Laboratory evaluation of a model for soil crumbling for prediction of the optimum soil water content for tillage

A model for soil crumbling, called the capillary crumbling model (CCM) was introduced by Aluko and Koolen [Aluko, O.B., Koolen, A.J., 2000. The essential mechanics of capillary crumbling of structured agricultural soils. Soil Till. Res. 55, 117–126]. According to the CCM, the optimum soil water content for tillage (θOPT) may be defined as the water content at which the capillary bonding strength between aggregates is minimum. The objective of this study was to evaluate the CCM for the arable layer of 10 agricultural soils (sandy loam to clay textures) from semi-arid regions in western Iran. The results were compared with conventional soil workability limits such as 0.85 of the soil plastic limit (0.85θPL), Proctor critical water content (θProctor), 0.6 or 0.7 of water content at matric suction of 50 hPa (0.6–0.7θ50 hPa), and the Kretschmer optimum water content (θKretschmer = θPL − 0.15(θLL − θPL)) where θLL is the soil liquid limit. Repacked soil cores were prepared from intact soil aggregates (0.50–4.75 mm) to 0.9 of the critical bulk density (to represent the soil conditions before tillage). Tensile strength and matric suction of the cores were determined at different soil water contents obtained by slow drying. The CCM provided evidence for the physics and mechanics of crumbling in the studied soils. It revealed that effective stresses are the dominant inter-aggregates forces, at least for the wet range of soil water content. A fall in strength of inter-aggregate bonds (i.e. tensile strength) was recorded due to water emptying from structural pores in a narrow range of matric suction (hOPT) which was consistent with the model. With increasing soil organic matter and clay contents the fall became more distinct, indicating increased structural stability. The θOPT values determined by the CCM were found in the hOPT range 551–612 hPa corresponding to 0.91–0.79θPL, which was in agreement with published values for the soil workability limit. Negative correlations between hOPT and clay and organic matter contents clearly confirmed the increasing effect of soil structure on the enlargement of inter-aggregate pores. High correlations were observed between θOPT and 0.85θPL, θProctor or 0.7θ50 hPa. The results showed that the CCM might be recommended as a physically based method for the determination of θOPT. Considering the 1:1 relationships between θOPT and 0.85θPL or θProctor, and easy determination of θPL and θProctor, use of these indices is recommended in situations where the CCM is not applicable.