Effect of broad beds and dolichos residue management on properties of an irrigated vertisol

Abstract The effects of mulching or incorporating residues of dolichos ( Lablab purpureus L.), sown in rotation with cotton ( Gossypium hirsutum L.), in broad (1.5 m wide) beds on soil properties of a Vertisol were studied in Northern New South Wales, Australia. Soil was sampled from the 0-0.10 m (bed surface) and 0.20-0.30 m (below bed) depths of edges and centres of broad beds during January 1993. Soil properties measured were particulate, mineral-associated and total organic matter, dispersion index, plastic limit, geometric mean diameter (GMD) of soil aggregates formed after puddling and drying at 40°C (soil reactivity), soil density, exchangeable cations and nitrate-N. Compared with mulching, incorporating dolichos residues resulted in a significantly lower dispersion index. Mulching also resulted in higher values of dispersion index below beds when compared with bed surfaces. Plastic limit at the centres of beds was significantly lower than that in the edges. Smallest GMD of soil aggregates occurred in the centre of mulched beds. Greatest soil compaction occurred at soil water contents ≤0.15 m 3 Mg −1 below beds when dolichos residues were mulched. Where dolichos residues were incorporated, at soil water contents ≤0.10 m 3 Mg −1 compaction in the soil surface was lower in bed centres when compared with those at the edges of beds. Residue management had no significant effect on soil organic matter fractions, although coarse (2 mm–212 μm), fine (212-53 μm) and total soil organic matter contents on bed surfaces were greater than those below beds, and coarse particulate organic matter at the edges of beds was greater than that at the centres. Greatest exchangeable K, and lowest exchangeable Na and exchangeable sodium percentage (ESP) occurred where dolichos residues were incorporated. In comparison with mulching, exchangeable Mg was higher and exchangeable Ca lower below beds with residue incorporation. Nitrate-N on bed surfaces was higher than that below beds with mulching. Mulching improved only friability of surface soil in bed centres, whereas indices of soil physical and chemical fertility such as aggregate stability, exchangeable cations, ESP and soil compaction in both bed surfaces and below beds were improved by incorporating dolichos residues. Better soil quality can, therefore, be maintained at this site by incorporating rather than mulching residues of dolichos sown in rotation with cotton.