AN EVALUATION OF NITROGEN MINERALIZATION INDEXES FOR ORGANIC RESIDUES

Organic residues without high levels of toxic metal or organic constituents are usually applied to agricultural soils at rates based on assumed contributions of available N to the growing crop. However, there is no accepted test for assessing potential available N supply from residues. Although the actual contribution will depend on soil characteristics and climate as well as residue characteristics, a chemical index might be useful in ranking residues according to potential available N contributions. An experiment was conducted to evaluate a number of chemical indices for N availability. The 19 residues used in the study consisted of seven types of manures, six sewage sludges, and six composted or mixed soil amendments. Organic residues were mixed with sandy top-soil and perlite and incubated in glass tubes in the laboratory for 67 d. Total N, total Kjeldahl N, organic N, and N released into the Walkely-Black acid-dichromate digest (WBN) were significantly correlated with the amount of N mineralized as well as the fraction of organic N mineralized during the incubation. The best relationship for the amount of N mineralized was with WBN (r2 = 0.89 for a quadratic relationship). The strongest of the correlations for fraction of organic N mineralized was also with WBN (r2 = 0.82).