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The Plant Availability of Various Sources of Phosphate Rock¹

ABSTRACT

Greenhouse experiments were conducted with buckwheat and alfalfa in which 10 sources of phosphate rock were compared as agronomic sources of phosphorus. Tunisian rock gave the largest plant growth response and supplied more phosphorus than other rocks of either foreign or domestic origin. Of the domestic sources, South Carolina land rock and 1 of 2 Florida land pebbles rated highest. However, South Carolina land rock was consistently superior to the Florida pebble. Idaho rock was the only material that exhibited a low initial response pattern that improved with successive cuttings of alfalfa. Virginia apatite was not appreciably better than the check. Although Tennessee brown rock and Montana rock supplied some phosphorus to the plant, yield responses and phosphorus uptake from these materials were low.

There was a close correlation between laboratory evaluations of some phosphate rocks and their actual value as shown by plant growth response in greenhouse tests. Ammonium citrate and citric-acid solubility tests seem to be equally efficient indexes for predicting the relative value of phosphate rocks as sources of phosphorus for plant growth. Both chemical and physical differences between materials afforded explanations for the differences in agronomic value. However, carbonate content of the rock seemed to be the most likely explanation for all rocks except the low fluorine curacao rock.

¹ Contribution of the Soil and Water Conservation Research Branch, A.R.S., U.S.D.A., Beltsville, Md.

² Agronomist and Soil Scientist, respectively.

³ The authors are indebted to the Fertilizer and Agricultural Lime Section, U.S.D.A., for the preparation of the fertilizer materials and their analysis.

Received for publication January 12, 1956. Accepted for publication June 14, 1956.