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The Absorption of Phosphorus from Radioactive Phosphate Fertilizers Applied to Established Meadows I. Effect of Source, Rate, and Time of Application¹

ABSTRACT

In the spring of 1950 radioactive superphosphate and fused tricalcium phosphate at rates of 40 and 120 pounds of P₂O₅ per acre were applied as a topdressing to alfalfa on two fields of Waukegan silt loam. Superphosphate was more effective than fused tricalcium phosphate in supplying phosphorus to the plant but differences became less with time. The yield and phosphorus content of alfalfa were increased by both sources of phosphorus.

Similar trials were conducted in 1951 on the same soil type with the added teatment of potassium to the heavier phosphate application. Again superphosphate was superior to fused tricalcium phosphate as a source of phosphorus for alfalfa. The uptake of phosphorus was increased by the addition of potassium to the soil.

Phosphate broadcast after removal of first-crop alfalfa was effective in supplying phosphorus to the plant (1951). In 1952, in a similar type of experiment, superphosphate at 40, 80, and 120 pounds of P₂O₅ per acre was broadcast after the removal of first crop alfalfa. Approximately one-half of the phosphorus in mature alfalfa came from the fertilizer. The addition of potassium increased the uptake of phosphorus. The phosphorus content of alfalfa increased progressively with increased rates of phosphate application.

The recovery of applied phosphorus was low regardless of kind, rate, or time of application. The best recovery was only 11.6% of the phosphorus from superphosphate applied at 120 pounds of P₂O₅ per acre.

¹ Paper No. 3100 of the Scientific Journal Series, Minnesota Agr. Exp. Sta. This study was supported in part by funds from the Graduate School, University of Minnesota. Presented before Div. IV, Soil Science Society of America, Dallas, Tex., Nov. 18, 1953.

² Associate Professor of Soils, Professor of Agricultural Engineering, Junior Scientist in Soils, and Associate Professor of Soils, University of Minnesota, respectively.

Received for publication December 29, 1953.