Computer modeling of trace metal speciation in soil solutions: Correlation with trace metal uptake by higher plants

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Abstract
The study of trace metal chemistry in soils which support crop plants is made complicated by the large number of reactions possible between trace metals and soil constituents and by the web of interrelations among trace metals and the macrocomponents of soil solutions which is created by these reactions. In a typical soil solution, there may be 10–20 different metal cations present (including trace metals) which can react with as many different inorganic and organic ligands to form 300–400 soluble complexes and up to 80 solid phases. In addition, redox, ion exchange, and specific adsorption reactions occur. GEOCHEM is a multipurpose computer program that calculates the chemical equilibrium in a soil solution where all of the reactions mentioned above can take place. The chemical principles and computational scheme on which this program is based are described. One important application of the program is to calculate the speciation of trace metals in the aqueous solution phase of an agricultural soil. This application is illustrated by a computer analysis of saturation extracts of representative soils which have received Cd either in sewage sludge or as Cd(NO3)2. The results of speciation calculations for Cd in the saturation extracts are presented and a discussion of the correlations possible between the principal soluble Cd species and the Cd taken up by a crop plant is given.