useful for some micronutrients, but most
deficiencies are due to other soil factors affecting nutrient availability
primarily soil pH. When a crop is deficient the foliar application of these

fertilizer materials can benefit the bottom line, but make sure the
deficiency exists before considering their application. Visual assessment
may not be adequate for diagnosing a micronutrient deficiency because many
plant stresses result in similar symptoms, therefore plant/soil sampling
should be used to confirm the problem.

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http://www.spraytec.com/articles/JuneJuly99/nutrients.asp

Plants absorb nutrients as well as other chemicals through their foliage to

varying degrees. Growers in most all types of agriculture apply foliar
nutritional sprays from time to time for various reasons. A basic
philosophy
many growers utilize is to apply what is believed to be required to the
soil
in the fertilization program, and use nutritional foliar supplements as a
tool to give crops any nutrients they may still be lacking. Even though
growers constantly use this technique as nutritional supplement, the
mechanism of foliar absorption of nutrients is not well understood.
In order to understand foliar absorption, we must first take a look at the
surface of a leaf. Moving from the outside. The leaf surface is composed of

layers of cuticular wax, followed by the cuticle or "skin" of the leaf. The

cuticle exudes the wax. Under the cuticle are the cell walls of various
types of leaf cells. Inside the cell walls are the plasma membranes of the
cells themselves. A foliar applied nutrient must pass through the cuticular

wax, the cuticle, the cell wall, and the membrane in that order. Sometimes
the nutrient will pass through these various layers, while other times it
may pass through the spaces between these layers. Such absorption involves
both active and passive processes of the leaf.

The second and most often the, major means of foliar absorption is through
the stomates, which are microscopic pores in the epidermis of the leaf.
When
the stomates are open, foliar absorption is often easier. Plant species
vary
widely in the, number of stomates per leaf area, and in their relative
distribution.

Some plants have more stomates on the lower leaf surface than on the upper
and some vice versa.

In simpler terms, some plants are, good at absorbing nutrients through
their
leaves, while others are not. The variables tend to be how many stomates
and
how they are distributed, and how thick the waxy cuticle of the leaf is.
Plants with large, broad soft leaves such as Spathiphyllum or many bedding
plant species are rather efficient at absorbing, foliar nutrients. Palms,
Avocados, Cucurbits, some Citrus and Zamias for example are not as adept at

this absorption, due to the thicker tougher nature of their foliage.

The speed of absorption of nutrients is quite variable according to the
nutrient, and to some degree the plant type. Rates of foliar absorption
have
generally not been studied in ornamental varieties.

ONE THING THAT IS NOT WIDELY KNOWN IS THAT NUTRIENTS ARE GENERALLY ONLY
ABSORBED WHILE THE SPRAY IS WET ON THE LEAF.

Once the spray has dried, absorption generally ceases until the leaves are
moistened again, either by the dew the next day or additional rainfall or
overhead irrigation. The various types of chelating agents are also not
equal in their ability to penetrate the leaf. Some chelating agents work
better on some types of plants, but not necessarily as well on others. The
best chelating agent will depend in part on what type of plant you are
spraying.

Another common misconception regards rates of foliar nutritional
applications.

Generally, there is a great deal of difference between the amount of
chemical it takes to maximize absorption and the amount it takes to burn.
Absorption is the limiting factor, so don't make your rates too high. You
may be able to double or triple the spray rate, but it won't necessary
increase absorption. It will increase risk of spray injury, so be
conservative in your foliar application rates.

There are a number of situations when foliar nutritional supplements are
especially useful. One is during propagation of slow rooting plant
material.
Long term mist propagation can leach nutrients severely, and foliar
nutritional sprays during that time are very helpful. Nutritional sprays
can
be used efficiently to overcome other problems. Another useful foliar
technique is during cold fronts. When a cold front comes down, frequently
you get heavy rain followed by several cold days. During this period, the
fertilizer is not releasing a great deal, and the plants are not feeding.
That is a good time to come in and apply some foliar nutrition to keep the
plants moving until things warm up.

Several techniques should be used when trying to maximize foliar absorption

of nutrients.

One is to try to maximize the time that the spray is wet on the foliage.
This preferably means early in the morning, when humidity is up, leaves are

wet with dew. Spraying in the middle of a hot day will give you reduced
effectiveness in absorption. It also helps to add urea or potassium nitrate

to nutritional sprays when applying trace elements. The mechanism is not
known, but there is substantial research that indicates applying these
materials with trace, elements increases trace element absorption. Try to
spray when the stomates are open, preferably. during a cooler time of day.
Some industries like to spray at night, and that can be useful in some
situations. Try also to coat both the upper and lower leaf surfaces where
practical, as many times the spray stays wet on the leaf

longer, and there are more stomates to facilitate absorption on the lower
leaf surfaces of many plant varieties.

The use of wetting agents or surfactants also aids in absorption, by
spreading out the spray from droplets into a broader shape, increasing
contact with the foliage. Surfactants also reduce the angle at which the
spray material enters the leaf, which can be useful. It is generally useful

to thoroughly wet the foliage when applying nutritional sprays.

Low volume sprayers may not be as effective in some cases. You should spray

to run off, and once again cover the lower leaf surfaces.

Finally, do not get too high on your rates. Going higher on the rates of
chemicals applied can actually reduce absorption, as can mixing too many
nutritionals in the tank at a time.

Foliar nutritional sprays can be a very useful technique, especially when
you understand the principles behind it. Nutritional sprays enable you to
correct deficiencies, strengthen weak or damaged crops, speed growth and
overall grow better plants, which is of course, the bottom line.

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Link for an interesting discussion of foliar fertilizer by gardeners -
including results of a replicated experiment

http://forums.gardenweb.com/forums/load/lab/msg041655342430.html?51

Link for an interesting short article on foliar applied biostimulants

http://www.ext.vt.edu/news/periodicals/commhort/pulledarticles/december03-1.
html