Kinetics of nonexchangeable-potassium release and plant response in some calcareous soils

Nonexchangeable K+ constitutes a slowly available reserve that may significantly influence K+ fertility of soils. Laboratory and greenhouse experiments were conducted to characterize the K+ supply and nonexchangeable K+-release kinetics in 10 calcareous soils using 0.01 M CaCl2 and 0.01 M oxalic acid extractions. Total K+ uptake by wheat (Triticum aestivum L.) grown in the greenhouse was used to measure plant-available K+. The release of K+ was characterized by an initial fast rate followed by a slower rate. The nonlinear relationship in the early stages of the K+ release may be attributed to the edge sites, and release of K+ from interlayer exchange sites may be responsible for the second part of the K+ release. Kinetics of K+ release was described best with power function, which showed the best fit of the four models tested. Parameters of kinetics models in 0.01 M CaCl2 were significantly related to K+ uptake by wheat. Potassium release was also correlated to initial NH4OAc-extratable K+ and to HNO3-extractable K+.