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

摘要

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 (i OPT) 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 (0.85i PL), Proctor critical water content (i Proctor), 0.6 or 0.7 of water content at matric suction of 50hPa (0.6-0.7i hPa), and the Kretschmer optimum water content (i Kretschmer = i PL 0.15(i LL i PL)) where i LL is the soil liquid limit. Repacked soil cores were prepared from intact soil aggregates (0.50-4.75mm) 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 (h OPT) which was consistent with the model. With increasing soil organic matter and clay contents the fall became more distinct, indicating increased structural stability. The i OPT values determined by the CCM were found in the h OPT range 551-612hPa corresponding to 0.91-0.79i PL, which was in agreement with published values for the soil workability limit. Negative correlations between h OPT 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 i OPT and 0.85i PL, i Proctor or 0.7i hPa. The results showed that the CCM might be recommended as a physically based method for the determination of i OPT. Considering the 1:1 relationships between i OPT and 0.85i PL or i Proctor, and easy determination of i PL and i Proctor, use of these indices is recommended in situations where the CCM is not applicable.