| This study constructs models to predict the distribution of Cd, Cu, Pb, Ni, Zn, and soil organic matter (SOM) between solid phases and solution phase in soil systems. The models emphasize prediction of metals that may ultimately become available to soil organisms. The characterization of metal species in a natural system is crucial for forming standards, criteria, and risk assessment policies. Forty-one widely varying soils were collected from around the world. These soils were extensively characterized in order to consider as many variables as possible for predictive model construction. Preliminary research focused on the selection of a facile, efficient chemical extraction that would include measures of soil metal intensity and capacity. The need to describe the plant availability of a certain soil system drove the search for a suitable extractant. Minimization of model independent variables was another key in model formulation. A dilute 0.01 M HCl extraction was employed because of its simplicity, relation to plant concentrations of the metals studied, and sensitivity towards soil acid neutralization capacity. In any perpetually dynamic system such as soil, the resistance to change in pH will play an important role in the plant availability of metals. Plant root proton release into the root microenvironment may be easily neutralized by soils with a high CaCO3 content, thus reducing plant available metal. Models describing the distribution of metals from soil to solid phase were then constructed based on a few easily measured soil parameters. Soil pH, 0.01 M HCl extraction equilibrium pH, percent organic matter, total recoverable metal, and 0.01 M HCl extract metal adequately described the distribution of the metals studied within an order of magnitude. Efforts then focused on the characterization of soil organic matter and the effects of the distribution of soil organic matter on the distribution of metals. For some metals, such as Cu, soil organic matter plays a dominant role in the behavior of the metal in environmental systems. |
