Field calibration of a neutron moisture meter in vertisols

Accurate calculation of water use efficiency is critical in evaluating inno vative crop management practices under limited moisture conditions. The use of a neutron moisture meter is advantageous where repeated, non-destructiv e measurements are needed, but it requires a particular calibration procedu re in Vertisols, The purpose of this study was to produce a field procedure for unbiased volumetric moisture content calculations using a neutron mois ture meter. Two methods of calibrating a neutron moisture meter in the fiel d were compared. The methods differed in their soil sampling procedures. Th e first method employs undisturbed core and clod sampling adjacent to the a ccess tubes. This method is destructive with respect to the access tube. Th e second method employs disturbed soil sampling using a screw auger a few m eters Born the access tubes, This method is less laborious and non-destruct ive. Undisturbed soil samples adjacent to the access tube are representativ e for the sphere of influence of the neutron moisture meter and their bulk density-moisture content relationships follow a normal one-dimensional shri nkage model, The precision of the calibration was increased by correcting f or changes of bulk density with changes of water content. The two methods o f calibrating gave highly significant (P < 0.001) correlation coefficients. The second calibration method, however, introduced a bias of less than or equal to 20% at the dry end of the moisture range, At wilting point this bi as was 10%, Separate calibration equations were necessary for the different horizons of the soil. Based on the calibration with correction for bulk de nsity effect, a polynomial function can be derived that relates neutron cou nt ratio with gravimetric water content; thus, we assumed one-dimensional s hrinkage, For volumetric moisture determinations at field level, gravimetri c water content was converted to volumetric on the basis of a three-dimensi onal isotropic shrinkage model.