Application of Kinetic Models in Describing Soil Potassium Release Characteristics and Their Correlations with Potassium Extracted by Chemical Methods

Potassium (K) release characteristics in soil play a significant role in supplying available K. Information on K-release characteristics in soils of central Iran is limited. The objectives of this study were to determine K release characteristics and correlations of K release rate constants with K extracted by different chemical methods in surface soils of ten calcareous soils of central Iran. The kinetics of K release in the soils was determined by successive extraction with 0.01 mol L−1 CaCl2 in a period of 2–2 017 h at 25±1 °C. Soil K was extracted by distilled water, 0.5 mol L−1 MgNO3, 0.002 mol L−1 SrCl2, 0.1 mol L−1 BaCl2, 0.01 mol L−1 CaCl2, 1 mol L−1 NaCl, 1 mol L−1 boiling HNO3, 1 mol L−1 NH4OAC, Mehlich 1, 0.002 mol L−1 SrCl2 + 0.05 mol L−1 citric acid, and ammonium bicarbonate-diethylenetriamine pentaacetic acid (AB-DTPA). A plot of cumulative amounts of K released showed a discontinuity in slope at 168 h. Thus, two equations were applied to two segments of the total reaction time (2–168 and 168–2 017 h). Cumulative amounts of K released ranged from 55 to 299 mg kg−1 in 2–168 h and from 44 to 119 mg kg−1 in 168–2 017 h. Release kinetics of K in the two time segments conformed fairly well to parabolic diffusion, simplified Elovich, and power function models. There was a wide variation in the K release rate constants. Increasingly higher average concentrations of soil K were extracted by distilled water, Mehlich 1, SrCl2, CaCl2, SrCl2 + citric acid, AB-DTPA, MgNO3, NaCl, NH4OAc, BaCl2, and HNO3. Potassium release rate constants were significantly correlated with K extracted. The results of this study showed that information obtained from mathematical modeling in two reaction time segments can help to estimate the K-supplying power of soils.