POTASSIUM FRACTIONS WITH OTHER NUTRIENTS IN CROPS - A REVIEW FOCUSINGON THE TROPICS

Potassium (K), a plant nutrient with diverse roles to play in plant me tabolism, is required in large amounts by most crops. It interacts wit h many other plant constituents to affect crop yield and quality. The magnitude of this interaction is high in areas of high cropping intens ity, as in the tropics. The interaction of nutrients with K may be in the soil or in plant. Potassium modifies ammonium (NH4(+)) ion fixatio n in soils to restrict nitrogen (N) availability. On the other hand, a n antagonistic effect between K and NH4 absorption has been suggested in which K absorption is restricted. Similarly, magnesium (Mg) or calc ium (Ca) deficiency occurs from ion antagonism in acid soils following K fertilization and in soils with high exchangeable K. Sulfur (S) has been reported to increase K absorption and productivity of oilseed cr ops. With increasing levels of applied or soil K, the severity of phos phorus (P)-induced zinc (Zn) deficiency in corn has been observed to d ecrease. Application of K decreases manganese (Mn) content and iron (F e) toxicity in rice. Application of K has been reported to decrease B levels in plants and to increase incidence of boron (B) deficiency. To p-dressing with K fertilizer was reported to lower the copper (Cu) con tent of alfalfa forage. In root, sugar-producing, or fiber-producing c rops, the sodium (Na) and K relationship is important with the specifi c response to either element depending on which element is in low or h igh supply. Molybdenum (Mo) stimulated K uptake in alfalfa and corn. I n intensive agriculture with high-yielding single crops or with multip le crops per year, farm management must include strategies to maintain substantial K reserves in the soil and to balance K nutrition with ot her fertilization practices.