Michigan State University Extension
Soils & Soil Management - Fertilizer - 06209710
07/10/97
Fluid Fertilizers--Liquids and Suspensions MSU Ag Facts Extension Bulletin E-933, December 1984, Revised, destroy previous editions
by R. E. Lucas and M. L. Vitosh, Department of Crop and Soil Sciences
Plant nutrients can be absorbed through the leaves, but the amount that can be applied without burn is limited! The minimum nutrient requirements for plants are rather specific, regardless of the method of application. Therefore, foliar feeding should not be expected to supply sufficient major nutrients for plant growth. Conversely, leaf feeding or any other program will not benefit plants if they already have ample amounts of nutrients. For example, ample nutrient levels for corn ear leaves at silking time are about 3 percent nitrogen, 0.3 percent phosphorus and 1.7 percent potassium. Plants will take up additional nutrients but without an increase in yield. We sometimes call this "luxury feeding."
Foliar application of micronutrients, such as manganese, zinc, copper, iron, boron and molybdenum, can be successful because of the small amounts required for normal plant growth. It takes less fertilizer to meet plant needs for micronutrients if fertilizer is applied to the leaves rather than the soil. (See Table 9, MSU Extension bulletin E-486, for the suggested rates and sources of secondary and micronutrients used for leaf application.)
Leaf feeding can be used to good advantage when spray programs are necessary for pest control and the fertilizer material is compatible with the pesticide. The practice of applying only the major plant nutrients (N-P-K) by leaf feeding generally gives poor returns, however, because of the relative high energy and labor costs per unit of plant nutrient applied. Extra field trips also increase soil compaction problems. Thus, leaf feeding should be considered a supplement to, not a substitute for, the regular soil application of fertilizers.