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A Comparison of the Effects of Monocalcium Phosphate and Diammonium Phosphate on Phosphorus and Calcium Availabilities¹

ABSTRACT

Because monocalcium phosphate (MCP) is often a superior phosphorus (P) source to diammonium phosphate (DAP) when used at high rates, a short-term, growth-chamber experiment was conducted to compare the two P sources. An acid, P-deficient Lucedale sandy loam (Rhodic Paleudult) was treated with all combinations of five lime rates and four P rates (0, 75, 150, and 300 ppm) as MCP and DAP. Other needed nutrients and a nitrification inhibitor were uniformly added, and a sorghum-sudangrass hybrid (Sorghum bicolor x S. sudanensis L.) was grown for 14 days. Soil pH was lower and soil-solution P and Ca were higher from MCP than DAP at each P rate. Several DAP-treated soils produced forage yields lower than that predicted from their P levels; the lower yields were positively correlated with lower soil-solution Ca. The critical ambient Ca level in DAP-treated soil was significantly lower for plants growing at the rate of 300 ppm P than at that of 150 ppm, possibly because high P levels enhanced the uptake or physiological utilization of Ca.

¹ Contribution from the Dep. of Agronomy and Soils, Auburn University, AL 36849. Published with the approval of the Director of the Alabama Agric. Exp. Stn. as Journal Series no. 3-810148.

² Professor of Soil Science.

Received for publication December 22, 1981. Accepted for publication April 12, 1982.