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Field Testing of Several Nitrogen Availability Indexes¹

ABSTRACT

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 0.01M CaCl₂ and 0.01M NaHCO₃-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 NH₄-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 NO₃^-, and H₂SO₄-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.

¹ Authorized for publication 21 Mar. 1978 as paper no. 5475 in the Journal series of the Penn. Agric. Exp. Stn.

² Assistant Professor and Research Aide, respectively, in the Dept. of Agronomy, The Pennsylvania State Univ., Univ. Park, PA 16802.

Received for publication April 7, 1978. Accepted for publication May 22, 1978.

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