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New evidence that adding clay to sandy soils can improve soil fertility and sequester carbon

Feb 1, 2019
Author: Amanda Schapel


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:

  • adding subsoil clay to sand permanently increased the soil’s clay content and raised its potential to store OC. Other benefits, such as overcoming water repellence and increased water and nutrient storage capacity, result in more biomass growth above and below ground.
  • the greatest potential for increased OC storage in clay-modified soil was at 10–30 cm depth.
  • the depth to undisturbed subsoil influenced OC stock, with a greater opportunity for increased OC stock where subsoil clay was deeper than 30 cm.
  • clay distribution deep in the profile (to at least 30 cm) through delving or spading results in higher OC stock than shallower distribution.
  • the greatest difference in OC stock between clay-modified and unmodified sites occurred when rainfall was between 350 and 450 mm. This suggests that the increased water holding capacity of clay-modified soil can substantially improve OC stock in this rainfall zone, where water is a limitation to biomass production.

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.

Contact Amanda Schapel (PIRSA-SARDI) for more information about the project or read the final project report here.