LIMITATIONS OF THE CHEMICAL POTENTIAL

Authors: COREY, A - RETIRED - U OF COLORADO; Logsdon, Sally

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2005
Publication Date: 7/5/2005
Citation: Corey, A.L., Logsdon, S.D. 2005. Limitations of the chemical potential. Soil Science Society of America Journal. 69(5):976-982.

Interpretive Summary: In order to understand how water moves into the soil, how it drains through the soil, and how it is used by plants, we need to know which way water moves in soil and how much water moves. This paper reviews theory about the state of water in the soil, and explains why we cannot add together influences such as gravity, small pore surfaces, and membranes that let water pass through more easily than the salts in the water. In order to know which way the water moves and how much water moves in the soil, we need to add the quantity and direction of soil water moved in response to each influence. This information is needed by scientists and water managers to assess processes such as infiltration of water into the soil, water drainage through the soil, and plant water uptake in a range of soil water content levels and salt concentration levels.

Technical Abstract: Potential theory is described in texts dealing with theoretical mechanics, mathematics, and physics. Limitations of potentials referring to soil-water solutions have been explained in soil-science literature. However, examples of the misapplication of potential theory continue to appear in the literature of soil and plant science. The purpose of this paper is to review the theory of potentials and explain why force potentials exist only for conservative force fields, and why component potentials can be summed only when they refer to a common reference element of a constituent or a solution as a whole. Forces acting on elements of a soil water solution or its constituents are conservative only when acting on an appropriate reference element, e.g., an element of volume or an element of mass. A common reference element for which all component forces are conservative does not exist. In particular, it is not possible to add a potential referring to an element of a constituent to a potential referring to an element of solution. Moreover, component forces, associated with the gradient of component potentials, induce resistance forces that are different functions of soil properties. Resistance forces are not described by thermodynamic functions but they affect the magnitude and direction of net flux resulting from component forces. Consequently, net flux resulting from the gradients of soil water potentials can be evaluated only by summing fluxes resulting from component forces, not by relating net flux to the gradient of a combined force potential.