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The Availability to Greenhouse Crops of Rock Phosphate Phorphorus and Calcium in Superphosphate-Rock Phosphate Mixtures¹

ABSTRACT

Two experiments were designed to test the effects of plant species, rates of fertilizer application, and superphosphate-rock phosphate mixtures on the availability of rock phosphate. Vigo wheat and Kenland red clover were grown for 3 months on Pearman silt loam and the above-soil-portion of the plants used for analyses. The availability of rock phosphate phosphorus was measured by plant yields, phosphorus content of plants, and uptake of P³² from irradiated rock phosphate and radioactive superphosphate.

The following are conclusions: 1. Clover is a better feeder on rock phosphate phosphorus than is wheat. 2. The amount of available rock phosphate phosphorus is increased by increasing the rate of application. 3. About three and one half to four times as much rock phosphate P₂O₅ as superphosphate P₂O₅ is needed to give equal yield and plant phosphorus contents when the phosphorus sources are applied separately. 4. A small application of superphosphate applied with relatively large amounts of rock phosphate increases the uptake of rock phosphate phosphorus but when superphosphate and rock phosphate are applied together in equal quantities the uptake of rock phosphate phosphorus is decreased. 5. Rock phosphate is of value as a fertilizer filler in increasing crop phosphorus content and yields as well as for use as a conditioner. 6. Phosphorus is taken up more readily from irradiated rock phosphate by clover and wheat than is calcium.

A discussion is included about the "A" value and some factors concerning it when more than two nutrient sources are present.

¹ Contribution from the Agronomy Department, University of Kentucky, Lexington, Ky. Published with the permission of the Director of the Kentucky Agr. Exp. Sta.

² Formerly Graduate Assistant, and Agronomist, respectively.

Received for publication October 16, 1954.