PHOSPHORUS SOURCE EFFECTS ON ALFALFA YIELD, TOTAL NITROGEN-CONTENT, AND SOIL TEST PHOSPHORUS

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 1,984, 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.