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Partially Acidulated Rock Phosphate as a Source of Phosphorus to Plants: I. Growth Chamber Studies¹

ABSTRACT

Rock phosphate was acidulated with H₃PO₄ at 0, 10, 20, 50, and 100% of that required to convert the rock phosphate to triple superphosphate. These materials plus a check (0 P) and H₃PO₄ alone were established as treatments on an acid soil (AB-38) in 1-gallon containers in a growth chamber. Representative treatments were also made to a Miami (County) soil. The phosphate treatments were applied both in bands and mixed with the soil. In general, German millet and alfalfa yielded about as much and contained as much P in the tissue from 10% acidulated phosphate as from 100% acidulated.

Corn seedlings took up P in proportion to the relative solubilities of rock phosphate, FePO₄, AlPO₄, Ca₃(PO₄)₂, and KH₂PO₄ in sand. Yet they took up much less P when the phosphates were added to the soil, the amount being nearly constant regardless of the phosphate source added. This seems to be evidence for depression by common-ion action of the availability of P in comparatively insoluble phosphates whether added to or formed in the soil.

Plant response to the partially acidulated phosphates and to H₃PO₄ was discussed in the light of recent reports on phosphate reaction products in the soil. This led to the suggestion that detrimental effects of the common ion may be minimized by the partial acidulation treatments such that the environment for the soluble phosphate may be largely defined by the unreacted rock phosphate.

The comparatively favorable plant response to the partially acidulated material and the economy of its production make partial acidulation appear to have very promising practical applications.

¹ Published with the permission of the Director of the Ohio Agr. Exp. Sta. as Journal Article No. 82-63. Presented before Div. S-2 Soil Science Society of America, Denver, Colo., Nov. 18, 1963. The results are part of a thesis to be presented in partial fulfillment of the requirements for Ph.D. degree, The Ohio State University, 1964.

² Professor and Graduate Assistant, respectively.

Received for publication December 2, 1963. Accepted for publication May 12, 1964.