Phosphorus source and surface fluid band spacing effects on irrigated alfalfa.

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 0.5x 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 0.5x 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.