Release characteristics of potassium from native reserves of some calcareous soils of Iran and their relationship with yield and potassium uptake by ryegrass (Lolium perenne L.)

Information on the release of potassium (K) reserves in soils can improve K-fertiliser management. The purpose of this study was to investigate the available K-release capacity of soils and its relationship with plant responses in three soil orders. Six consecutive ryegrass cultivations were conducted on nine soil samples from surface (0-30 cm) horizons of Vertisols, Alfisols, and Inceptisols of three sites with different temperature and precipitation under greenhouse conditions. The K release was investigated using 0.2 M sodium tetraphenylboron extractant during 0.2-144 h. The cumulative release of K ranged within 243-895 mg kg(-1) at 0.2 h and 620-1788 mg kg(-1) at 144 h. Kinetics of K release from soils at two time intervals (0.2-2 and 2-144 h) were described well by power function and pseudo second-order equations. The growth of ryegrass was somewhat stunted after six consecutive cultivations. Significant correlations were observed between dry matter and K uptake of ryegrass. The parameters of the best fitted kinetic models were significantly correlated with plant responses, especially the uptake of K by ryegrass.