EFFECT OF BROAD BEDS AND DOLICHOS RESIDUE MANAGEMENT ON PROPERTIES OFAN IRRIGATED VERTISOL

The effects of mulching or incorporating residues of dolichos (Lablab purpureus L.), sown in rotation with cotton (Gossypium hirsutum L.), i n broad (1.5 m wide) beds on soil properties of a Vertisol were studie d 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 c entres of broad beds during January 1993. Soil properties measured wer e particulate, mineral-associated and total organic matter, dispersion index, plastic limit, geometric mean diameter (GMD) of soil aggregate s formed after puddling and drying at 40 degrees C (soil reactivity), soil density, exchangeable cations and nitrate-N, Compared with mulchi ng, incorporating dolichos residues resulted in a significantly lower dispersion index. Mulching also resulted in higher values of dispersio n index below beds when compared with bed surfaces. Plastic limit at t he centres of beds was significantly lower than that in the edges. Sma llest GMD of soil aggregates occurred in the centre of mulched beds. G reatest soil compaction occurred at soil water contents less than or e qual to 0.15 m(3) Mg-1 below beds when dolichos residues were mulched. Where dolichos residues were incorporated, at soil water contents les s than or equal to 0.10 m(3) Mg-1 compaction in the soil surface was l ower in bed centres when compared with those at the edges of beds. Res idue management had no significant effect on soil organic matter fract ions, although coarse (2 mm-212 mu m), fine (212-53 mu m) and total so il organic matter contents on bed surfaces were greater than those bel ow beds, and coarse particulate organic matter at the edges of beds wa s greater than that at the centres. Greatest exchangeable K, and lowes t exchangeable Na and exchangeable sodium percentage (ESP) occurred wh ere dolichos residues were incorporated. In comparison with mulching, exchangeable Mg was higher and exchangeable Ca lower below beds with r esidue incorporation. Nitrate-N on bed surfaces was higher than that b elow beds with mulching. Mulching improved only friability of surface soil in bed centres, whereas indices of soil physical and chemical fer tility such as aggregate stability, exchangeable cations, ESP and soil compaction in both bed surfaces and below beds were improved by incor porating dolichos residues, Better soil quality can, therefore, be mai ntained at this site by incorporating rather than mulching residues of dolichos sown in rotation with cotton.