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Dissolution and Availability to Plants of Rock Phosphates of Igneous and Sedimentary Origins¹

ABSTRACT

Three studies were conducted to test the P supplying power of Missouri apatite of igneous origin relative to that of Florida and Arkansas rock phosphates of sedimentary origins. The analyses of the materials showed that Missouri apatite contained as much as 15.4% P, while Florida and Arkansas rock phosphates contained 14.5% and 9.6% P. Arkansas phosphate contained almost 20% CaCO₃, Flordia had 9% CaCO₃, most of which was incorporated in the apatite structure, and Missouri apatite had only 5.6% CaCO₃ as an impurity.

The absence of carbonate from the Missouri apatite crystal resulted in a very strong crystalline structure. Incremental dissolution by dilute HCl as well as a greenhouse study with corn and soybeans revealed that this igneous apatite released its P very slowly. The rock phosphates of sedimentary origin with their small degree of crystallinity were decomposed more easily. Florida rock phosphate was most effective in supplying the plant with P while the low P as well as the high lime contents of Arkansas rock reduced the effectiveness of this material as a P fertilizer.

¹ This study was supported by the International Minerals and Chemical Company, Skokie, Ill. Contribution from the Missouri Agr. Exp. Sta. Journal Series no. 5178. Approved: H. J. L'Hote, Assistant Director. The research reported was submitted by the senior author in partial fulfillment of the requirements for the M.S. degree.

² Graduate Assistant in Soils, and Professor of Soils, Univ. of Missouri. The senior author is now with the Department of Agr., Cornell Univ., Ithaca, N. Y.

Received for publication June 21, 1967. Accepted for publication October 2, 1967.