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Availability of Residual Phosphorus from Long-Time Rock Phosphate and Superphosphate Applications to Groseclose Silt Loam¹

ABSTRACT

The availability of residual phosphorus from long-time superphosphate applications to Groseclose silt loam exceeded that of rock phosphate. Based on A values, residual phosphorus from superphosphate was approximately four times as available as that from rock phosphate.

Data on total, Truog, and NaHCO₃-soluble phosphorus showed that: (1) more of the superphosphate had reacted with the soil than had rock phosphate; (2) more of the phosphorus applied as superphosphate had accumulated in the silt and clay factions of the soil and, based on solubility in NaHCO₃, was relatively available; and (3) more of the phosphorus applied as rock phosphate had accumulated in the sand fraction of the soil and was relatively unavailable, as judged by solubility in NaHCO₃.

Apparently, some of the rock phosphate had not reacted with the soil and was still in its original molecular combination. X-ray analysis revealed the presence of apatite or a mineral similar to apatite in the sand fraction of soil that had received rock phosphate. The mineral, which gave pronounced reflections at the same values obtained from rock phosphate fertilizer, was not found in soil receiving superphosphate.

Approximately three-quarters of the phosphorus applied over a 40-year period from either source was present in the soil as residual phosphorus. This quantity nearly doubled the total phosphorus content of the soil. Crop yields on plots receiving the two fertilizers have been equal except for the first few years when superphosphate produced higher yields.

¹ Contribution from Agronomy Department, Virginia Agr. Exp. Sta., Virginia Polytechnic Institute, Blacksburg, Virginia. Presented before Div. IV, Soil Science Society of America, Nov. 14, 1956, Cincinnati, Ohio.

² Assistant Professor's of Agronomy, and Professor of Agronomy, respectively.

Received for publication July 13, 1956. Accepted for publication December 27, 1956.