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The Effects of Exchangeable Calcium on the Retention and Fixation of Phosphorus by Clay Fractions of Soil¹

ABSTRACT

Laboratory studies were made of the phosphorus retaining and fixing properties of four Kentucky soils. The soils studied were: Armour, Pembroke, and Memphis silt loams, and Loradale silty clay loam. All work was done with the clay fraction of the soils. The effect of percent calcium saturation of the clay, rate of phosphorus application, and initial saturating cation were studied. Retained phosphorus was considered to be that portion of the added phosphorus removed from a water solution by the clay separate. Fixed phosphorus was considered to be that portion of the retained phosphorus not recovered from the clay separate by extraction with 0.005N H₂SO₄. Radioactive phosphorus was used in the experiment. Phosphorus retention and fixation were determined by a solution counting technique.

With a rate of 100 parts of P₂O₅ per million parts of clay, phosphorus retention and fixation by the four initially acid saturated clay fractions increased as the percent calcium saturation of the clay was increased. No sizeable increase occurred until the clay separates were about 60% calcium saturated. When phosphorus was applied to Loradale clay separate at a rate of 800 or more parts of P₂O₅ per million parts of clay, fixation decreased with increasing percentages of calcium saturation. This would indicate that fixation studies should be carried out with rates of phosphorus application comparable with those used on field crops if laboratory results are to be extrapolated to field conditions.

Initially aluminum saturated clay separate retained much more of the added phosphorus but fixed no more of it than initially acid saturated clay separate.

Phosphorus fixation by clay separates with "natural" base saturation decreased with increasing pH, whereas fixation by initially acid saturated clay separate increased or remained unchanged with increasing pH. This may indicate that fixation studies made with initially acid saturated clay separates are not accurate indications of fixation as it occurs under natural soil conditions.

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

² Formerly Research Assistant, and Agronomist, respectively. Appreciation is expressed to the National Science Foundation and the Tennessee Valley Authority for partial financial support of this investigation.

Received for publication September 25, 1956. Accepted for publication February 5, 1957.