Field Calibration of a Neutron Moisture Meter in Vertisols

Accurate calculation of water use efficiency is critical in evaluating innovative crop management practices under limited moisture conditions. The use of a neutron moisture meter is advantageous where repeated, non-destructive measurements are needed, but it requires a particular calibration procedure in Vertisols. The purpose of this study was to produce a field procedure for unbiased volumetric moisture content calculations using a neutron moisture meter. Two methods of calibrating a neutron moisture meter in the field were compared. The methods differed in their soil sampling procedures. The first method employs undisturbed core and clod sampling adjacent to the access tubes. This method is destructive with respect to the access tube. The second method employs disturbed soil sampling using a screw auger a few meters from the access tubes. This method is less laborious and non-destructive. Undisturbed soil samples adjacent to the access tube are representative for the sphere of influence of the neutron moisture meter and their bulk density-moisture content relationships follow a normal one-dimensional shrinkage model. The precision of the calibration was increased by correcting for changes of bulk density with changes of water content. The two methods of calibrating gave highly significant (P < 0.001) correlation coefficients. The second calibration method, however, introduced a bias of ≤20% at the dry end of the moisture range. At wilting point this bias was 10%. Separate calibration equations were necessary for the different horizons of the soil. Based on the calibration with correction for bulk density effect, a polynomial function can be derived that relates neutron count ratio with gravimetric water content; thus, we assumed one-dimensional shrinkage. For volumetric moisture determinations at field level, gravimetric water content was converted to volumetric on the basis of a three-dimensional isotropic shrinkage model.