Field calibration of a neutron-gamma probe in three agriculturally important soils of the Lower Macquarie Valley

Abstract

A neutron-gamma probe was field calibrated in a grey clay, a grey-brown clay and a red-brown earth near Trangie, New South Wales. These are the main soils used for irrigated agriculture in this region. Accurate but simple calibrations and sampling procedure\ are required by commercial irrigators and researchers. Bias in the estimation of soil water content was minimised by using separate calibrations for each soil and depth where equations were significantly different. Precision tended to be higher where an equation combining all depths and soils was used. but the introduction of large bias at some depths suggests that soil-specific equations should be used where possible. Calibration error generally was a smaller component of total error than location error, particularly where the combination equation was used. The errors associated with commercial sampling methods are discussed. The relationship between gamma count rate ratio and soil wet bulk density was poor, particularly in the grey and grey-brown clays, and this technique was considered to be inadequate for the measurement of bulk density in the field.