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Granular and Fluid Phosphorus Sources for Irrigated Alfalfa

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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