Moisture limits for grassland soil to avoid structural damage due to machine trafficking

Soil compaction is one of the primary threats to soil degradation in Europe; however, data to guide grassland farmers on how to avoid traffic induced soil compaction is limited. In grassland systems, soil moisture regimes are measured by daily soil moisture deficit (SMD) values and, when coupled with soil physical indicators could help safeguard the soil physical quality (SPQ). The objective of this study is to investigate how soil physical quality changes across different induced traffic compaction events at targeted SMD. A field study at Johnstown Castle Beef Farm (Wexford, Ireland) investigated the severity of soil physical changes caused by machine trafficking across different targeted soil moisture regimes. A tractor and a fully loaded slurry tanker trafficked moderately drained soil plots at SMD targets of 10 (dry (D)), 0 (moist (M)) and – 10 (wet (W)) mm. Compaction events simulated four passes across one year of grassland management: at the time of the first silage cut, in April; after the first cut silage harvest, in June; before the slurry spreading opening season, in October; and at the beginning of the slurry spreading period, in January. Soil bulk density (BD) samples were taken in the middle of the tyre marks at different depths (0-10, 10-20 and 20-30 cm). To examine indirect soil physical quality of various treatments, soil physical data was used to calculate the S value (S_i) using the SawCal model. Initial results showed that the progressive increase in the number of trafficking events occurring above SMD 0 mm  led to major compaction, which significantly increased (P<0.05) compared to trafficking at SMD 10 mm. The cumulative effect of the four passes showed a significant difference from M to D and W, with M’s BD increasing by 22.2% compared to the control. D and W BDs remained similar ranging between 1.10 and 1.11 Mg m^-3. Accordingly, S_i values were indicative of very poor (i.e. degraded; S < .035) SPQ in M only. These early results indicate that the most severe degradation occurred in the 0 – 10 cm depth. Forecasting soil moisture is a valuable tool for the protection of SPQ and to enable farmers to minimise any soil degradation due to trafficking.

Keywords: soil compaction, grasslands, soil moisture, field traffic, soil physical quality indicators