NITROGEN ENHANCEMENT OF SURFACE APPLIED FERTILIZER PHOSPHORUS UPTAKE BY FORAGE SPECIES

Abstract

Nitrogen, supplied as NH4NO3 to timothy (Phleum pratense L.) and bromegrass (Bromus inermis L.) or as N derived from alfalfa (Medicago sativa L.) growing in association with timothy, enhanced the utilization of surface applications of pelleted, 32P-labelled calcium phosphate. The proportion of the P in timothy derived from a single application of 60 kg P/ha increased from a range of 16% to 21% without N to a range of 38% to 43% with an application of 60 kg N/ha before spring growth and repeated after the first and second harvests. At the first two harvests, growing timothy in association with alfalfa further increased the proportion. The increase was greater at the second harvest and at the lower rates of N, amounting to an increase of one-third at applications of 15 and 30 kg N/ha. The proportion of P in the first harvest of bromegrass which was derived from a surface application was increased 17% to 26% as the rate of P was increased from 40 to 120 kg P/ha; however, N fertilizer as NH4NO3 increased fertilizer P utilization from 17% to 32% as the rate of N was increased from zero to 120 kg N/ha at the 40 kg P/ha rate, and from 26% to 57% with increasing N at the 120 kg P/ha rate. Similar data were obtained when the rates of N application were repeated for the second and third harvests. A relationship, developed between the N concentration in the grass species and the percent utilization of fertilizer P, would suggest that enhancement in utilization of P was, in part, the result of an increase in the assimilated N in the plant. The experiments illustrate that it is imperative to provide adequate N for efficient utilization of surface-applied P by perennial forage grasses.