ARS | Publication request: Gravimetric Measurement of Soil Water

Submitted to: Encyclopedia of Water Science
Publication Type: Book / Chapter
Publication Acceptance Date: December 18, 2002
Publication Date: July 22, 2003
Citation: Pikul Jr, J.L. 2003. Gravimetric measurement of soil water. In Stewart, B.A. and Howell, T. (Editors), Encyclopedia of Water Science. New York, NY: Marcel Dekker, Inc. p. 879-881.

Interpretive Summary: Water limits crop production in most agricultural soil and directly or indirectly affects soil physical, chemical and biological properties and processes. Quantity of water held in soil is commonly determined by measuring the mass of water relative to the mass of dry soil. The ratio is called gravimetric soil water, oven-dry, or soil water content. This measurement has been a mainstay of many field studies and is generally accepted as a calibration standard for many indirect soil water measurement methods. A brief review of literature from 1907 to 1930 (Agronomy Journal, American Society of Agronomy) revealed that scientists then, as now, rarely provide detail on methodology used to measure soil water content. In the current literature, the term "gravimetric water content" is commonly used to identify the base in which soil water content is being reported (gravimetric versus volumetric base) and to suggest that a standard procedure was followed. Unfortunately, the term "gravimetric water" was not defined in the Soil Science Society of America, Glossary of Soil Science Terms 1996. Some text books and Methods of Soil Analysis: Part 1 include a definition of gravimetric water. Other terms commonly used are "oven-dry water", "soil moisture", "soil water content", and "gravimetric procedure". Often there is no reference to a standard method. In the reporting of soil water content, it is imoprtant to identify that a standard method was followed.

Technical Abstract: Sampling methods that enable simultaneous measurement of both bulk density (BD) and gravimetric water content (GWC) are desirable because BD is essential for converting GWC to a volumetric water content (VWC). Expression of soil water as VWC enables calculation of several fundamental attributes of soil water condition related to volume fraction. Special attention must be given to samples collected for both BD and GWC because there is serious error associated with cutting soil cores improperly. Missing or excess soil results in an error in BD and consequently VWC. For samples with the same diameter, a small cutting error is more serious in cores of short length rather than long length. For example, on 76 mm long soil cores, an error of about 13 percent on both BD and VWC would occur if these cores were cut 10 mm short (or long). In contrast, a 10 mm under-cut (or over-cut) would result in an error of only 3 percent for cores having an intended length of 305 mm. Soil water content plays a vital role in respect to biological activity and it is important to accurately determine BD and VWC for shallow soil depths. Unfortunately, the investigator is most apt to accrue serious errors because of poor sampling technique of thin layers.