An evaluation of phosphate sources for alfalfa production was made on P-deficient Superstition fine sand under arid conditions. Factors studied included particle sizes, and methods, rates, and frequencies of application. These factors were evaluated under field conditions in terms of (a) hay yields, (b) P composition of plants, (c) total P removed from the soil by the hay, and (d) estimated residual P in the soil over a 3-year cropping period using radiophosphorus techniques.
Supplemental P applications increased hay yields an average of 38%, P percentage 25%, P yield 66%, and A values or residual P, 126%.
A 3-year summary demonstrated consistently that dicalcium phosphate, if mixed thoroughly with Superstition soil, was at least as efficient as concentrated superphosphate in increasing alfalfa hay yields, P percentage of the hay, total P removed from the soil, and available residual P. The two particle sizes of dicalcium phosphate were equally effective for the 3-year period, but the finer, with greater surface area, appeared more efficient the first year.
The higher rate of application, 131 pounds P per acre, was 7 to 71% more effective than the 66-pound rate in increasing yield, P percentage, P yield, and A values. A single initial P application for the 3-year period was as effective as the same total amount applied annually. When applied properly, both particle sizes of dicalcium phosphate, the phosphoric acid, and concentrated superphosphate all appeared to be satisfactory phosphate sources.