Phosphorus Source and Surface Fluid Band Spacing Effects on Irrigated Alfalfa

Abstract

Subsurface banding of P fertilizer is more effective than broadcast and incorporation for annual crops on P‐deficient sites. Subsurface banding in established alfalfa (Medicago sativa L.) may damage plants. This study compared spring‐applied, broadcast dry monoammonium phosphate (MAP) and fluid ammonium polyphosphate (APP), and APP applied in surface bands with 15‐, 30‐, or 60‐cm spacing, at rates equivalent to 0.5× and 1× the soil test recommendation for yield, tissue P concentration, and P uptake at six sites. There was no difference in soil test P measured after the first cutting between broadcast MAP or APP. Yield responses to P occurred at all sites and ranged from a low of 0.7 Mg ha−1 (7%) averaged across P rate, to 2.6 Mg ha−1 (46%) at the 0.5× rate and 5.4 Mg ha−1 (95%) at the 1× rate above the 0 kg P ha−1 treatment. Yield was higher with MAP than with APP for five of 19 individual harvests, while total yield was 1.0 Mg ha−1 higher with MAP than with APP at one site‐year. Phosphorus source effects on P concentration and uptake were similar to those on dry matter yield. There was a linear decrease in yield with increasing band spacing for one site‐year and a quadratic response for another. Responses to band spacing in tissue P concentration for nine harvests suggest that band spacing of 15 and 30 cm may enhance P availability relative to broadcast or 60‐cm spacing. In only one site‐year did we find a yield advantage of MAP over APP, or of surface band over broadcast APP.