Potassium release kinetics and its correlation with pinto bean (Phaseolus vulgaris) plant indices

The rate of potassium (K) release from soils is a dynamic process, and it is important to the evaluation of soil K availability to plant. The objectives of this study were to determine K release characteristics and the correlation of these parameters with bean plant indices in surface soils of 15 calcareous soils. The kinetics of K release was determined by successive extraction with 0.01 mol/L CaCl2 in a period of 2-2017 h at 25 +/- 1 degrees C Pc. The results showed that the cumulative K release ranged from 111 to 411 mg/kg. A plot of cumulative amount of K released showed a discontinuity in slope at 168 h. Thus, two equations were applied to segments of the total reaction time (2 to 168 and 168 to 2017 h). The amounts of K released during 2-168 and 168-2017 h ranged from 55 to 299 and 44 to 119 mg/kg, respectively. Release kinetics of K conformed fairly well to parabolic diffusion, simplified Elovich and power function models in two segments. Potassium released after 2-168 and 168-2017 h and K release rate constants were significantly correlated (P < 0.05) with bean plant indices, while correlation between K extracted by using 1 mol/L NH4OAc (ammonium acetate) and plant indices was not significant. The results of this research showed that information obtained from K release studies in laboratory and kinetics equation parameters can help to estimate the K supplying power of soils.