Degradation and restoration of soil structure in a cracking grey clay used for cotton production

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Abstract
A field experiment involving irrigated cotton investigated the effect of tillage on a self-mulching cracking grey clay at three different soil water contents: dry (close to permanent wilting point to over 1 m depth), moist (subsoil close to the lower plastic limit) and wet (just trafficable). These treatments had been repeated over three years prior to the plant and soil measurements reported in this paper. Compared with tillage of dry soil, tillage of moist or wet soil depressed lint yield by 35% (P<0.001). Shrinkage curves of resin-coated, intact soil clods showed lower clod bulk density at a standard water content in the dry treatment than in the wet treatment (P<0.001). This difference was most marked in the 0.2-0.3 m depth, where clods from the wet treatment had a bulk density approaching that of clods made from hand-remoulded soil. Subsequent restoration treatments showed that, although wheat improved soil physical condition, the yield of a following cotton crop was reduced due to lower nitrogen uptake (P<0.001). Deep tillage alone increased clod bulk density but deep tillage after wheat decreased density (interaction P<0.001 for the 0.2-0.3 m depth, P<0.05 for the 0.3-0.4 m depth). These effects of deep tillage were not reflected in yield of cotton. Effects of the previous dry, moist and wet tillage treatments persisted but there was some improvement, even in the absence of restoration treatments. To preserve the structure and productivity of cracking clay soils, they should be tilled only when dry to permanent wilting point through the full depth of tillage.