Chemicals in soils tend to establish a balance between the amount on the solid surfaces and the amount in solution. Some chemicals exist primarily in the liquid phase while others are strongly adsorbed and exist primarily on the solid surfaces. Molecules tend to move from one phase to another to maintain this balance. The manner in which the molecules are partitioned into the solid and liquid phases depends upon both soil and chemical properties. These relationships, called sorption isotherms, can be determined experimentally. The figure below from Means et al., (1993) (In Hassett, John J. and Wayne L. Banwart. 1989. The sorption of nonpolar organics by soils and sediments. In Reactions and Movement of Organic Chemicals in Soils B.L. Sawhney and K. Brown, Editors, Soil Science Society of America Special Publication 22. p. 35.) illustrates isotherms for pyrene on different soils and sediments.

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. When chemical partitioning can be described by a straight line through the origin, it can be represented as

S = Kd C

where S is the concentration of pesticide in the solid phase (typically with units of grams of pesticide per gram of soil), C is the concentration of pesticide in the liquid phase (units of grams of pesticide per millilitre of soilwater), and Kd is the partition coefficient (units of mllilitre of soilwater per gram of soil) for this chemical in this soil. For many soils and pesticides, the partition coefficient can be estimated using the equation

Kd = Koc OC

where Koc is the organic carbon partition coefficient (units are milliliter soilwater per gram organic carbon) and OC is the organic carbon content (units are grams of organic carbon per gram of soil) of the soil. 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. Bentazon sodium salt has a Koc value of 34 ml/g OC, acifluorfen sodium salt has a Koc of 113 ml/g OC, and metolachlor has a Koc of 200 ml/g OC. 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.

Note: This figure simply shows the location of the centre of the chemical pulse in the soil. It does not provide any information on the amount of chemical still remaining in the pulse. That must be observed in the figure showing degradation. If 1000 g of each product were applied, only 7 g of acifluorfen, 31 g of bentazon, and 463 g of metolachlor would remain after 100 days due to differences in their degradation rates.