Effect of soil characteristics and potassium application rate on the plant-absorbable potassium forms and transport mechanisms in soil

Although the demand for saving potassium (K) fertilizer application is increasing, the evaluation of available K by exchangeable K that is operationally defined as 1 M ammonium acetate-extractable K has limitations. In order to investigate the contribution of soil K forms and transport mechanisms for K uptake, pot experiment was performed using two soils with varying K application rate. The spinach grown in vermicu-lite-rich soil absorbed considerable amount of K from nonexchangeable fraction and the transport mechanism was mainly by diffusion, whereas the plant cultivated in volcanic ash soil, which contain less vermiculite, mostly absorbed exchangeable K and the contribution of mass flow was substantially high. Exchangeable K and soil solution K in vermiculite-rich soil did not much increased at the high rate of K application. A part of applied K might become readily absorbable- nonexchangeable K. To evaluate K supplying capacity of soil, it is necessary to quantify not only exchangeable K but also the K fixation and the amount of K released from nonexchangeable sites. In soil whose K fixation capacity is low, the transport of K from soil might be modeled by solute transport and cation exchange.