The release rate of nonexchangeable K (NEK) plays a significant role in supplying available K, particularly in soils containing K bearing minerals. Information concerning NEK release rate in Gilan soils is limited. The objective of this research was to study the dynamics of NEK in 10 different soils from Gilan province through successive extraction with 10 mM citric acid over a period of 2500 h and the correlation of kinetic equations rate constants with soil properties and alfalfa plant indices. The amount of NEK released after 2500 h ranged from 134-6 19 mg kg’ , in which the highest and the lowest amount of NEK was in soil no 3 and no2 respectively. The amount of NEK released after 2500 h was significantly correlated with NEK content (R=0.83). The kinetics of cumulative NEK released were evaluated using the kinetic equations. First order, parabolic diffusion law and power function equation described well the NEK release kinetics, based on their highest determination coefficient and their lowest value of the standard error of the estimate. Rate constants particularly in first order and parabolic diffusion equations were different from soil to soil. These variations were attributed to differences in particle size distribution and to the type and content of clay minerals. In correlation study, the rate constant of the power function equation was not significantly correlated with soil properties. The rate constants of first order equation was significantly correlated with clay(r=0.59*), sand(r=0.68*), CEC(r=0.63*) and pH(r=0.64). The rate constant of parabolic diffosion equation was significantly correlated with clay(r=0.76*), sand(r=0.66*), CaCo3(r=0.62*) and CEC (r=0.63*). The rate constant of the power function equation was not significantly correlatrd with plant indices. The rate constants of the other equation were significantly correlated with concentration and uptake indices, but these constants were not significantly correlated with yield.
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