The Baker Soil Test, abbreviated BST, evolved from the concept that the availability of an ion to plants is related to its activity or relative partial molar free energy within the substrate. From the concepts of Relative Partial Molar Free Energy and the Gibbs-Duhem equation, it becomes obvious that plant availability of an ion is affected by all factors affecting ionic activity. For soil systems, these factors include water properties, mineral solubilities, ionic adsorption, and other entropy changes. A perfect soil test would measure the true ionic activity or relative chemical potential of labile ions, analogous to pH and acidity (pCd2+ and labile cadmium for cadmium).
The BST includes measurements and interpretations of the extractable amounts (Q), the negative logarithm of the activity of ions associated with diethylene triaminepenta acetic acid (pCd2+), and concentrations of cadmium and other metals by the U.S. Environmental Protection Agency (1986) method.
The determination of the intensity and buffer capacity (dI/dQ) of Cd2+ and other metals is important in order to identify soils with low potential cadmium uptake by crops, those with high cadmium uptake, and those for which the desired reduction of cadmium uptake by plants can be obtained by liming and other treatments which have been found to increase the soil buffer capacity. Reaching equilibrium after treatment takes time, and the required time can be expected to be different for different soils. The intensity and buffer capacity, predicted by the BST, should therefore be obtained repeatedly over time for soil management of cadmium and other elements for crops in the food chain.