Gravimetric method to determine volumetric soil water content in situ

Abstract

A simple procedure and corresponding soil corer to determine volumetric soil water content (VSWC) by weight, directly in the field, is described. VSWC is determined by sampling and weighing an undisturbed soil core of known volume, then comparing this weight to that of a similar core of known moisture content, which needs to be pre-established for the same field and soil depth as a reference. Two types of reference cores, and thus applications, are suggested: a) dry weight or absolute reference, established thermogravimetrically, the use of which allows to know by difference in  weight the absolute VSWC value of  a sample; and, b) relative references, which require to pre establish the weight of reference cores at chosen levels of soil moisture, e.g. field capacity or a “need-to-irrigate” low level, allowing to determine by weight the difference in VSWC between a sample and that of the reference. Using this procedure and a locally built steel corer containing a 0.5 m deep soil core with a volume of 0.001 m³, a sizable sample that reduces variability, VSWC values determined for 3 tropical soils in Costa Rica were not significantly different from, and were significantly correlated to, standard thermogravimetric determinations of soil moisture converted to VSWC using bulk density, thus contributing to validate the method.