CAUSAL-MODELING PH AND PHOSPHATE EFFECTS ON MOLYBDATE SORPTION IN 3 TEMPERATE SOILS

Causal modeling was used to study mechanisms involved in pH, P, and Mo interactions in three soils, to determine the relative importance of several variables to an endogenous variable and to decompose the simpl e Pearson correlations into direct, indirect, simple noncausal, and sp urious effects. Solutions varying in pH and concentrations of P and Mo were reacted with a clay, a loam, and a sand soil. Supernatant was an alyzed for pH and concentrations of Ca, Si, Mo, P, Fe, and Al. Dissolu tions of Ca (Ca(D)), dissolution/desorption of Si (Si(D)), and sorptio n of P (P(S)) and Mo (Mo(S)) were calculated. Logarithm values Of Ca(D ), Si(D), and Mo(S) and pH were used to build the structure equations using P(S) and Mo(S) as endogenous variables. The causal modeling show ed that there existed reciprocal relationships between Mo(S) and P(S) and that Mo(S) was influenced more by P(S) than vice versa. Significan ce of such reciprocal effects varied in the three soils. In general, d ecreases in Mo(S) did not occur until P(S) increased to 17, 16, and 15 mmol kg-1 soil in the clay, the loam, and the sandy soil, respectivel y. The effects of pH and Si(D) on Mo(S) were more significant in the s andy soil than in the loam and/or in the clay soil. The relative impor tance (absolute standardized coefficient) of variables to Mo(S) was in the decreasing order of P(S), Mo, pH, Ca(D), and Si(D) in the clay, o f pH, Mo, Si(D), P(S), and Ca(D) in the loam, and Of Si(D), pH, P(S), Mo, and Ca(D) in the sandy soil. In some cases, the direct effects of the pH, Si(D), Ca(D), Mo, and P on P(S) or Mo(S) were counterbalanced by their indirect and/or noncausal effects, resulting in small Pearson correlation coefficients. The causal modeling and the ion activity pr oducts of some Ca-P and Ca-Mo minerals suggested that Mo retention was primarily a process of adsorption in the three soils and that P adsor ption was the principal reaction in the clay soil, while both adsorpti on and precipitation controlled P reaction in the loam and the sandy s oils.