Saturday, 15 July 2006

Effect of Irrigation-Induced Salinity and Sodicity on Sugarcane Yields, Soil Chemical and Microbial Properties.

Richard John Haynes and Diana N. Rietz. Univ of KwaZulu-Natal, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa

The effects of irrigation-induced salinity and sodicity on sugarcane yields, soil chemical and microbial properties were investigated in vertic soils on a Zimbabwean sugar estate. Furrow-irrigated fields were selected which had a gradient of salinity and sugarcane yield ranging from good cane growth at the upper ends to dead and dying cane at the lower ends. Soils were sampled under dead and dying cane, poor, satisfactory and good cane growth and from adjacent undisturbed sites under native vegetation. The calcareous, vertic soils in the study area under undisturbed veld were found to have high pH values (9 to 9.5), very high exchangeable Ca and Mg concentrations and there was evidence of accumulation of soluble salts in the surface 0.15m. Under sugarcane high values for electical conductivity, sodium absorption ratio and exchangeable sodium percentage were generally encountered in the surface 0-0.3 m of the profile.In addition, the pH values under sugarcane were often between 9 and 10 particularly in profiles where sugarcane grew poorly or died. As expected, pH was positively related to exchangeable sodium percentage and sodium absorption ratio but negatively related to electrical conductivity. Sugarcane yields were negatively correlated with exchangeable sodium percentage, sodium absorption ratio and pH, but were not related to electrical conductivity. Results thus suggested that sodicity was a more limiting factor for sugarcane than salinity. There was a significant negative exponential relationship between electrical conductivity and microbial biomass C, the percentage of organic C present as microbial biomass C, indices of microbial activity (arginine ammonification and fluorescein diacetate hydrolysis rates) and activities of the exocellular enzymes glucosidase, alkaline phosphatase and arylsulphatase but the negative relationships with sodium absorption ratio and exchangeable sodium percentage were best described by linear functions. By contrast, the metabolic quotient increased with increasing salinity and sodicity, exponentially with electrical conductivity and linearly with sodium absorption ratio and exchangeable sodium percentage. Potentially mineralizable N, measured by aerobic incubation, was also negatively correlated with indices of salinity and sodicity. These results indicate that increasing salinity and sodicity resulted in a progressively smaller, more stressed microbial community which was less metabolically efficient. The results demonstrate that agriculture-induced salinity and sodicity not only influence soil chemical and physical characteristics but also reduce soil quality by inhibiting soil microbial and biochemical processes. This will, in turn, decrease fertility and nutrient supply.

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