Effect of access hole properties on soil water content determination by neutron thermalization

Installation of snugly fitted neutron access tubes in uniformly sized holes is difficult to achieve in many soils. This is especially true for areas not readily accessible by heavy machinery. We evaluated the effect of access hole diameter and a kaolinite soil slurry placed around loosely fitted access tubes on soil water content determination by neutron thermalization. Uniformly sized 5.1 (control hole), 7.6, and 9.1-cm diam. holes, and a nonuniformly sized 5.5- to 6.75-cm diam. hole were constructed at the vertices of five 80- by 80-cm square plots on a Cecil soil (clayey, kaolinitic, thermic Typic Hapludult) within a small pasture. An additional uniformly sized 5.1-cm diam. hole was constructed 80 cm from the control hole at each plot. A 2-in. Al irrigation pipe was used as access tubing. The access tubes were fitted tightly in the 5.1-cm holes and were placed at the center of the larger holes. A kaolinite-soil slurry was placed around the access tubes in the nonuniformly sized holes. Count ratios obtained at various depths in each hole type were linearly regressed against the count ratios for the corresponding control hole at each plot. While there was a statistically significant one to one relationship between the count ratios in the control and 5.1-cm holes, the coefficient of determination (r2) was only 0.69. The r2's for the relationship between the count ratios for the control hole and the other three hole size treatments were between 0.61 and 0.72, and the regression coefficients (b) decreased with increasing access hole diameter. The calibration curve r2's for the 5.1-, 7.6-, 9.1-cm holes and the slurry backfilled holes were 0.71, 0.62. 0.76, and 0.61, respectively. The slope of the calibration curves generally increased with increasing hole size indicating some loss of sensitivity with the larger holes.