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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #364687
Research Project: Agroecosystem Benefits from the Development and Application of New Management Technologies in Agricultural Watersheds

Location: Agroecosystems Management Research

Title: Should upper limit of available water be based on field capacity?

Author
item Logsdon, Sally

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2019
Publication Date: 10/10/2019
Citation: Logsdon, S.D. 2019. Should upper limit of available water be based on field capacity? Agrosystems, Geosciences & Environment. 2(1):1-6. https://doi.org/10.2134/age2019.08.0066.
DOI: https://doi.org/10.2134/age2019.08.0066

Interpretive Summary: Understanding how much soil water is available for crop use is important for crop management. This study showed that plants start taking up water even while the water is still draining. Plants are able to extend their root zone as surface soil dries out. So in summary, plants can use more soil water than previously realized. This information is important for crop water managers and their advisors.

Technical Abstract: "Plant available water" is usually determined from laboratory 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 study is to demonstrate data inconsistent with the concept of field capacity 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 with a somewhat more shallow water table showed that tensiometer pressure heads 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 "field capacity"; therefore, field capacity should not be considered the upper cutoff of available water.