Soil Salinization

by

Pichu Rengasamy

• LAST REVIEWED: 25 November 2014
• LAST MODIFIED: 25 November 2014
• DOI: 10.1093/obo/9780199363445-0008

Introduction

Soils are made up of inorganic and organic compounds inclusive of living organisms. Soil salinization is the accumulation of water-soluble salts within soil layers above a certain level that adversely affects crop production, environmental health, and economic welfare. Soil salinity is described and characterized in terms of the concentration and composition of the soluble salts. Even though soluble salts are inherent in all soils, there are many processes that can contribute to the build-up of salts in a given soil layer. Weathering of soil minerals, salts added through rain, agronomic practices such as fertilizer and pesticide application, saline groundwater intrusion with water table fluctuations, irrigation with saline water sourced from bore, recycled or waste waters, dumping of industrial and municipal wastes into soils, and other soil conditions leading to reduced leaching of salts from the soil layer—all can lead to soil salinization. Seawater intrusion onto land when sea levels increase can deposit a large amount of salts in soils of coastal lands. The particular processes contributing salt, combined with the influence of other climatic, hydrological, and landscape features and the effects of human activities, farming practices, and plant interactions, determine where salinization occurs. Worldwide, more than 800 million hectares of land are estimated to be salt-affected, covering a range of soils defined as saline, saline-sodic and sodic in every climatic zone in every continent except Antarctica. All soil types with diverse morphological, physical, chemical, and biological properties may be affected by salinization. Generally, salt-affected soils are predominant in arid and semi-arid regions.

Processes of Soil Salinization

Soil salinization is normally classified as “primary” and “secondary” salinity, based on whether salt accumulates by natural phenomena or as a consequence of the management of natural resources. There are three major types of salinity based on soil and groundwater processes found all over the world. They are: (1) groundwater-associated salinity, or fluctuations in groundwater (usually saline) leading to water and salt discharge on soil surface layers; (2) non-groundwater-associated salinity, or poor hydraulic properties of soil layers (commonly found in sodic soils) resulting in reduced leaching; and (3) irrigation-associated salinity, where salts introduced by irrigation water are stored in soil layers because of insufficient leaching. During these processes, the secondary salinization is usually accelerated or enhanced by several human activities. With the introduction of agriculture and clearance of perennial native vegetation, the equilibrium levels of the water tables change. In low-lying regions, where water tables are shallower, more unused water by crops and pastures, with salt, leaks from the upper soil layers and raises the levels of groundwater. As the groundwater approaches soil surface, salt accumulates, aided by evaporation. In coastal aquifers, the salt water from the sea flows inland into freshwater aquifers, causing saltwater intrusion, a result of the density difference caused by salinity levels between seawater and freshwater, as defined by the Ghyben-Herzberg principle. This process is enhanced by the extraction of fresh groundwater. In irrigated regions, the quantity and quality of salt in irrigation water leads to the accumulation of salts when drainage conditions do not meet leaching requirements. Salt deposition from wind-blown materials from desiccated lakes and seepage from urban infrastructure also lead to soil salinization.

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  DOI: 10.1002/gea.3340030306

  Artzy and Hillel 1988 is a paper that gives an explanation of the fundamental processes of soil salinization and degradation induced by irrigation of poorly drained river valleys in arid regions, and it explains why these processes were practically uncontrollable under the circumstances of ancient southern Mesopotamia.

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  Beresford, et al. 2001 is a paper that deals with the secondary salinization and the impact of stream salinization in relation to landscapes in Western Australia and the role of communities and politics.

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  Ghassemi, et al. 1995 is a book that reviews the global and regional extent of salinization of land and water resources. The authors describe salinity problems in thirteen selected countries, including Argentina, Australia, China, Egypt, India, Iran, Pakistan, South Africa, Thailand, and the United States, as well as processes of salinization in land and water resources and management strategies.

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  Hillel 2005 is a paper that discusses historical and contemporary perspectives on soil salinity. This paper is useful to understand the general nature of salinization through generations.

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  Jacobsen and Adams 1958 is a short paper which gives an account of salinization problems in an ancient civilization. Shows that the problem of salinity is not new, and how salinization can destroy an entire civilization.

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  Rengasamy 2006 is a paper that reviews the different types of salinity found globally, and specifically distinguishes the transient salinity not associated with groundwater. This paper also examines the subsoil constraints associated with salinization of Australian soils and the soil processes that dictate the edaphic environment important for root functions.

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  Salama, et al. 1999 is a paper that explains salinization by two main chemical models developed by the authors: weathering and deposition. Provides details on case studies at a small catchment scale in South and Western Australia. The authors compare the situations in Australia, the United States, and Sudan.

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  Schofield, et al. 2001 is a paper that presents the results from the study of three contrasting salt-affected landscapes. Irrigation-oriented salinity in Spain, the natural occurrence of salt-affected soil in Hungary, and the catenary relationship to salinity in Tunisia are discussed to demonstrate various causal processes of soil salinization.

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  Vengosh 2003 is an essay which recognizes that salinization is a global environmental phenomenon affecting many aspects of human life. The author explains in detail river salinization, lake salinization, groundwater salinization, salinization of dryland environment, and anthropogenic salinization. He also elucidates the sources of salinity.