Soil structure in permanent beds under irrigated cotton-based cropping systems in a Vertisol

Much of the research pertaining to soil structure in Australian cotton (Gossypium hirsutum L.) farming systems has been conducted using cropping systems where cotton was followed by a single rotation crop such as wheat (Triticum aestivum L.) and a lengthy fallow. Research on systems where the fallow was very short or absent is sparse. The objective of this study was to quantify soil structure within beds in an irrigated Vertisol by assessing the changes in shrinkage curve indices caused by (a) imposing permanent bed systems in a site that had been under intensive tillage, and (b) eliminating or shortening fallow periods in cotton-based crop rotations by sowing vetch (Vicia spp.). Shrinkage curve indices in Vertisols are indicators of soil structure degradation such as compaction. The experimental treatments sown on permanent beds were: cotton monoculture, CC; cotton-vetch (Vicia spp.), CV; cotton-wheat, CW where wheat stubble was incorporated into the beds after harvest with a disc-hiller; and cotton-wheat-vetch, CWV where wheat stubble was retained as an in-situ mulch into which the following vetch crop was sown. Soil clods were extracted during September 2002 from soil pits. Clods were also sampled with a spade during September 2009 and 2010 from the beds in every plot. The field-moist clods were coated with saran resin dissolved in ethyl-methyl ketone (2002) or paraffin wax (2009, 2010). As the clods dried, soil specific volume was determined at various soil water contents. Soil specific volume was plotted against soil water content, and linear functions fitted to the zones of structural, normal and residual shrinkage, and several shrinkage indices determined. Average soil specific volume of oven-dried soil was 0.58 m3 Mg−1 with CV, 0.65 m3 Mg−1 with CC, 0.62 m3 Mg−1 with CW and 0.58 m3 Mg−1 with CWV (SEM = 0.005, P < 0.001). Soil compaction in beds was less in cropping systems that had a lengthy fallow (CC, CW) than in those that had no or a very short fallow (CV, CWV). Higher compaction in the latter systems may be due to management practices associated with the vetch component. The virtual elimination of the fallow was, however, beneficial in wet years as it reduced potential soil structural degradation under wet conditions, presumably due to the drier conditions under a growing crop.