Soil organic carbon (SOC) is an important determinant of soil health and fertility. Clay minerals (i.e. phyllosilicates) and metal oxides in soil are known to significantly influence SOC accumulation and stabilisation by adsorbing organic carbon onto their surfaces. Sandy soils are typically low in both clay and organic carbon. Thus, amending these soils with clay could enhance fertility and productivity. This project evaluates whether by-products from kaolin mining can stabilise SOC in the sandy agricultural soils of southwest Western Australia. However, before their application to soil, understanding their composition and carbon-binding potential is crucial.

To this end, the by-products were characterised using multiple complementary analytical techniques, followed by adsorption-desorption experiments with organic carbons. The two tested by-products, Kaosil and Overburden, were primarily composed of kaolinite (47% and 60%, respectively) and quartz (49% and 37%), with minor quantities of muscovite (and trace amounts of iron oxides in the overburden). Both by-products had low cation exchange capacity and specific surface area. Despite this, adsorption trials using plant-derived organic carbons showed a potential for stable carbon binding, as minimal desorption was observed. This research highlights the potential of repurposing waste by-products to enhance SOC sequestration in weathered, sandy soils, which can lead to improved soil health and agricultural productivity while also contributing to minimising the carbon footprint of agriculture.