LIGNOSULFONATE EFFECTS ON PHOSPHATE REACTIONS IN A CLAY SOIL - CAUSAL-MODELING

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 beta = 0.79), Ca (beta = 0.30), and organic C (beta = -0.39), and retained LS-C (beta = 0.32). Supernatant pH was mainly a function of retained LS-C (beta = -1.22), and concentrations of Ca (beta = 0.49) and P (beta = -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.