The effect of bulk density on neutron meter calibration

Abstract

The effect of dry bulk density on neutron transport in soil is discussed in relation to the calibration of the neutron moisture meter. Olgaard's model based on three group diffusion theory predicts count rate with acceptable accuracy in soils with an absorption cross section less than 0.004 cm2 g-l. For soils with a high absorption cross section, which are common in Australia, the model fails to predict the plane of the calibration line. The theoretical model predicts that, at constant total water content (available plus constitutional water), the count rate increases with increasing bulk density; for all soils for values of total water content less than 0.5 a correction factor based on this prediction significantly reduces scatter in the experimental calibration points; at water contents higher than this, clay soils are intractable, and it has not been possible to assess the density effect. An alternative, more simple procedure of correcting for the effect of bulk density on count rate is proposed; the method is based on the assumption that count rate at constant total water content is proportional to the square root of bulk density.