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 (θWTL), optimum water content for tillage (θOPT) and the water content at the dry tillage limit (θDTL). 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: θWTL is estimated as soil water content at an air-filled porosity of 0.10 m3 m–3, and θDTL as soil water content at tensile strength of 50 kPa. The θOPT 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 to 0.033 kg kg–1), and soil from Lerbjerg (DK) with a range of clay contents (0.12 to 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 (>θWTL) or too dry (<θDTL). For both Highfield and Lerbjerg soils, θWTL 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. On the average, there were 21 and 25 workable days in the spring and the autumn, respectively for soil with low clay more than soil with high clay. The approach used for estimating θWTL and θDTL 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 θWTL, θDTL and workable days on more soils.