Foliar Nutrition of Plants

    It has been known for many years that plants are able to absorb essential elements through their leaves.  The absorption takes place through the stomata of the leaves and also through the epidermis.  Movement of elements is usually faster through the stomata, but the total absorption may be as great through the epidermis.  Plants are also able to absorb nutrients through their bark.

    The following elements have been successfully used to supply nutrients for plant growth by apply them as foliar sprays to the leaves:

    One difficulty in using foliar sprays to supply essential elements to crops is that translocation of the applied element may not be rapid enough for increasing crop yields.  With some plants this problem is more difficult than with others.  For example, the relative mobility of essential nutrients in bean plants when applied as a foliar spray in order of decreasing mobility, was as follows:

    Nitrogen fertilizer compounds have been used for several years as foliar sprays.  Sodium nitrate, ammonium sulfate, potassium nitrate, and urea have all been used experimentally, but only urea gives satisfactory results.  The other fertilizers cause the burning of leaves, due partly to the high osmotic concentration of the spray solution.

    Urea has been successfully sprayed on apple trees, tomatoes, celery, lima beans, potatoes, cantaloupes, cucumbers, and sugar cane.  Amounts up to 15 pounds of urea per acre at one spraying have been used with beneficial results on apple trees.  Higher concentrations burn the leaves.  The usual concentration for apple trees is five pounds of urea per 100 gallons of water.  This is commonly mixed and applied with the regular spray materials at weekly intervals early in the growing season.

    The application of urea fertilizer to leaves of plants has given response approximately equal to that of fertilizer applied to the soil.  The uptake of urea is faster when it is sprayed on the leaves, but it is cheaper to apply it to the soil.

    Phosphorus is capable of being utilized by the plant when it is sprayed on the leaves.  Although the practice is not common, there are many good reasons for predicting that there may be an increase in the foliar application of phosphorus.

    One reason is that in most soils only a small percentage of phosphorus fertilizers is recovered by the plant (averaging about 20 percent for the first year); whereas, when phosphorus is sprayed on the leaves, nearly all of it is absorbed.  In one experiment, approximately three pounds of P₂O₅ sprayed on tomato leaves gave a greater early growth than did 135 pounds of P₂O₅ applied to the soil.  The yield of tomatoes, however, was 12 percent greater when the 135 pounds of P₂O₅ was sprayed on the leaves.

    Potassium applications as foliar sprays have been made, using potassium sulfate fertilizer.  Some leaf injury resulted, and the conclusion was reached that soil applications are far more satisfactory.

    Magnesium is now commonly applied to plant foliage as solutions of magnesium sulfate (Epsom salts).  One reason for the popularity of the practice is that soil applications of magnesium commonly take three years to correct magnesium-deficiency symptoms of such perennials as apple trees, whereas foliar sprays are effective within a few days after application.
   
A foliar application of a two per cent solution of MgSO₄ to tomatoes, oranges, and apples has relieved magnesium deficiency and has increased crop yields.

    Calcium is seldom applied as a foliar spray because it can be efficiently applied to the soil.  If CaCO₃ is too slow in reaction, then CaO or Ca(OH)₂ can be applied to the soil. CaCl₂ is primary method of applying Ca to foliage.

    Sulfur sprayed on leaves is readily absorbed by the plants.  This fact was demonstrated, however, in connection with the study of the influence of certain sulfur sprays when used as a fungicide.  Although there have been no reports of a sulfur deficiency being relieved by sulfur sprays, the practice may become established because it is physiologically sound.

    Iron has been sprayed on foliage since about 1916 to relieve chlorosis.  The first of such research work was carried out with chlorotic pineapples growing on highly alkaline soils in Hawaii.  Periodic sprays of five percent ferrous sulfate are now common practice on Hawaiian pineapple  plantations.  The biggest obstacle to this practice is the fact that, even though the iron moves readily into the leaves, it is translocated very slowly.  As a result, after spraying with ferrous sulfate, chlorotic spots may still be in evidence in places which did not receive some of the iron spray.  Iron chelates have also been successfully used as a spray.

    On alkaline soils where iron chlorosis is common, applications of iron compounds to the soil have not been very successful because the iron is soon rendered insoluble.

    The leaves of chlorotic grain sorghum on calcareous soil in Tulare County, California, were sprayed with 40 gallons per acre of three percent ferrous sulfate solution about one week before heading, at a cost for materials of 50 cents per acre.  The yield of grain sorghum was increased from 540 pounds of grain on the untreated plot to 1,774 pounds on the treated plot, an increase of 222 percent.

    Applications on the soil of more than 3,000 pounds per acre of ferrous sulfate were required to accomplish similar increases in yields.

    Manganese.  While soil manganese becomes less available in alkaline soils, many states in more humid regions of the country often report manganese deficiencies in peat and muck soils and in local areas of alkaline soils.  Manganese deficiencies are frequently corrected by spray applications of manganese sulfate, usually five to 10 pounds per acre.  Manganese sulfate is also applied to the soil at rates of from 20 to 150 pounds per acre.  Manganous oxide is also used to correct manganese deficiencies.  In alkaline soils an acid-forming material, usually fertilizer, is applied to prevent fixation of the applied manganese.  NH₄⁺ applied H⁺ released.

    Zinc is often sprayed on the leaves of apple and pear trees to relieve "leaf rosetting," a symptom of zinc deficiency.  Approximately 25 pounds of zinc sulfate in 100 gallons of water (roughly a three per cent solution) applied to apple trees just before the buds open has corrected zinc deficiency.  Zinc sulfide, zinc oxide, and zinc carbonate have all been successfully used as sprays.  Driving galvanized (zinc-coated) nails in trees also relieves zinc deficiency.

    Boron, as boric acid or borax (sodium tetraborate), used as a foliar spray has proved to be a successful method of application.  Internal cork of apples has been controlled by spraying the foliage with eight pounds of borax in 100 gallons of water.  As little as two pounds of borax per 100 gallons of water has checked "cracked stem" of celery.  Boron has been satisfactorily applied to the soil, either alone or in mixed fertilizers.

    Copper deficiency has been controlled by spraying the leaves with a mixture of eight pounds of CuSO₄ plus eight pounds of Ca(OH)₂, in 100 gallons of water.  Without the calcium hydroxide, the copper sulfate injures the foliage.  Copper oxide has also been used successfully as a spray.

    Molybdenum, as sodium molybdate, 1 ounce in 100 gallons of water, has eliminated deficiency symptoms in citrus trees.  Somewhat like iron, however, molybdenum does not seem to be readily translocated within the plant.  Spraying only the lower half of a citrus tree that showed molybdenum deficiency did not cure the deficiency symptoms on the upper half of the tree.

    In highly acid soils, molybdenum is sometimes fixed in an unavailable form, thus causing deficiencies, particularly for legumes.  The amount of molybdenum in soils and the amount required by plants is very small.  In addition to sodium molybdate soil application of 0.5 to two pounds per acre, a commercial seed-coating preparation (Molygro) for some legumes, applied at about two ounces per acre, is used to correct deficiencies.  Broadcast applications are best mixed with limestone on very acid soils to prevent fixation.