SUSTAINABILITY OF SOIL-STRUCTURE QUALITY IN RICE PADDY SOYBEAN CROPPING SYSTEMS IN SOUTH SULAWESI, INDONESIA

In South Sulawesi, paddy rice (Oryza sativa L.) is generally double cropped with soyabean (Glycine max L.) in the dry season. Decline in soya-bean yield and response to fertilisers after some years indicates that these cropping systems may not be sustainable in terms of soil structural properties. We investigated the deterioration of soil structure by measuring penetration resistance, air-filled porosity, bulk density and plant available water content in rice paddy-soya-bean rotations of 10, 16 and 21 years duration. These results were compared with those from a 21 year rotation in which rice was followed by grass fallow. The non-limiting available water range was used to assess cropping system differences. Rice-soya-bean cropping systems were detrimental to soil physical conditions in the long term. Bulk density and soil strength increased and air-filled porosity decreased significantly over a period of 21 years. At this time, aeration porosity at the upper drained limit was below the critical value for adequate aeration, soil penetration resistance exceeded the limits for root growth within the plant available water content range, and the non-limiting range of plant available water was narrowed. These results indicate that double cropping is unsustainable in terms of soil structure quality. Shorter periods (10 years) of double cropping did not cause the same level of deterioration and the grass fallow system had no discernible limitation to available water as a result of poor aeration or high soil strength. We conclude that grass fallow cropping systems appear to be more sustainable and in long-term double cropped systems, a period of grass fallow is probably required to restore soil structure to a favourable state.