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Phosphate Rock Dissolution and Availability in Cerrado Soils as Affected by Phosphorus Sorption Capacity¹

ABSTRACT

The reactivities of two phosphate rocks (PR), North Carolina and Patos de Minas, were compared in seven acid soils from the Cerrado of Brazil during 158 days of laboratory incubation. Four soils are classified as Typic Acrustox and the remaining three Typic Haplustox, Aeric Tropaquept, and Ustoxic Quartzipsamment. Dissolution was accompanied by measurements of resin- and Olsen-extractable P and exchangeable Ca. North Carolina PR was more reactive than Patos de Minas PR. Changes in Olsen-P were more indicative of the PR dissolution process than resin-P. Maximum PR reactivity, as measured by increases in calcium, occurred in those soils maintaining the lowest amounts of Olsen-extractable P. Such soils have the highest clay and free Fe₂O₃ contents and, consequently, the highest P sorption capacities. Maintenance of low levels of solution P is considered an important driving force for PR dissolution in these acid soils. Consequently, high P sorption capacity favored the dissolution of PR in such soils. The "i" modifier in the Fertility Capability Classification system separated those soils in which phosphate rocks reacted rapidly from those that reacted at a slow rate.

¹ Paper no. 6,987 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27650. Based on a portion of the Senior Author's Ph.D. thesis sponsored by the Tropical Soils Research Program under Contract AID/ta-C-1236 with the U. S. Agency for International Development, conducted in cooperation with the Centro de Pesquisa Agropecuária dos Cerrados, EMBRAPA, Brasília, Brazil.

² Visiting Assistant Professor and Professor, respectively, Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27650.

Received for publication June 26, 1981. Accepted for publication November 1, 1981.

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