Amid a growing world population and diminishing living space, the discerning and beneficial use of steel plant slag as liming material in agriculture has become a viable option. However, until the potentially detrimental effect on human health and environmental quality of heavy metals contained within the matrixes of slag has been assessed, the use of slag in agriculture cannot be deemed sustainable. This study aims to correlate the mechanisms by which chromium(III) and nickel are sorbed in Eastern Highveld soils to easily classifiable soil constituents. In addition, the influence of an external source of silicon, as would be the case in dissolving slag, on Cr(III) and Ni mobility was investigated. The study consisted of an investigation into the mechanisms by which Cr(III) and Ni are sequestered in soil within a single wetting and drying cycle, as well as over a period of five rewetting and drying cycles. The effect of Si was observed for the latter and a detailed mineralogical study conducted as part of the former. Additionally, the reactivity of and heavy metal (Cr and Ni) release from two commercially available slag samples used as liming material in a highly buffered soil was investigated. Five main conclusions were drawn: (i) whereas Cr(III) is effectively sorbed (even at near water saturated conditions) and precipitation ascribed to bulk-solution saturation, Ni is not sequestered as effectively, and initially precipitates owing to the influence of soil mineralogy; (ii) conclusive evidence could not be gathered for the further immobilisation after sorption of Cr(III) and Ni over a short time period; (iii) both Cr(III) and Ni associate mainly with the amorphous Fe oxide mineral phase; (iv) an external source of Si renders Ni less mobile in soil that does not sequester the element effectively, but more mobile in soil that does tend to sequester the element to some degree; and (v) although neither Cr(III) nor Ni mobility could be explained using field classifiable soil characteristics (soil colour and texture), soil mineralogical and detailed surface charge characterisation did provide explanations for sorption behaviour. Copyright / Dissertation (MSc(Agric))--University of Pretoria, 2009. / Plant Production and Soil Science / unrestricted