Field Testing of Several Nitrogen Availability Indexes

With no quick, reliable soil N availability test for humid region soils, N fertilizer recommendations in these areas are being made on the basis of crop N requirements. When the variability in N‐supplying capability of soils is not taken into account, most N fertilizer recommendations will be inaccurate, leading to inefficient use of N, less economical crop production, and the potential for N pollution of the nation's air and water. Although several N availability indexes have proved to be well correlated with the N‐supplying capability of a soil in greenhouse or laboratory experiments, very little testing of these indexes has been done with field experiments. Eight N availability indexes were correlated with the capability of eight Pennsylvania soils to supply N to field‐grown corn ( Zea mays L.) in 1976 and 1977. Boiling 0.01 M CaCl 2 and 0.01 M NaHCO 3 ‐extractable N were both significantly correlated at the 1% level ( r = 0.86 and 0.77, respectively) with the N‐supplying capability of the soils from the combined 1976 and 1977 experiments. Autoclave‐extractable NH 4 ‐N and total soil N were significantly correlated with the soil supplying capability at the 5% confidence level ( r = 0.70 and 0.68, respectively). Walkley‐Black soil organic matter, soil NO 3 ‐ , and H 2 SO 4 ‐extractable and KCl‐extractable N were not significantly correlated with N availability in the field. Though four of the indexes were well correlated with the N availability in the soil, time and expense necessary for these analyses may preclude their being used routinely by soil testing laboratories.