Special Publication 22. The horizontal axis shows the concentration of pesticide in solution. The vertical axis shows the concentration in the solid phase. Each line represents the relationship between these concentrations for one soil. A straight line through the origin represents the data well for the range of concentrations shown.
Isotherms sometimes deviate from a straight line for high concentrations.
This is a useful tool for estimating Kd from the known Koc of the chemical and the organic carbon content of the soil horizon of interest. The figure above illustrates the impact of different partition coefficients upon the depth of movement in an irrigated Eufaula soil of Caddo County Oklahoma. The figure illustrates that the centre of the chemical pulse on a particular day is greater for pesticides with smaller partition coefficients than for those with larger ones. This is because chemicals with smaller partition coefficients are less strongly adsorbed to the soil solids and can move more freely with the soil water. It does not provide any information on the amount of chemical still remaining in the pulse.