QUANTITY-INTENSITY RELATIONSHIPS OF SOIL AMMONIA IN LONG-TERM ROTATION PLOTS

Reports of release of nonexchangeable NH4+ from soils under some conditions and reports that management practices can change the affinity of the exchange complex for NH4+ indicate a need to reevaluate the commonly accepted practice of determining exchangeable NH4+ to assess the availability of NH4+ 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 NH4+. Potassium-exchangeable NH4+ concentrations were well correlated with measurements of labile NH4+ (r = 0.91, P < 0.01) and equilibrium activity ratios for NH4+ (r = 0.89, P < 0.01). No significant relationship was found between relative affinity of the exchange complex for NH4+ and organic C within each soil. Soil management practices influenced NH4+ availability as evaluated by KCl-exchangeable NH4+ and Q/I parameters, but did not change the relative affinity of the exchange complex for NH4+. Overall, indexes of NH4+ availability based on determinations of K-exchangeable NH4+ were similar to those derived from studies of Q/I relationships.