Assessment of the amino sugar-nitrogen test on Iowa soils: II. Field correlation and calibration

There has been growing interest in using the amino sugar-nitrogen test (ASNT) to improve N fertilization of corn (Zea mays L.). The ASNT is intended to measure the soil organic N fraction that contributes to plant available N. The objectives of this study were to correlate the ASNT to corn N response measures and calibrate the test to Iowa soils and climatic conditions. Soil samples were collected in the fall, early spring, and late spring at the 0- to 15-cm and 0- to 30-cm sample depths. No significant correlation could be found between the ASNT and relative leaf chlorophyll meter value, relative grain protein, relative grain yield, grain yield response to applied N, and economic optimum N rate (EONR). The ASNT was not able to differentiate sites that were responsive or nonresponsive to N fertilization and could not be calibrated to EONR. There were strong linear correlations between the ASNT and total soil N (TSN), hydrolyzable NH4-N, and hydrolyzable NH4 + amino sugar-N. The ASNT was not significantly correlated to hydrolyzable amino sugar-N. The soils tested in this study had large amounts of hydrolyzable NH4-N relative to hydrolyzable amino sugar-N, which may partially explain the poor results with the ASNT. Also, liberation of a constant proportion of TSN by the ASNT procedure explains the inability of the test to estimate a specific portion of soil N that contributes to plant available N. Based on the results of this work, the ASNT is not recommended in Iowa for estimating corn N responsiveness or adjusting N application rate.