Can foliar fertilization improve crop yield where no signs of nutrient deficiency can be seen? This is an oft asked question which unfortunately does not have a black and white answer. The objective of this paper is to briefly highlight what is known about leaf functions and provide an overview of the performance of foliar fertilizers.

Supplying nutrients to plants is a primary function of roots and not leaves. Though not a primary function, nutrients may enter a plant through leaves. Nutrient uptake by leaves is much less than roots, but like roots, many factors impact the uptake of nutrients from leaves. Marschner (1994) provides a list of concerns related to foliar application of nutrients: (1) Low nutrient penetration rates, particularly in plants with thick cuticles; (2) Runoff from hydrophobic leaf surfaces; (3) Washing off by rain; (4) Rapid drying of spray solutions; (5) Limited rates of translocation of some nutrients; (6) Limited amounts of macronutrients that can be supplied by one foliar application; and (7) Leaf damage. Marschner (1994) also provides some guidance on where foliar nutrient applications may be beneficial. They include: (1) Soils where nutrient availability is low. This is particularly true for micronutrients on soils with a high pH and high organic matter content. (2) Conditions where dry topsoil limit nutrient availability. (3) At the onset of reproduction when root activity decreases and nutrient uptake is reduced.