Assessment of the Amino Sugar–Nitrogen Test on Iowa Soils: I. Evaluation of Soil Sampling and Corn Management Practices

Abstract

A soil N test capable of measuring the soil organic N fraction that contributes to plant available N would be useful to corn (Zea mays L.) producers as they make N fertilizer rate decisions. The objectives of this study were to evaluate the effects of soil sampling time, sampling depth, long‐term crop rotation, and long‐term N fertilizer application on the amino sugar–N test (ASNT). Soil samples from 43 N rate trials were analyzed using the ASNT procedure. The ASNT was also determined on soil samples from two sites having several crop rotations and N fertilizer rates in place for the past 18 and 24 yr. The ASNT results were consistent over time but show variation from week to week and were more variable at sites with high soil organic C or manure application history. Amino sugar–N test values from the 0‐ to 15‐cm soil depth were usually greater than the 15‐ to 30‐cm or 0‐ to 30‐cm depths. Crop rotations that included alfalfa (Medicago sativa L.) resulted in greater ASNT values than continuous corn and corn rotated with soybean [Glycine max (L.) Merr.]. Therefore, sampling depth and rotation should be taken into consideration when calibrating the ASNT. Long‐term N fertilization rate did not consistently influence ASNT values. Differences between soils had larger effects on the ASNT than corn management practices. Sampling before N application in the fall, early spring before planting, or late spring during the growing season should provide similar ASNT results and flexibility in sample collection before N application.