Potential of clay‐rich waste and industrial by‐products as effective inorganic soil amendments to enhance soil physical properties

Clay minerals play a crucial role in regulating the structure and functions of soils because of their chemical nature and their interactions with other soil constituents. In this context, a key objective of the research described in this article was to assess the extent to which the addition of various types of recycled clay‐rich materials to soils prone to crusting and erosion increase their clay content and improve their physical properties. Excavated soil (ExS), considered waste under European Union law, and quarry sludge (QS), an industrial by‐product of the washing process in mining operations, were used as soil amendments. ExS is dominated by swelling clay mineralogy such as interstratified illite‐smectite, whereas QS contains high level of non‐swelling clay minerals such as kaolinite and illite, and a low level of interstratified illite‐smectite. Lysimeters were filled with topsoil (0–30 cm) from an agricultural field. Treatments where the soil was amended with these materials were compared with a control without amendment. Lettuce was cultivated for three growing seasons. After 250‐days, the aggregate stability and water retention of the topsoil were measured. The equivalent pore‐size distribution (PSD), derived from the soil water retention curve, was analysed to predict soil water and aeration status. The application of QS to the topsoil enhanced soil structure without compromising lettuce productivity compared with the control and other treatments. However, amending topsoil with ExS decreased the stability because of the abundance of swelling clay minerals. Soil water retention at saturation increased from 27% in the control to 42% and 36% with ExS and QS amendments, respectively. At a pressure head >15,000 cm (residual pores <0.2 μm), water retention rose from 6% in the control to 19% with ExS and 14% with QS. The application of the clay‐rich material increased small pores compared with larger ones. This led to a minor increase in porosity responsible for water movement, aeration and root growth. This observation highlights the limitations of clay application benefits, which depend on the quantity and mineralogy of the clay and the presence of other binding agents. Further research is needed to investigate the combined effects of organic matter and clay‐rich materials on improving the soil structure and optimizing the pore size distribution for multiple functions.