By Robert L. Ellsworth, Ph.D.

Foliar feeding has one major advantage over root feeding; the supplying in essential minerals more rapidly. The supply is small, and thus temporary (except, perhaps, for the trace minerals) and the supply also needs to be available from the soil for complete correction. Therefore, foliar sprays with the major and minor nutrients should be considered as stimulants to promote better health and new root development. The foliar application should be allowed to contact the soil supply as well as top growth or fruiting promotion. It can also be a valuable way to help control the quality of the finished products.

Stomata are sites of gas exchanges in the epidermal cell layer of the leaf. CO_2, O₂, SO₄, NH₄, and NO₃  may also enter the leaf primarily through the stomata. In some industrial areas, plants may even absorb an excess of sulfur to the point of toxicity. In pastures, the intake of Nitrogen as NH through the leaves has been estimated to be as high as 10# per acre.

In higher plants, the uptake of minerals through the leaves may be restricted by the outer walls of the epidermal cells, otherwise known as the cuticle. This layer is designed to slow or prevent the excessive loss of moisture. It also acts to prevent loss by leaching of organic and inorganic solutes caused by rain or sprinkler irrigation systems. 

The cuticle also functions as a weak cat ion exchanger, partially due to the negative charge of the pectin material contained therein. The movement of solutes through the cuticle takes place in cavities called actodesmata. They serve to provide channels for the mineral elements to enter from outside the plant via foliar sprays. The rate at which minerals are taken through the leaves is dependent to some extent on the mineral balance within the plant. Deficiencies increase rate of uptake. The rate is also dependent upon the age of the leaf tissue. The rate declines with age. In contrast to root uptake of minerals, which occurs primarily in the dark, the uptake by leaves is stimulated by light. This is least partially due to the ATP cycle of energy flow during daylight hours. ATP is formed during light hours in the chloroplasts, and is thus available to provide energy needed for mineral transfer.  However, the increase in temperature during daylight hours will usually decrease humidity and thus slow mineral uptake via leaves because of evaporation of moisture from the foliar spray. Therefore, the uptake of foliar sprays needs to be rapid to avoid loss of the moisture necessary for induction.

Foliar feeding becomes particularly important when plants are stressed by such problems as extreme heat, large mineral deficiencies, Nitrogen excess, disease, and maturity. Root activity slows as the reproductive stage progresses, therefore, the application of ViTech Foliars to aid in quality is very important.

Summary: 1) Foliar spray minerals enter the leaf largely through the ctodesmata in the leaf epidermis. 2) It is good to use an “entry promoting” material such as ViTech ViCare with each spray to limit evaporation and speed uptake 3) Temperature has a dramatic affect on absorption. 4) Mineral foliar sprays with ViTech ViGorator  carrier are particularly important as the fruit is setting, and then again in maturation.