Availability and Release Kinetics of Nonexchangeable Potassium in Some Calcareous Soils of Fars Province

Sum of exchangeable and solution forms of soil potassium is widely used to determine potassium availability for plants. Reliability of these methods is not enough in soils that contain 2:1 phyllosilicates. Additional to exchangeable potassium, nonexchamgeable potassium also has an important role in plant nutrition. Limited information about availability and release kinetics of nonexchangeable potassium in calcareous soils of Fars province is available. For this purpose, some extractants including ammonium acetate, boiling nitric acid, 0.1M nitric acid, 2M sodium chloride and water were evaluated to prediction of potassium availability for corn in 10 calcareous soils of Fars province. Release kinetics of nonexchangeable potassium was studied using 15 successive 15-min extraction with 0.01M calcium chloride. Kinetics models describing nonexchangeable potassium release rate including zero order, first order, second order, third order, parabolic diffusion, power function and ellovich were evaluated. Results showed that 1M neutral amonium acetate, 0.1M aitric acid, water and 2M sodium chloride extractants had high correlation with corn potassium uptake. Amount of potassium released among studied soils was vary in the range of 243 to 814 mg kg-1. According to R2 and SE, kinetics of nonexchangeable potassium release was described with power function, parabolic diffusion and ellovich equations satisfactorily. According to this fact that constant rate of parabolic diffusion and ellovich models had significant correlations with corn potassium uptake, it is recommended that these two models are suitable for use in these studied soils.