Kinetics of Soil Potassium Release Under Long-Term Imbalanced Fertilization in Calcareous Soils

Knowledge on potassium ion (K+) release from soils makes K fertilizer recommendation more efficient and profitable. Kinetics of K+ release under continuous fertilization of no fertilizer (CK), urea (N), triple superphosphate (P), and urea + triple superphosphate (NP) without K fertilizer was investigated in calcareous (chloritic and kaolinitic) soils on the Miandarband Plain in Kermanshah Province of Iran. The results showed that the kinetics of K+ release included an initial reaction and a slow reaction. The phosphate- and NH4+-induced K+ release followed the same rate process during the rapid (2-192 h) and slow release periods (192-1 090 h). There were no significant differences in the cumulative K+ released from the chloritic and kaolinitic soils among all the treatments. The cumulative K+ released was positively correlated with P adsorption capacity for the chloritic (r = 0.461, P < 0.05) and kaolinitic soils (r = 0.625, P < 0.01), and negatively correlated with K fixation potential for the chloritic (r = 0.720, P < 0.01) and kaolinitic soils (r = −0.513, P < 0.01). There was a significant (P < 0.001) interactive effect of K fixation potential × P adsorption capacity on the cumulative K+ released for both soil groups. The initial release rate (IRR) index (a·b, where a and b are the rate coefficients of the power function equation) for the chloritic soils was significantly (P < 0.05) higher under applications of P and NP than N and CK. The IRR index values among different fertilization treatments were in the order of NP = P > N = CK for the chloritic soils, and N = P > NP > CK for the kaolinitic soils. This study showed that K fixation potential and P adsorption capacities controlled K+ release from soils. This information will be helpful for precise fertilizer recommendations for the studied soils.