Neutron moisture meter calibration equations for soil water assessment in the sugar industry

Calibration equations to determine volumetric soil moisture using a neutron moisture meter (NMM) were developed for a range of soils cropped by the sugar industry. Both linear and non-linear calibration equations were developed from a pooled data set of 1128 observations along with individual site calibrations and linear calibrations obtained for six soil textural classes. The accuracy of these equations in determining soil moisture were compared to the general calibration equations supplied with the neutron moisture meter. Both the pooled data set calibration equations and the supplied calibration equations were found to over-estimate the relative differences in volumetric soil moisture on light textured soils by up to 11.3% and to under-estimate the differences on heavy clays by up to 7.1 %. Errors of up to 11.5% volumetric moisture were also found in absolute soil moisture determination using these equations. The calibration equations developed for individual sites were found to produce an average deviation in absolute soil moisture of 1.6% while equations developed using data grouped according to soil texture produced an average deviation from the measured soil moisture of 0.58%. Relative differences in volumetric soil moisture of 0.1 % to 3.0% were found using the individual site calibrations while relative differences of 0.4% to 1.9% were found using the textural class equations. These results indicate that textural class calibrations may be used instead of individual site calibrations for some NMM applications in the sugar industry.