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Soil compaction negatively affects soil productivity, fertilizer use efficiency and water infiltration. The extent of compaction is dependant on soil strength, which is influenced by the soil moisture content. The purpose of this study was to determine the extent of soil compaction (measured by changes in soil bulk density and shear strength) and soil deformation incurred due to a single pass of a tractor and a fully loaded slurry tanker over grassland soils at a range of soil moisture deficits (SMD). The study should identify threshold values of SMD at which adverse soil compaction becomes significant for the soil-crop system. These values may be incorporated into the forecasting and decision making process for slurry spreading. SMD was used as a proxy for volumetric water content. Treatments of a single pass by a Landini Vision 105 tractor and a loaded 7.2 m single axle slurry tanker (total weight of c. 18 tonnes) were conducted on well, moderate and poorly drained grassland soils at forecasted SMD of 0, 5, 10 and 20 mm. The moderately drained soil was classified as a loam, while the well and poorly drained sites were classified as sandy loams. Changes in soil bulk density and torsional shear strength were used as indicators of compaction, with rut profile measurements taken to measure the extent of surface deformation, which is often the most visible indicator of compaction on the soil surface. Grass yields were measured at 30 and 60 days subsequent to trafficking. Results showed that SMD at the time of traffic had an effect on the changes in bulk density, shear strength and the extent of soil rutting following wheel traffic. Preliminary results indicate that higher SMD at the time of trafficking resulted in smaller changes to soil characteristics and more rapid recovery from surface deformation than when trafficking occurred at lower SMD. Trafficking at an SMD of 20 mm led to mean increases in soil bulk density of 8% and formation of ruts with cross sectional areas in the range of 29.4 cm to 98.3 cm. Trafficking at 0 SMD (field capacity) led to mean increases in bulk density of 15% and the formation of rut profiles in the range of 91.6 cm to 197.9 cm. These preliminary results indicate that forecasted SMD provides a valuable tool to determine the suitability of the soil for supporting farm vehicle operations such as slurry spreading. This study is still ongoing, with more detailed results and analysis to be forthcoming.