Phosphorus source effects on alfalfa yield,total nitrogen content,and soil test phosphorus

Abstract

Recent studies showing a lack of response by alfalfa (Medicago sativa L.) to phosphorus (P) fertilization have raised concerns about the efficacy of commonly used P fertilizer materials in the southwestern United States. Studies were conducted between 1982 and 1985 in southeastern New Mexico to evaluate (i) alfalfa yield response to different P sources, (ii) the effect of P fertilizer source on forage total nitrogen (N) content and leafiness, and (iii) the effect of P fertilizer source on soil test P. Nine P sources were applied annually from 1982 through 1984 at a rate of 58 kg P/ha. Alfalfa was grown during 1985 without fertilization to examine the residual effect of P sources. Triple superphosphate (TSP) and monoammonium phosphate (MAP) gave the highest alfalfa forage yields over the 3‐year application period, but no residual fertilizer effects were observed when fertilization was discontinued. Fertilization did not affect forage leafiness. Except for a small N response in forage total N content in 1984, neither P nor concomitant applied N had a significant effect on forage N content over the 3‐year period. Phosphorus fertilization significantly increased average soil test P for the 3‐year period, but there were no significant differences between treatments. Currently used P materials (TSP and MAP) still appear to be the most efficacious for alfalfa production.