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Effects of Phosphorus on Some Physical and Chemical Properties of Clays¹

ABSTRACT

Montmorillonite, kaolinite, and a 1:1 mixture of them were treated with phosphorus at rates as high as 1,600 ppm using phosphoric acid and monocalcium phosphate. After 3 months the clays were examined for pH, zeta potential (charge), swelling, hydration in suspension, modulus of rupture of briquets, and water retention at 5 and 15 bars pressure. The H₃PO₄ decreased pH in all clays; the CaH₄(PO₄)₂ decreased it in montmorillonite and in montmorillonite-kaolinite, but increased it in kaolinite. Both sources of phosphorus caused alternate increases and decreases in the charge of the clays at rates up to 200 ppm and swelling and hydration of the clays were closely correlated to the charge. In general, water retention at 5 bars alternately increased and decreased with increasing rates of phosphorus but did not always exactly parallel the changes in charge, hydration, and swelling. The highest rate (1,600 ppm) caused a significant decrease in the 5- and 15-bar percentages in all instances except the H₃PO₄ on kaolinite. This exception may have been due to chemical alteration of the kaolinite produced by the very low pH. Modulus of rupture was significantly reduced by both sources of phosphorus. Essentially all of the P-induced physical and chemical changes would be beneficial in agricultural soils, and they are economically important because the greatest changes were produced by feasible rates of phosphorus.

¹ Paper no. 4447 of the Journal Series of North Carolina Agr. Exp. Sta., Raleigh, NC 27607. Part of a thesis presented by the junior author in partial fulfillment of the requirements for the Ph.D. degree.

² Professor Emeritus of Soil Science and Graduate Research Assistant, respectively. Dr. Haque is now Professor of Soil Science and Head of the Soil Science Department, University of Lyallpur, Lyallpur, West Pakistan.

Received for publication August 16, 1974. Accepted for publication September 24, 1974.