Should Upper Limit of Available Water be Based on Field Capacity?

Abstract

Core Ideas Plants can use water while soil drains. Extra water can move up into the root zone. Field measurements of water use are not restricted by boundary values. Plant-available water is usually determined from laboratory measurements as the difference between field capacity (undisturbed soil samples) and wilting point (disturbed samples), both of which are determined at arbitrary applied pressures. How much soil water is actually available to plants? The purpose of this report is to demonstrate that data are inconsistent with the concept of field capacity at a pressure head being the upper limit of available water. Existing field studies were re-examined. Literature shows that a considerable portion of transpired water often comes from below the root zone. A site that had water table depths ranging from near the surface to 3 m deep showed that tensiometer pressure heads in the root zone rarely got as dry as −333 cm pressure head (approximate limit of field capacity). Another study showed that as surface soil dried, the roots were able to take up water from deeper soil, showing the flexibility of the root zone depth. Still other studies suggest that transpiration might start between 3 and 10 h after a rain ends, even while the soil is still draining. Together these studies strongly suggest that roots take up water under conditions wetter than −333 cm pressure head; therefore, laboratory-determined field capacity should not be considered the upper cutoff of available water.