Written by: Katie Christison
Australia is dominated by expansive deserts and thriving rainforests; harsh environments in which its native flora and fauna have adapted too throughout thousands of years of evolution1. When the Europeans colonised Australia, they began to remove the native flora and replaced it with shallow rooting European plants and crops2. This has led to the Australian water table rising and bringing with it, thousands of years of salt stores.
To understand the movement of salts to the surface, first we must understand what the vadose zone and water table are. The vadose zone includes surface soil, unsaturated subsurface materials, and a capillary fringe. The water table sits directly below the vadose zone7. A simple way to picture these zones is like a sponge half submerged in water. The bottom half is saturated, all the pore space has been used, this is the water table. The top half of the sponge is permeable, it is still able to absorb water, and can disperse it throughout the dry areas of the sponge, this is the vadose zone.
The vadose zone in Australia naturally stretches very deep, this is due to a combination of regional weather, topography, flora and geology. Salt has accumulated in the vadose zone over the geological history of Australia from the weathering of marine base rocks and transportation of salt water by wind and infiltration.
Australia’s native plants had to adapt to have deep taproots which could reach the water table. This maintained the water table level far underground, below the salt stores6. However, when the Europeans colonised Australia they set out to “improve” the productivity of the land by removing the native plants and replacing them with shallow rooting European plants and crops2. With nothing drawing from it, the water table began to rise. As this happened the salt which was stored in the vadose zone dissolved and came with it6.
When these salt stores reach the surface soil, they draw water away from plants. This is because plants rely on having a higher salt content than the soil that surrounds them. Water will move via osmosis to an area of less water and higher salt concentration. If the salt in the soil increases too much, a plant will struggle to retrieve and retain water and if the problem is not remedied than it will wilt and die5.
This is a significant financial problem for Australia, in fact, by 2050 the NLWRA have expected the total area of affected land to rise from 5.7 million hectares to 17 million hectares4. This will put significant economic strain on Australia’s agriculture, infrastructure and environment as its estimated that every 5000 hectares of visibly affected land costs the country $1 million dollars each year3.
The issue of dry salinity is a complex one, with the cost to Australia’s economy substantial. The Australian government is taking a multivariable approach to the issue depending on risk factors of different areas. There have been projects to replant native species, and research into developing salt tolerant plants and crops3 but only time will tell if these methods are effective.
- Australian Government. (2019). Our natural environment. Retrieved from https://www.australia.gov.au/about-australia/our-country/our-natural-environment
- Janson, L. (2016). The impact of Australia’s distinctive nature and ecology on imperial expansion in the first years of settlement in New South Wales. Merici, Volume 1, 2015
- Office of environment and heritage. (2013). Costs of salinity. Retrieved from https://www.environment.nsw.gov.au/salinity/basics/costs.htm
- NLWRA (National Land & Water Resources Audit) (2001). Australian dryland salinity assessment 2000: extent, impacts, processes, monitoring and management options, NLWRA, Canberra.
- Zhu, J. (2007). Plant Salt Stress. Riverside, California: University of California. Encyclopedia of life sciences
- Podmore, C. (2009). Dryland salinity – causes and impacts. Department of Industry and investment. Retrieved from https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/309381/Dryland-salinity-causes-and-impacts.pdf
- Holden, P. A. Fierer, N. (2005) VADOSE ZONE | Microbial Ecology. New York, USA: Encyclopaedia of Soils in the Environment.