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

We propose the soil strength and air capacity (SSAC) approach, which was compared with alternative water retention curve (WRC) and the soil consistency (SC) approaches for estimating the water content at wet tillage limit (θ_WTL), optimum water content for tillage (θ_OPT) and the water content at the dry tillage limit (θ_DTL). Unlike the WRC and SC approaches, the SSAC approach uses a fixed value of air-filled porosity and tensile strength of soil aggregates to determine the wet and dry tillage limits: θ_WTL is estimated as soil water content at an air-filled porosity of 0.10 m³ m^–3, and θ_DTL as soil water content at tensile strength of 50 kPa. The θ_OPT is estimated using the double-exponential model as soil water content at the local minimum of the pore size distribution between the peaks of the textural and structural pores. The three approaches were compared using soils from the Highfield long-term field experiment (UK) with a range of soil organic carbon (SOC, 0.009–0.033 kg kg^–1), and soil from Lerbjerg (DK) with a range of clay contents (0.12–0.45 kg kg^–1). Workable days for the soils in spring and autumn between 2014 and 2018 was estimated using a decision support tool for assessing soil workability. Workability was limited where soil conditions were either too wet (>θ_WTL) or too dry (<θ_DTL). For both Highfield and Lerbjerg soils, θ_WTL for the SC approach was generally drier than that for WRC and SSAC approaches. The number of workable days per season were influenced by SOC and clay contents. Soil with higher SOC had average of 36 and 23 workable days in the spring and the autumn, respectively, more than soil with low SOC. The number of workable days per season decreased with increasing clay content. Soil with lower clay had average of 21 and 25 workable days in the spring and the autumn, respectively, more than soil with high clay. The approach used for estimating θ_WTL and θ_DTL has impact on the number of workable days. For both soils studied, the number of workable days per season were more for WRC and SSAC approaches compared to the SC approach. While we believe the SSAC approach to be based on sound physical principles, further studies are needed to test the practical application of the SSAC approach for estimating θ_WTL, θ_DTL and workable days on more soils.