POTASSIUM DYNAMICS IN SOIL AND SOYBEAN PO TASSIC NUTRITION

Potassium deficiency in soybeans has spread iii Sao Paulo State, Brazi l, as a consequence of soil K reserve depletion. Exchangeable and none xchangeable K dynamics in soil were investigated in order to understan d the reserve exaustion and its significance to soybean nutrition. Soy bean was grown in 8 1 pots with five soils differing in K and clay con tents, in presence or absence of K fertilization. Soil samples were ta ken at 20 days interval and soybean was harvest at R6. Exchangeable (H 2SO4 0,05 N) and nonexchangeable K (boiling HNO3) were estimated in so il sample. Two of the soils were very poor in exchangeable K, and even so there was no response in dry matter due to K fertilization. In spi te of some differences in K contents in some of the soybeans parts, ge nerally K absorption was not affected by K fertilization, except in gr ains. Potassium acumulation in soybean plants were affected by soils, but the response was not closely related to exchangeable K in soil. Th e main K source to the plants was the HNO3 extrated K, showing that th is form is not at all nonexchangeable. After the period of maximum pla nt demand there was a sharp increase in extractable K in soil showing a tendency to a natural equilibrium.