Field calibration and use of the neutron moisture meter on some nigerian soils

This investigation was carried out because of a need to monitor soil moisture content periodically in the field. The neutron probe equipment (Troxler, model 104 A) was therefore calibrated in the field on four soil profiles of contrasting soil physical properties. The effects of gravel content and gravel mineralogical composition on the thermalization of neutrons were also investigated in the laboratory. An oil drum was filled with mixtures of gravel and a subsoil fine earth fraction. Gravel concentrations of 0, 50, and 75 percent were used. Quartz gravel and iron/manganese concretions were used separately. The correlation coefficient of the regression equation, Y = a + bx, where Y is the count ratio and x is the moisture content, which was obtained for the different soil layers in the field ranged between 0.84 and 0.98. The lower correlation coefficient, obtained mostly in the gravelly layers, was attributed to both spatial variability of the soil physical properties and the inherent problem associated with gravimetric moisture determination in gravelly soils. Although gravel concentration, gravel mineralogical composition, and total bulk density increased the thermalization of neutrons, the slope b of the calibration curve was not significantly affected. Sandy soils had lower intercepts a than clayey soils. For a soil-gravel mixture made in the laboratory that has a gravel concentration of 50 percent (g/100g) and a moisture content of 0.15 cmcm, the count ratio was 0.50 and 0.67 for quartz gravel and iron/manganese concretionary gravel, respectively. Unlike the field calibration curves, the slope b of the laboratory calibration curve was identical to the factory calibration curve. Although it was possible to establish one single calibration curve for all soils and depths (r = 0.89), the use of the curve to infer absolute moisture content would lead to an appreciable error. Also, the factory calibration curve will find its best use only in interpreting relative changes in moisture content.