Alfalfa Yield and Soil Phosphorus Increased with Topdressed Granular Compared with Fluid Phosphorus Fertilizer

Fluid P fertilizers are commonly applied to established alfalfa (Medicago saliva L.) despite their higher cost compared to granular forms. The objectives of this experiment were to compare effects of fluid and granular P fertilizer on alfalfa yield, and availability and movement of P in the soil. The P fertilizers compared were fluid ammonium polyphosphate (APP, 10-34-0) applied in irrigation water and granular monoammonium phosphate (MAP, 11-52-0) topdressed and incorporated by flood irrigation on a calcareous soil at Maricopa, AZ. Fertilizer P was applied each December at rates of 1,2,3, and 5 g P m -2 . The soil was sampled about 1 mo later. In the first four cuttings of the first year, hay yields were 1358 g m -2 for APP and 1501 g m -2 for MAP. No differences in yield due to P source were measured in the remaining four cuttings of the first year, or in any cutting the second or third year. In the surface soil (0-7.6 cm), the bicarbonate extractable soil P averaged 44 mg kg -1 for APP and 7.1 for MAP. The depth of movement of the fertilizers was similar except in the first year where MAP moved deeper into the soil profile than APP. In this study, the higher cost of fluid APP compared with granular MAP was not recovered by increased yield. However, at low P rates, water-run APP may be more economical than topdressed MAP due to its low application cost.