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

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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.

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