Agricultural soils are important not only for their critical role securing food supplies in a resource limited world but for their contribution to the balance of greenhouse gas emissions.
A recently released Goyder Institute for Water Research report outlined a novel approach to investigating how to increase a soil’s carbon content and storage capacity by assessing the potential of an existing agricultural practice—adding sub-soil clay to sandy soils. Increasing organic carbon in sandy soils by adding clay improves soil physical, chemical and biological characteristics. In combination, these develop healthier soils, resulting in increased crop and pasture productivity. This could provide farmers with additional income while mitigating climate change effects.
The project titled Offsetting greenhouse gas emissions through increasing soil organic carbon in SA clay-modified soils: knowledge gap analysis was funded by the Institute and the Department for Environment and Water and led by Amanda Schapel and Professor Jim Cox (PIRSA-SARDI).
The project team collated and analysed data from existing clay-modified and unmodified field sites across South Australia. They found that clay modification increased soil organic carbon (OC) stock in the surface 30 cm of soil by an average 4.9 t/ha-1, with a 4–8 t/ha-1 range depending on rainfall zone. The analysis also identified factors that influence OC storage in clay-modified soil, including rainfall and water storage and depth to subsoil clay (Table 1).
Table 1. Summary of key factors or practices that influence carbon stock and the primary limitation in sandy soils with clay addition. Number of ticks indicates the level of influence on OC stock
The research project found that:
A literature review also found that subsoil clod size inversely influenced OC concentration, and a greater number of subsoil clods positively influenced OC stock.
The project has provided South Australian researchers, farmers and natural resource managers with an evidence base to improve soil fertility and sequester carbon in agricultural soils and contribute to mitigating the effects of climate change.
The project’s findings will help to develop guidelines for clay-modification techniques for soil carbon sequestration in South Australia and guide further research in the area.