Soil Water Retention of a Bare Soil with Changing Bulk Densities.

Tillage changes the bulk density of the soil, lowering the density initially after which it increases as the soil settles. Implications of this for soil water content and soil water potential are obvious, but limited efforts have been made to monitor these changes continuously. We present in-situ measurements of soil water retention using Thermo-TDRs and micro-tensiometers. These measurements are compared to water retention curves of soil cores obtained at different times following a tillage event, measured under controlled conditions in the laboratory. Initial results indicate that the lowest bulk densities are associated with lower water retention and a larger range of plant available water. Bulk densities were uniform with depth immediately following tillage. In the top 5 to 10 cm soil layer, bulk densities remained low throughout the season while in layers below 10 cm bulk densities increased, strongly increasing water retention and reducing the range of plant available water. Actual available water in the field appeared to be highest in the 10-15 cm layer; where high water content was associated with high pressure potential. The low density of the 0-10 cm layer allowed water to drain relatively quickly, while at 15-20 cm depths the higher density resulted in a smaller range of plant available water. The implications of water retention differences for plant available water and evaporation will depend on the matric potentials typically found in the field. The comparison between laboratory and in-situ generated data will be highlighted.