Variations in sludge effects on selected properties of four soil types and vegetable yield

This study investigated the degree of modification of selected properties of soils in a semi-arid environment as a result of sludge application, in an endeavour to understand variations in percentage improvement with sludge application rate (SAR). Sludge was applied to a calcic luvisol (luvisol 1), a ferric luvisol (luvisol 2) an arenosol, and a vertisol at different rates and after three months, each soilsludge mixture tested. Spinach (Spinacea oleracea) was then planted to test sludge effects on spinach yield. Addition of sludge reduced the pH of luvisol 1, luvisol 2, arenosol and vertisol by an average of 0.8, 1.1, 0.4 and 1.0 pH units, respectively. Mean concentrations of phosphorus, total kjeldahl nitrogen, and plant available nitrogen were highest in luvisol 1 (91.1 mg/kg), vertisol (2.3%), and vertisol (718.3 mg/kg) respectively. Concentrations of all plant nutrients increased with SAR in all soils. Percentage increase however varied, being highest in the luvisols at lower SAR, and shifting to the arenosol as SAR increased. Highest spinach yield was obtained from luvisol 2 with mean values of 49.2 gm, 19.7 gm and 179.7 cm2 for fresh biomass, dry biomass and leaf area of spinach respectively. Spinach grown on the sludge-amended vertisol had the highest correlation coefficients between leaf area (LA) and SAR (0.92), fresh biomass (FBM) and SAR (0.90) and dry biomass (DBM) and SAR (0.95). Possible causes of the observed differences in sludge effects on the different soil properties are discussed.