Sludge of two different ages (3 years old; Type 1 sludge, and 3 months old; Type 2 sludge) were applied to the following soil types: two luvisols, an arenosol and a vertisol at different rates to determine whether sludge application had any effect on the partitioning and consequently, bioavailability and mobility of Cu, Pb and Zn. A five- stage sequential extraction procedure was used to determine the concentration of metals in the different fractions of the soil after sampling and three months after sludge application. Type 1 sludge had higher values for Cu, Pb, and Zn (115.9, 295.5 and 400 mg kg⁻¹, respectively) than Type 2 sludge (110.8, 228.6 and 341 mg kg⁻¹ respectively). These values were significantly higher than values for the background concentrations of the heavy metals in the different soils. Pseudo-total concentration of Cu, Pb and Zn in the soils therefore increased with increase in sludge application rate. Mobility and bioavailability of Cu, Pb, and Zn were affected by interactions between sludge application rate and soil type with sludge age having no significant effect. Some precaution should be taken when the period between sludge application and planting is relatively short to minimize heavy metal mobility and bioavailability.
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