A Comparison of Liquid Phosphoric Acid and Dry Phosphorus Fertilizer Sources for Irrigated Alfalfa Production on Calcareous Soils

Abstract

Alfalfa production in the Western United States requires large quantities of phosphorus (P) fertilizer. In 1998, phosphoric acid (PA) became available as a fertilizer source. Phosphoric acid was more expensive per kg of P than other fertilizers, but was promoted on the basis of convenience of application when tank mixed with other chemicals, and superior agronomic performance (higher yield per kg P applied) in alkaline soils due to the neutralizing effect of the acid. The objective of this study was to compare the effects of PA, triple superphosphate (TSP, 0-45-0), and monoammonium phosphate (MAP, 11-52-0) on alfalfa yield and soil and plant tissue nutrient levels. Phosphorus was broadcast at rates based on soil test recommendations for two years at two sites having calcareous soils typical of alfalfa production systems in the Western U.S. At one site, potassium chloride (KCl, 0-0-60) and zinc sulfate were also mixed with the PA based on soil test recommendations. Responses to fertilizer ranged from less than 1 Mg/ha (7 to 10%) to above 6 Mg/ha (44 to 62%) relative to the control. Yield and soil test P were not significantly different among fertilizer sources in either year or location. At one location where more intensive soil and plant tissue testing was done, there were no differences among fertilizer sources in soil or tissue test K or micronutrient concentrations. These results indicate that PA does not have an agronomic (yield) benefit relative to traditional dry P fertilizers for alfalfa produced on calcareous soils. Therefore, the higher cost per kg of P with PA would have to be justified by considerations other than agronomic performance.