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Pyrite Enhancement of Phosphorus Availability from African Phosphate Rocks: A Laboratory Study

Soil, Crop, and Atmospheric Sciences Dep., Bradfield Hall, Cornell Univ., Ithaca, NY 14853

R. R. Weil

Agronomy Dep., Univ. of Maryland, College Park, MD 20742

*Corresponding author.

ABSTRACT

Phosphate rock (PR) and pyrite deposits, indigenous to many African countries, may be useful to resource-poor farmers for increasing the P and S fertility of highly weathered tropical soils. Combination of PR and pyrite was examined as a means to increase the availability of P from five PR of African origin. Pyrite and PR were combined in several ratios (P/S) and incubated. Mixtures were periodically leached on a vacuum extractor with 0.01 M KCl for a 6-mo period. Soluble P and pH of the leachate were measured. The pH of the leachates decreased with increasing levels of pyrite. In all cases, soluble P measured in the leachate increased with increasing levels of pyrite. Soluble P measured in the leachate was greatest from Togo and Uganda PR mixtures, much less from Zimbabwe PR, and virtually nil in all but the highest pyrite treatments for both Tanzania and Malawi PR mixtures. Citrate-soluble P was a less reliable predictor of P release than total P and the percentage of CO₂, Al, and Fe in the PR and associated minerals. High pyrite levels with low-quality rocks generated P release comparable with that from untreated high-quality rocks. The addition of Fe from the pyrite apparently did not lead to precipitation of substantial amounts of P as it was released from PR. The rocks responded very differently to the pyrite treatment. Although the method is promising for some rocks (Togo and Uganda), it is obviously not useful for other rocks (Malawi).

The work was completed at the Univ. of Maryland. Scientific Article no. A6589 and Contribution no. 8801, Maryland Agric. Exp. Stn., College Park, MD 20742.

Received for publication April 8, 1994.