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Application of the Potential Concept to Soil Water Equilibrium and Transport¹

ABSTRACT

This study examines the view that a "total soil water potential" can be defined such that (i) the water component will always move from regions of higher to regions of lower potential, and (ii) that if the total potential is constant at all points, the water component is in equilibrium. Soil science literature is reviewed to discover why such a concept is widely accepted. It is found that many analyses leading to the total potential concept contain shortcomings which have resulted in invalid conclusions. The key shortcoming is a failure to distinguish between components of a potential that apply only to elements of the soil solution and those that apply only to the water component. Concepts from texts dealing with transport processes and thermodynamics are used to show that there is no single potential that is a function of the state of the soil solution only, whose gradient will always indicate the direction of net transport of the water component, or which if constant in all parts of the system will ensure that equilibrium exists.

¹ Contribution of Rocky Mountain Area Agricultural Research Service, USDA, in cooperation with the Colorado State Univ. Agric. Exp. Stn.

² Professor Emeritus, Colorado State Univ., Fort Collins, CO. 80523; Professor, Dep. of Agronomy, Colorado State Univ.; and Soil Scientist, USDA – ARS, Fort Collins, CO 80523, respectively.

Received for publication December 5, 1983. Accepted for publication September 5, 1984.

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