Granular and Fluid Phosphorus Sources for Irrigated Alfalfa

Scientists from Arizona State University compared fluid ammonium polyphosphate (10-34-0) applied in irrigation water with granular monoammonium phosphate (MAP) broadcast and irrigated into the soil as phosphorous (P) sources for high-yield alfalfa (eight cuttings per year) and to study movement and availability of P in a calcareous soil. They reported the following results:

• Alfalfa yield for all cuttings during the three-year period was slightly greater for top-dress MAP. Much of the response was measured from the first four cuttings of the first year. The authors believed this was caused by reduced root interception and diffusion of P on the APP plots resulting from low soil temperature during crop establishment.

• P movement into the soil was greater for MAP than ammonium polyphosphate (APP). Soil samples from the top three inches of soil averaged about 60 percent higher P test levels for MAP than APP. The scientists noted no differences in P concentration in plants sampled in May of each year. Soil P data suggests that MAP can, under some circumstances, move deeper in the soil than APP, rather than the reverse, as is often presumed.

• The economics of P fertilizer use for alfalfa showed that when P₂O₅ rates were higher than 65 pounds per acre, granular MAP was more economical while at lower rates, and liquid APP was the best choice. Since the rate of P₂O₅ applied to alfalfa annually in the U.S. Southwest is usually about 100 pounds per acre P₂O₅, MAP becomes the more economical choice.

The authors reported that fluid APP applied in irrigation water, or sprayed on the soil surface, was less effective than top-dressed MAP, since it resulted in less hay yield and lower soil P values. Fluid forms of P fertilizer have the advantage of ease of handling, versatility of application, and a lower cost when applied in irrigation water.

Source: M. J. Ottman, T. L. Thomson, and T. A. Doerge, Agron. J. 98: 899-906, 2006

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