Soil salinization: putting food security at risk
The problem of increasing soil salinity poses major challenges to farmers and scientists worldwide. Throughout the world, the land area of salt-affected soils is expanding as a result of decades of mismanagement and industrial agriculture. Most crops do not tolerate salt. Food security and the proper functioning of ecosystem services are therefore at risk. And climate change will make things worse. In this article, we explore what soil salinization is, what it is caused by, which countries are affected, and what can be done about it.
Author: Kathelijne Bonne.
Soil salinization refers to the increasing concentrations of salt in soils, which makes it difficult for the roots of plants to absorb water. As a result, soil productivity and soil health deteriorate. In a salt-affected soil, plant growth decreases, the soil loses its good structure, cracks form, and white spots and even crystals begin to form in the soil profile and on the surface. At the first signs of salinization, remediation is still possible, but after a while, the problem becomes irreversible (unless complicated and expensive means are used). Farmers then abandon their fields to avoid even greater losses. Of course, this is a vicious circle in which especially poor, small farmers become entangled.
Soil salinization is one of the main causes of desertification. Studies show that 833 million hectares of land (four times the area of India, and 9% of the total land surface) are affected by salt, due to human activity. Since the 90s, this has been about 5,000 hectares a day. In the face of this alarming trend, the Food and Agriculture Organization of the United Nations has created awareness on soil salinization and tried to educate the public and policy makers about this phenomenon, in the run-up to World Soil Day on 5 December 2021. Soil salinity was mapped and the Global Map of Salt-Affected Soils was published, available to everyone.
Circle of life
But in order to understand soil salinization, let's first focus on the importance of soils in general. Soil is the thin, outer layer of the earth's crust in which plants can grow. Soil is literally the basis of all terrestrial ecosystems. In the soil, the hard rock that makes up the earth's crust has been weathered by chemical interactions with water, air and life, and is broken down into loose sediment particles. This loose sediment is held together - in healthy soil - by soil moisture, living micro-organisms, decomposing organic matter and humus. Humus consists of humic acids. These are the remains of living organisms that can remain stable for thousands of years. Humic acids give the soil its typical black or dark brown color and spongy structure. Invisible to the naked eye are the billions of micro-organisms, such as bacteria, that perform a myriad of vital tasks in every teaspoon of soil. One of these tasks, but certainly not the only one, is to process dead plant material and to release the constituents of these plants back into the soil so that they can be taken up again by a new generation of plants via the roots. This is how the cycle of life unfolds within the soil.
But in so many places, this healthy, crucial cycle has been pushed to its limits. Soil salinization is one of the many forms of soil degradation. In short, soil degradation occurs when the soil loses its capacity to carry out vital ecosystem functions. As a result, it also has a reduced ability to produce biomass and agricultural crops.
Unsustainable soil management
Salts are naturally present, but normally the concentrations are low. Salty groundwater is usually located deep enough so it does not pose a danger to plant roots. The causes of soil salinization are the practices that cause the salt in the deep groundwater to rise to the root zone; that deter salt from being washed away with rain and groundwater; and that cause salt to become concentrated in the soil. Amongst causes are irrigation, deforestation, intensive farming (including plowing), grazing, groundwater pumping, and use of pesticides and fertilizers.
In dry regions, crops are often irrigated with salty water. And because this irrigation water is also used wastefully (because it is cheap), the fields get more water than necessary. This excess water evaporates, leaving a more concentrated salty soil solution behind. Irrigation with good-quality water can also cause salinization. Deeper, saline groundwater (which normally has no effect on vegetation) can rise to the root zone because the soil is wetted from above, and a continuous water column is established between the moist soil and the deep groundwater. Osmosis then causes this deep salt to rise. Deforestation also causes salinization. The mass disappearance of these 'lungs' changes precipitation patterns, it rains less, salts are less likely to be flushed away with groundwater, and there is more evaporation. Pumping groundwater near the sea causes brackish or saline groundwater to flow inland. Irresponsible use of fertilizers and pesticides changes the chemistry and structure of the soil, making it more prone to salinization. Uncontrolled grazing of large, trampling herds destroys the vegetation (grass), in turn destroying the soil structure, increasing evaporation and salinity. Removing the deeply rooted plants can cause the water table to rise, causing salts to reach top layers. As the more soil is exposed to the air, through deforestation, by ploughing and by fields lying bare, salinity will only continue to increase.
A salt-affected soil is more prone to erosion, its permeability reduces, the biodiversity decreases, and the functioning of the biogeochemical cycles weakens. Ultimately, all these problems lead to rising prices of essential goods and forced migration of peoples.
