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Relative Value of Calcium Metaphosphate and Superphosphate as Sources of Phosphorus for Plants on Different Soils¹

ABSTRACT

Availability-coefficient ratios (ACR) of phosphorus in calcium metaphosphate (CMP) to phosphorus in concentrated superphosphate (CSP) applied to 11 soils were obtained from data on yields of phosphorus in plants in a greenhouse experiment. ACR values ranged from 0.61 to 1.25 and were lower for calcareous than for acid soils. The best index of ACR values was provided by the increase in orthophosphate in the soils as a result of addition of CMP and partial hydrolysis of the condensed phosphate it contains. The increase in orthophosphate was estimated by (a) adding different quantities of P as KH₂PO₄ to soil, (b) incubating the soil under the same conditions employed for CMP, (c) finding the increase in orthophosphate by isotopic dilution of P³²O₄ in the presence of (CH₃)₄N-resin, (d) plotting the observed increases in orthophosphate against the quantities of P added as KH₂PO₄, and (e) finding by interpolation in the plot of (d) the quantity of P added as KH₂PO₄ to which the observed increase in orthophosphate with CMP was equivalent. The relative increases in orthophosphate present in water extracts of the soils as a result of application of CMP or CSP provided a good index of ACR values in most soils but not in all. The gross movement of P from CMP into surrounding soil provided a relatively poor index of the ACR values. The results indicate that in soils treated with CMP, the P absorbed by plants comes mainly from the orthophosphate and not directly from the condensed phosphates.

¹ Journal Paper No. J-4161 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Project No. 1183. Contribution from the Dept. of Agronomy. This work was done in cooperation with the Division of Agricultural Relations, Tennessee Valley Authority.

² Former Graduate Assistant (now Soil Scientist, ARS, USDA, Morris, Minnesota) and Professor of Soils, respectively.

Received for publication September 28, 1961. Accepted for publication April 9, 1962.