Using Soil Moisture Deficit to Predict Grassland Trafficability.

Soil compaction beneath agricultural tires is a prominent form of structural damage, causing prolonged effects on drainage and crop yield. The vulnerability of a soil to compaction is strongly influenced by its water content at the time of trafficking. The hybrid soil moisture deficit (SMD) model predicts soil wetness relative to field capacity (θ_fc) (represented as 0 mm SMD). Increasing soil dryness is expressed as millimeters of precipitation required to bring the soil to θ_fc. A randomised block experiment was conducted on well, moderate and poorly drained grassland fields to determine the vulnerability of soil to surface compaction during slurry spreading at various SMD values. A single pass of a fully loaded Landini Vision 105 tractor and 2000 gallon single axle-slurry tanker (combined weight c. 18 tonnes) was made at predicted SMD of 0, 5, 10 and 20 mm. Soil bulk density (B_d) prior and subsequent to trafficking was used as an indicator of compaction, and was further monitored at 30 and 60 days post-treatment. Rut profiles were also measured at these intervals to indicate surface deformation and natural  alleviation over time. Grass dry matter yield was compared between trafficked and non-trafficked areas. Results found that SMD at the time of trafficking had a significant effect (P<0.05) on the magnitude of B_d changes, rut dimensions and on the dry matter yield of the trafficked area. Trafficking at greater than 10 mm SMD allowed a return to pre-traffic conditions by 60 days post-treatment. Consequently, a threshold of 10 mm SMD is proposed for safe slurry spreading.