Saline and sodic soils (two types of soils with high salinity) occur naturally in very arid regions, but due to human activity, they now also become common in rainy parts of the world. Natural salinity is usually found along coasts where the sea breeze blows salt onto the land and where flooding is common. Or in inland areas that have been inundated by the sea in the geological past and therefore have a high salt content, e.g., in Australia and the south-east of Spain. And further inland, where lakes have dried up due to past climate changes, such as the high plains of the Andes, the Himalayas and Mongolia, which also have extremely dry climates. In such naturally saline areas, well-adapted animals, plants (halophytes) and other organisms thrive, finding refuge in salt plains, forming delicate ecosystems. But the vast majority of living creatures succumb to salt, as do all commercial crops (wheat, rice, maize, rye, grass, sorghum, soya, fruit, vegetables, etc.) with which we feed mankind, and the livestock bred for mankind.
To the oceans
Salts form through weathering of rocks during soil-forming processes. Elements such as sodium, potassium, calcium, magnesium, chlorine, bicarbonate, sulphate, etc. are released into the soil moisture. These elements react with each other forming salts. All living beings need a small amount of salt, but too much will cause problems. Salts are very soluble, and therefore a liquid quickly becomes saturated with salt. The negative effect of salt is that it makes water very difficult for plant roots to absorb. The plants then suffer from osmotic stress. If we take in too much salt, our blood pressure goes up and our heart has to pump harder.
If there is enough precipitation, the salts that are naturally present in the soil are washed away with the groundwater. Groundwater replenishes rivers that flow to the ocean. Thanks to this influx, the seas are rich in salts and minerals, having been replenished continuously over millions of years.
Many of Australia's soils are severely affected by salt. The causes of soil salinization in Australia are a combination of unsustainable farming and soil management, an arid climate not suitable for crops, and its geological past. Since the British colonization and the rise of modern industrial agriculture, the deep salt has percolated upwards, through massive irrigation (often with salty water) and by leaving fields devoid of any vegetation during the non-productive season.
The losses in Australia through soil salinization are immense. In addition to the obvious losses through reduced agricultural productivity, the quality of the drinking water of the cities deteriorates. Australia now has to desalinate seawater to meet its drinking water demands. The salt is also hugely damaging to infrastructure. But luckily, Australian scientist, farmers and policy makers have realized decades ago that a shift to nature-friendly methods of soil management is imperative. In the 90s, the Government has launched major programmes addressing soil salinity, such as the National Dryland Salinity Program (1993-2004) and the National Action Plan for Salinity and Water Quality. Today, sustainable land management is supported by the National Landcare Program. (You can read more about Australia's salt-related problems in the book "Collapse" by Jared Diamond)
Soil salinization also occurs elsewhere, e.g., in the Indus Plain, the Middle East, the Nile Delta, the Ebro Delta (Spain), the Netherlands, Peru, and parts of the Argentine pampas. The Aral Sea, a very serious case, was once very large, but has now almost completely dried up due to climate change. The emerged bottom of this inland sea was first found to be fertile and cultivated. But irrigation with salty water caused salinization and the area surrounding the remaining pools of the Aral Sea has lost its productivity.
There are different types of salt-affected soils (saline and sodic), each requiring its own specific approach. There are high-tech desalination methods, but a first step is to switch to sustainable soil management, regenerative agriculture and nature-friendly precision agriculture. Irrigation, pumping, fallow, chemical fertilization and pesticide use must be halted. Drip irrigation, natural manure, increasing biodiversity and growing salt-tolerant crops can help restore natural balances.
The solutions to soil salinization are hence part of a worldwide need for sustainable solutions to guarantee food security in the future.
Joint Research Centre (JRC): Sustainable agriculture and soil conservation, Soil degradation processes: Salinization and sodification: https://esdac.jrc.ec.europa.eu/projects/SOCO/FactSheets/ENFactSheet-04.pdf
Nature, Planetary limits to soil degradation: https://www.nature.com/articles/s43247-021-00323-3
FAO, Global Soil Partnership, Global Map of Salt-Affected Soils: https://www.fao.org/global-soil-partnership/gsasmap/en/
FAO, Voluntary Guidelines for sustainable soil management: https://www.fao.org/3/bl813e/bl813e.pdf
UN Natural Resources Forum, Salt-induced land and water degradation in the Aral Sea basin: A challenge to sustainable agriculture in Central Asia: https://onlinelibrary.wiley.com/doi/10.1111/j.1477-8947.2009.01217.x
Earth Observation System, Soil Salinization: https://eos.com/blog/soil-salinization/
JRC, World Atlas of Desertification, Soil Salinization: https://wad.jrc.ec.europa.eu/soilsalinization
Diamond, Jared. Collapse. Penguin Books, 2011.
Photo Salty Soil EU JRC: https://wad.jrc.ec.europa.eu/sites/default/files/subchapters/11_4_Salinisation/11_4_Salinization_img6.jpg
Article written by Kathelijne Bonne, geologist, soil scientist and editor of GondwanaTalks.