Quantity‐Intensity Relationships of Soil Ammonia in Long‐Term Rotation Plots

Abstract

AbstractReports of release of nonexchangeable NH+4 from soils under some conditions and reports that management practices can change the affinity of the exchange complex for NH+4 indicate a need to reevaluate the commonly accepted practice of determining exchangeable NH+4 to assess the availability of NH+4 to plants. To address this need, quantity‐intensity (Q/I) relationships were studied in soil samples from two Iowa soils, a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll) and a Galva silty clay loam (fine‐silty, mixed, mesic Typic Hapludoll) in long‐term experiments having various crop‐rotation and N‐fertilizer treatments. The observed Q/I relationships gave no evidence for release of nonexchangeable NH+4. Potassium‐exchangeable NH+4 concentrations were well correlated with measurements of labile NH+4 (r = 0.91, P < 0.01) and equilibrium activity ratios for NH+4 (r = 0.89, P < 0.01). No significant relationship was found between relative affinity of the exchange complex for NH+4 and organic C within each soil. Soil management practices influenced NH+4 availability as evaluated by KCl‐exchangeable NH+4 and Q/I parameters, but did not change the relative affinity of the exchange complex for NH+4. Overall, indexes of NH+4 availability based on determinations of K‐exchangeable NH+4 were similar to those derived from studies of Q/I relationships.