Alternative or complementary role of foliar supply in mineral nutrition

Morphology and organisation of leaf tissue is such that it accomodates the uptake of gaseous plant nutrients, whilst that of roots the uptake of water-soluble solutes. These water soluble plant nutrients are mainly supplied with fertilizers. Only in exceptional cases where nutrients are strongly fixed by soils or where aerial nutrient requirement of a crop is higher than the root uptake rates, foliar application can be adopted as a routine fertilization measure. The efficiency with which foliar applied nutrients are utilized depends on the mobility of the specific nutrient throughout the entire plant, mobility comprising long distance transport especially phloem transport as well as the symplastic transport. Potassium and nitrogen are examples of nutrients showing high mobility and when taken up by leaves they can be rapidly distributed throughout the entire plant. Calcium, sulfur, and iron show a low mobility and Ca2+ taken up by leaves cannot be transported to younger tissues or fruits where it may be required. Micronutrient requirements can generally be better met by foliar application than requirements of macronutrients because in absolute terms higher quantities of macronutrients are needed. Iron in the leaf apoplast can be mobilized by a foliar application of diluted acids.