Structure repair of a compacted vertisol with wet-dry cycles and crops

We hypothesized that the four rotation crops: wheat (Triticum aestivum L.), sorghum [Sorghum bicolor (L.) Merr.], lablab [Lablab purpureus (L.) Sweet] and mung bean [ Vigna radiata (L.) R. Wilczek] differ in their ability to repair soil structure. The study was conducted on a Typic Haplustert, Queen sland, Australia, locally termed a Black Earth and considered a prime cropp ing soil. Large (0.5-m depth by 0.3-m diam.) soil cores, collected from com pacted wheel furrows in an irrigated cotton (Gossypium hirsutum L.) field, were subjected to three, six, or nine wet-dry cycles that simulated local f lood irrigation practices. After each cycle, soil profiles were sampled for clod bulk density, image analysis of soil structure, and evapotranspiratio n. Generally, all crops improved soil structure over the initial field cond ition but lablab and mung bean gave improvements to greater depths and more rapidly than wheat and sorghum. Mung bean and lablab caused up to a threef old increase in clod porosity in the 0.1- to 0.4-m soil layer after only th ree wet-dry cycles, whereas sorghum required nine wet-dry cycles to increas e clod porosity in only the 0.2- to 0.3-m layer, and wheat gave no improvem ent even after nine wet-dry cycles. Image analysis of soil structure showed that lablab and mung bean rapidly (by three wet-dry cycles) produced small er peds with more interconnected pore space than wheat and sorghum. By nine wet-dry cycles, sorghum achieved deep cracking of the soil but the materia l between the cracks remained large and dense. Evapotranspiration was doubl e under lablab and mung bean compared with wheat and sorghum. Our results i ndicate greater cycles of wetting and drying under lablab and mung bean tha n wheat and sorghum that have led to rapid repair of soil compaction.