Use of a surface gamma-neutron gauge for in situ measurement of changes in bulk density of the tilled zone

Short-term temporal changes in bulk density and related soil-water properties of a tilled soil may appreciably influence the processes of infiltration, soil water storage, runoff and erosion. Using a properly-calibrated surface gamma-neutron gauge, the changes in bulk density and moisture content within the topsoil layer can be measured in situ and at a large number of locations, with a minimum amount of time and expense. In this study on a Bernow loam soil (Typic Paleudult), factory calibration for either the neutron or the gamma component of a Troxler gauge was found unsatisfactory when compared with soil cores. Field calibration was obtained for both these components. Two different methods tried for gamma calibration gave satisfactory and nearly the same results. These findings generally agreed with the results for two other soils, whose data were available from an earlier study. Using field calibrations, soil bulk density was measured weekly at several sites within 4 freshly-tilled plots, one left bare, two planted to corn and one to soya bean, in depth intervals of 0–10, 10–20 and 20–30 cm. The plots were irrigated 3 days before each measurement. Over a 15-week period, the major changes in bulk density occurred only within the 0–10-cm layer, and these changes were strongly correlated with the amount of water applied. The presence of crops did not significantly influence these changes measured in the interrow areas. However, some additional data in the 0–10-cm layer indicated that roots may modify soil bulk density in the crop row and interrow areas differently. Measurements of this type serve to provide important information for improving soil and water management.