Soil water contents for tillage : a comparison of approaches and consequences for the number of workable days

Knowledge of soil water content for tillage is important in planning and scheduling tillage operations to avoid tillageinduced soil structural degradation, creating undesirable seedbed for crop establishment, and using high energy inputs because soil is not workable. We propose a new approach (NA), which we compare with the water retention approach (WRA) and the consistency approach (CA) for estimating the wet tillage limit, optimum water content for tillage and the dry tillage limit. The new approach uses soil water content at an air-filled porosity of 0.10 m3 m–3 to estimate the wet tillage limit and an aggregate tensile strength of 50 kPa to estimate the dry tillage limit. The optimum water content for tillage is estimated using the double-exponential water retention function as the soil water content at the break point between the textural and structural pores where in general, the structural pores have drained whereas the textural pores remain filled with water. The three approaches were compared using Highfield soil with a range of soil organic carbon contents (SOC), and Lerbjerg soil with a range of clay contents. The number of workable days for these soils were estimated using a decision support tool for assessing soil workability. Workable days were limited by the soil being either too wet or too dry. For the Highfield soils, the wet tillage limit estimated by NA was generally wetter than that for WRA and CA, whereas for Lerbjerg soils NA was identical to values estimated by WRA. Workable days were strongly influenced by SOC content, clay content, and the approach used for estimating tillage limits. The estimated average workable days in the spring and the autumn seasons over the period from 2014 to 2018 increased with increasing SOC content (from 2 days for BF to 40 days for G treatment), but decreased with increasing clay content (from 38 days for L12 to 0 day for L45) although there was an exception for WRA. Average workable days for the period investigated were more for the tillage limits estimated by WRA and NA compared to CA. Further studies are needed to test the use of fixed values defined in NA for estimating the wet and dry tillage limits for all soils. Future studies could also investigate whether the fixed values of 0.10 m3 m–3 and 50 kPa need to be refined. Field validation on a range of soil textures and in different climates could be the subject of further research to investigate the robustness of the approaches for estimating tillage limits.