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Phosphate Transport in Calcium-Saturated Systems: II. Experimental Results in a Model System

Dep. of Soil Science, Univ. of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

*Corresponding author.

ABSTRACT

This study investigated the diffusive transport of phosphate in a cation-exchange-resin model system. The objective was to demonstrate the effect of cation-exchange reactions on the movement of the phosphate ion. To a cylindrical wax column of Ca-saturated resin, 0.2 g of KH₂PO₄ was surface applied to simulate one-dimensional diffusive transport. Other ions of interest were Ca²⁺, K⁺, and H⁺. Extraction and analysis of 2-mm increments of the column after the desired incubation period (3, 7, and 14 d) revealed the following results. The pH of the system became low as a result of the interaction between added phosphate and Ca²⁺ from the exchange resin. The higher the cation-exchange capacity (CEC), the lower the pH became. Calcium ions, present on the exchange sites, were displaced into solution by cation exchange with K⁺. The higher the CEC of the column, the higher was the amount of Ca²⁺ displaced into soil solution. The movement of K⁺ and phosphate ions was retarded in the system, and the degree of retardation increased with increasing CEC of the soil column. The retardation of K⁺ movement was due to ion-exchange interaction with exchangeable Ca²⁺, while P movement was retarded as a result of chemical interaction with the displaced Ca²⁺. The experimental results showed the effect that the K-Ca exchange reaction has on the diffusive movement of P. There is a need to take this into consideration in any model simulating P transport in the soil.

Received for publication March 15, 1990.