Evaluation of a Method to Predict Nitrogen Mineralized from Soil Organic Matter Under Field Conditions

Methods to estimate the amount of N mineralized from soil organic matter are needed for accurate fertilizer recommendations. We conducted laboratory and field studies to evaluate the method of Stanford and Smith for predicting N mineralized under field conditions. Soil samples were incubated to determine their N mineralization potentials and first‐order rate constants of mineralization. To predict N mineralized in the field, the rate constants of mineralization were adjusted by soil temperature, and the amount of N mineralized predicted with the N mineralization potentials and adjusted rate constants were further adjusted by soil water content. Nitrogen mineralized in the field was measured in sorghum [Sorghum bicolor(L.) Moench] plots established on Haynie (coarse‐silty, mixed, calcareous, mesic Mollic Udifluvent), Kahola (fine‐silty, mixed, mesic Cumulic Hapludoll), and Richfield (fine, montmorillonitic, mesic Aridic Argiustoll) soils in 1983 and 1984. Nitrogen mineralized was also measured in fallow plots on Haynie and Wymore (fine, montmorillonitic, mesic Aquic Argiudoll) soils in 1984. The method only accurately predicted the amount of N mineralized in 104 d in fallow plots on Haynie soil. In all other cases, the method significantly overpredicted by 67 to 343% the amounts of N mineralized in the field. Among possible causes for the overpredictions may be an improper soil water content factor and the drying and sieving of the samples before incubation.