The respective roles of roots and earthworms in restoring physical properties of Vertisol under a Digitaria decumbens pasture (Martinique, WI)

Sloping Vertisols having large exchangeable sodium contents are erodible. In Martinique, French West Indies, intensive market gardening during 15 years decreased their organic matter content, biological activity (roots, earthworms, mesofauna, microorganisms), soil porosity, and increased soil losses due to erosion. Conversely, old irrigated pastures (Digitaria decumbens, Pangola grass) on these soils maintained organic matter content, biological activity, soil porosity and aggregate stability. A research programme was set out in 1991 in the south-eastern part of Martinique in order to study the respective roles of grass roots and earthworms in the restoration of the properties of a degraded Vertisol. Plots with and without plants, with and without earthworms were tested and physical properties were followed during 4 years and compared with properties existing under market crops and old pastures. The restoration of physical properties was more rapid and more important in treatments with plants than in treatments without plants. Plants played a dominant role through rhizosphere effects and possible carbon rhizodeposition. The effect of earthworms was less important. Earthworms particularly increased the stability of aggregates of 200–500 μm size, and decreased clay dispersion.