SOS Salinisation and Sodification
Salinisation and sodification are processes that involve salts in the soil. Salinisation is the accumulation of salts whereas sodification is an accumulation of sodium on clay exchange sites leading to structural decline; sodium is transported into the soil via the sodium chloride salt. Soil salinisation occurs when salt stored underground is drawn into topsoils and subsoil by the movement of water (dryland salinity), or when land is irrigated with saline water (irrigated salinity), or if excessive amounts of fertiliser is applied (Eugenio et al. 2018). Irrigation-driven salinity is a particular issue for areas with poor drainage conditions which prevent salt removal from the profile (WA EPA 2007, ITPS 2015). Depending on an individual species salt tolerance, soil salinisation can negatively affect plants/crops via osmotic stress leading to a reduction of plant water intake (Rengasamy 2016). Sodic soils also have negative effects on plant growth as they disperse when wet with low salinity water (such as rainfall). Dispersion is a degradation of clay structure and causes clay microaggregates to break apart and the clay particles to ‘disperse’ into the soil water. Clay particles then block soil pores, leading to poor infiltration, waterlogging when wet and hardsetting when dry. Due to their poor hydraulic properties, sodic soils are vulnerable to water erosion via gullying (Wong et al. 2010, Rengasamy 2016); gullies in sodic subsoils are particularly difficult to stabilise.
The movement of salt into the root zone in dryland salinity is driven by rising water tables; this is often caused by a change in the vegetation from deep rooted perennial species to shallow rooted annual species. This land use change allows groundwater to rise to a point (approx. 1.5 m depth) where capillary action due to evaporation draws the water (and the salt) to the soil surface (Rengasamy 2016). This process can take several decades before being detected and local geology, topography and groundwater aquifer characteristics also play an important role in groundwater upwelling (WA EPA 2007). The loss of deep rooted perennial species continues in Australia through both natural (fire, drought) and human induced pathways (disease, urban development), furthering the spread of salinisation into susceptible areas (WA EPA 2007, Rengasamy 2016). Although drought decreases the number of deep rooted perennial plants, it can also have a positive effect on salinisation rates by decreasing groundwater recharge (WA EPA 2007, Campbell 2008).
Salinisation and sodification are widespread issues in Australia. Although no accurate and recent statistics are available, about 16% of Australia’s total cropping area has been estimated to be affected by dryland salinity; it is expected to increase from 5.7 million hectares to 17 million hectares by 2050 (ITPS 2015, Rengasamy 2016, Metcalfe and Bui 2017). Approximately 75% of Australia’s dryland salinity occurs in Western Australia, including about 1.1 million hectares of land in the South West; over 14 000 hectares of land is lost to land salinisation each year in Western Australia (WA EPA 2007). About 40% of the Swan Coastal Plain (Western Australia) is at risk of salinisation (WA EPA 2007). The extent of soil salinisation in Victoria is currently unknown as it has not been mapped since the Millennium Drought (VIC EPA 2018). Although it hasn’t been recently estimated for the nation, salinity costs to water, infrastructure and agricultural land within the Murray-Darling Basin have been estimated at $305 million AUD per year (Campbell 2008). The lost opportunity in Australia associated with soil sodicity and salinity for wheat production is estimated to be worth A$1,300 million and A$200 million per annum respectively (Orton et al. 2018).
Campbell, A. 2008. Managing Australia’s soils – a policy discussion paper. Prepared for the National Committee on Soil and Terrain (NCST) through the Natural Resource Management Ministerial Council (NRMMC).
Eugenio, N. R., M. McLaughlin, and D. Pennock. 2018. Soil pollution – a hidden reality. Food and Agriculture Organization of the United Nations, Rome.
ITPS. 2015. Status of the world’s soil resources technical summary. FAO – Intergovernmental Technical Panel on Soils, Italy.
Metcalfe, D. J., and E. N. Bui. 2017. Australia state of the environment 2016: land. Australian Government Department of the Environment and Energy, Canberra.
Orton, T., T. Mallawaarachchi, M. Pringle, N. Menzies, R. Dalal, P. Kopittke, R. Searle, Z. Hochman, and Y. Dang. 2018. Quantifying the economic impact of soil constraints on Australian agriculture: A case‐study of wheat. Land Degradation and Development 29:3866-3875.
Rengasamy, P. 2016. Salt-affected soils in Australia. GRDC, Adelaide.
VIC EPA. 2018. Victorian State of the Environment 2018 scientific assessments. Authorised by the Commissioner for Environmental Sustainability, Melbourne.
WA EPA. 2007. State of the Environment Report: Western Australia 2007. Perth.
Wong, V. N. L., R. S. B. Greene, R. C. Dalal, and B. W. Murphy. 2010. Soil carbon dynamics in saline and sodic soils: A review. Soil Use and Management 26:2-11.
Soil Science Australia acknowledges the traditional owners of the land and pays its respects to their Elders, past, present and future.