Potassium Fractions in Soils as Affected by Monocalcium Phosphate,Ammonium Sulfate,and Potassium Chloride Application

Soil potassium （K） deficiency has been increasing over recent decades as a result of higher inputs of N and P fertilizers concomitant with lower inputs of K fertilizers in China; however, the effects of interactions between N, P, and K of fertilizers on K status in soils have not been thoroughly investigated for optimizing N, P, and K fertilizer use efficiency. The influence of ammonium sulfate （AS）, monocMcium phosphate （MCP）, and potassium chloride application on K fractions in three typical soils of China was evaluated during 90-d laboratory soil incubation. The presence of AS significantly altered the distribution of native and added K in soils, while addition of MCP did not significantly affected K equilibrium in most cases. Addition of AS significantly increased water-soluble K （WSK）, decreased exchangeable K （EK） in almost all the soils except the paddy soil that contained considerable amounts of 2：1 type clay minerals with K added, retarded the formation of fixed K in the soils with K added, and suppressed the release of fixed K in the three soils without K added. These interactions might be expected to influence the K availability to plants when the soil was fertilized with AS. To improve K fertilizer use efficiency, whether combined application of AS and K was to be recommended or avoided should depend on K status of the soil, soil properties, and cropping systems.

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 phosphateand NH_4^+-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.

Comparison of potential potassium leaching associated with organic and inorganic potassium sources in different arable soils in China

Potassium (K) leaching is detrimental to the maintenance of sustainable arable soil K fertility,especially in low-K fixation soils.It is not known whether the application of inorganic fertilizers with lower K mobility or crop straw can reduce potential K leaching in low-K fixation arable soils.The potential K leaching of 14 representative arable soils with different K fixation capacities in China was evaluated with or without the addition of K under two rainfall intensities (90 and 225 mm),and then potential K leaching was assessed in relation to five K sources (KCl,K_(2)SO_(4),KH_(2)PO_(4),maize (Zea mays L.) straw,and rice (Oryza sativa L.) straw).Without K addition,K leaching mainly occurred in sandy soils at 90 mm of rainfall and in soils with greater organic matter at225 mm of rainfall.With K addition,the leaching percentage of exogenous K ranged from 0.6%to 11.6%at 90 mm of rainfall and 1.2%to 21.2%at 225 mm of rainfall.The greatest K leaching occurred in soils with fewer K-bearing minerals and lower pH at both rainfall intensities.In most cases,KH_(2)PO_(4),which has lower K mobility,markedly reduced K leaching in both high-and low-K leaching soils at the two rainfall intensities.Maize and rice straw reduced K leaching only in soils with high K leaching,regardless of rainfall amount,whereas more K was leached in soils with lower K leaching at high rainfall intensity.In conclusion,KH_(2)PO_(4) and straw should be preferred for reducing K leaching in low-K fixation arable soils.