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Calcination Effect on the Agronomic Effectiveness of Apatitic North Carolina Phosphate Rock

Agro-Economic Division, International Fertilizer Development Center (IFDC), P.O. Box 2040, Muscle Shoals, AL 35662

L. L. Hammond

Texasgulf, Inc., Glenwood at Glen Eden, P.O. Box 30321, Raleigh, NC 27622-0321

*Corresponding author.

ABSTRACT

Calcination of apatitic phosphate rock (PR) is one of the means to increase the grade (total P content) of the mineral. The objective of this study was to investigate the effect of calcination on the solubility and agronomic effectiveness of apatitic North Carolina PR in a greenhouse experiment. A Hartsells silt loam (fine-loamy, siliceous, thermic Typic Hapludult) with pH 4.8 was used. Fertilizers were mixed with the soil at P rates of 0, 50, 150, and 300 mg P kg^–1. Funks G5757 maize (Zea mays L.) was grown for 6 wk for each of two crops. The results showed that the degree of carbonate substitution for phosphate in the apatite structure was decreased from 1.20 to 0.32 mol after calcination. The corresponding decrease in neutral ammonium citrate solubility was from 2.7 to 0.8%. Dry-matter yield and P uptake by maize in two successive crops also decreased due to calcination. Across the range of the P rates used in two maize crops, an average reduction in dry-matter yield of 77% was calculated for the calcined PR relative to uncalcined PR. Thus, apatitic PR used for direct application should not be calcined, even though calcination can increase its total P content.

Received for publication November 16, 1990.