Long term effects of rice growing on some chemical properties of two soils of northern Australia

Abstract

Solodic soils (Natrustalfs) of the Lower Burdekin Valley, Qld, and grey-brown clays (Chromusterts) of the Ord Irrigation Area, W.A., of varying rice cropping histories, were analysed before and after a laboratory flooding experiment to determine whether any long-term changes in chemical fertility had occurred. In the solodic group changes in the extractability of iron and manganese in acetate and oxalate reagents indicated that oxides of these elements may be more easily reduced in soils that had grown nine or more rice crops than in soils which had not been used for rice growing. The flooding experiment showed that manganese and iron reduction and the mobilization of phosphorus were more rapid in the solodic soils which had been used for rice than those which had not. Iron reduction did not occur in the flooded grey-brown clays, and manganese reduction was apparent only in three soils, but was most rapid in those which had grown 15 rice crops. Extractable phosphorus levels decreased during flooding of the grey-brown clays, but this was not related to the cropping history. There were small increases in sodicity of the subsoil (40-50 cm) of the solodic soils which had been used for rice growing. It was concluded that for the solodic soils rice growing leads to soils more favourable for further rice growing but less favourable for non-flooded crops, whereas this effect was not shown for the grey-brown clay soils.