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Lignosulfonate Effects on Phosphate Reactions in a Clay Soil: Causal Modeling

Environmental Research and Engineering Dep., Alberta Research Council, P.O. Box 8330, Postal Station F, Edmonton, Alberta, Canada, T6H 5X2

A. F. MacKenzie and I. P. O'Halloran

Dep. of Renewable Resources, Macdonald College, SteAnne de Bellevue, Quebec, Canada, H9X 1C0

* Corresponding author.

ABSTRACT

Phosphate adsorption reduces the effectiveness of P fertilizers. Lignosulfonate (LS) is expected to compete with P for adsorption sites on soil particle surfaces and reduce P retention. A Dalhousie clay soil (a fine, mixed, nonacid, frigid Typic Humaquept) was incubated with various amounts of CaLS (0–150 g kg^–1 soil) and phosphate (0–792 mg P kg^–1 soil) for 240 h under moist conditions. A subsample was extracted with deionized water and the supernatant analyzed for P, Ca, pH, and organic C. Additions of CaLS increased P retention, compared with zero addition, with maximum increase occurring with LS at 50 g kg^–1 soil. Adsorption of organic C was greater with nondesugared (BD) than with desugared CaLS (SF), but P retention was greater with SF than with BD. Recursive causal modeling indicated that, with LS additions, P retention was mainly related to concentrations of P (path coefficient ß = 0.79), Ca (ß = 0.30), and organic C (ß = –0.39), and retained LS-C (ß = 0.32). Supernatant pH was mainly a function of retained LS-C (ß = –1.22), and concentrations of Ca (ß = 0.49) and P (ß = –0.27). The data suggested that Ca added with CaLS precipitated P, probably forming hydroxapatite [Ca₅(PO₄)₃OH]. Further studies with NH₄LS showed low P retention with high NH₄LS additions after soluble native Ca had been removed. The results confirmed that P retention was decreased by LS under the conditions that soil Ca was below the level required to form Ca-P precipitates and that organic C had been adsorbed on the soil surface.

Contribution from Dep. of Renewable Resources, Macdonald College.

Received for publication November 3, 1989.

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