THE AVAILABILITY OF RESIDUAL FERTILIZER NITROGEN IMMOBILIZED AS CLAY-FIXED AMMONIUM AND ORGANIC N

Abstract

A study of plant uptake, incubation-extraction and acid hydrolysis was carried out on soil samples from a field study using 15N. The samples had varying proportions of residual fertilizer 15N (15Nex) as clay-fixed ammonium and organic N. Availability of 15Nex to plants was positively correlated with percent of 15Nex as clay-fixed ammonium, and negatively correlated with percent of 15Nex as organic N. A similar relationship was noted for recovery of 15Nex in acid hydrolysates. The relationship was reversed for recovery of 15Nex in aqueous extracts following incubation, although this may have been due to lack of removal of N by plant uptake or leaching, and limitation of microbial activity by lack of readily available carbon. This direct comparison of availability of residual fertilizer N as clay-fixed ammonium N and organic N demonstrates that fertilizer N, once incorporated into organic forms, is much less available to plants than fertilizer N in the form of clay-fixed ammonium.