Relationships of Total Soil Nitrogen to Several Soil Nitrogen Indices

The usefulness of soil nitrogen (N) tests or indices for predicting crop N requirements has been controversial. This paper shows how the relationship between soil indices and total soil N varies using simple linear regressions. The soil N indices included measurement of extractable NH4 + following autoclaving or anaerobic incubation, or following “hot” (100°C) KCl extraction. Also, extractable NO3 − levels sampled four times during the early growing season (May/June) were correlated to total N. Total soil N was considered to be a relatively stable “bench mark” for this study. Soil samples (0–30 cm) were collected from field trials that focused on response of corn to N with different red clover cover crop management practices on different landscape locations over a 3-year period. Management practices included comparisons between no tillage and conventional tillage done in either the late fall or spring. It was observed that the correlations between total N and any of the soil N indices were extremely variable from year to year with each cover crop management practice. The correlations also varied with respect to slope location from year to year in an unpredictable manner. It was concluded that N indices only partially and variably reflect the N released from the large humus N pool in the soil, and are therefore unpredictably related to other more labile N pools (e.g., residues) in the soil.