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C-Change in GEES Human Pressures on the Environment Session 5: Salinization Session 5
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Session Outline • The Process of Salinization • Human Causes of Salinization • Management of Salinization
Soil Salinity Salt affected soils are caused by excess accumulation of salts, typically most pronounced at the soil surface. Salts are transported to the soil surface by capillary transport from a salt laden water table and accumulates due to evaporation. As soil salinity increases, salt effects can result in degradation of soils and vegetation. Salt is a natural element of soils and water. The ions responsible for salinization are: Na+, K+, Ca 2+, Mg 2+ and Cl-. As the Na+ (sodium) predominates, a salt affected soil is often also a sodic soil. Salinization is a natural process that results from: • high levels of salt in the soils. • landscape features that allow salts to become mobile (movement of water table) • climatic trends that favour accumulation
Soil Salinity What is Salinization? Salinization is the process by which watersoluble salts accumulate at the surface of soil. Salinization is a resource concern because excess salts hinder the growth of crops by limiting their ability to take up water. Salinization may occur naturally or because of conditions resulting from management practices. Atacama desert, salt flats Photo: palegoldenrod (flickr. com) Any process that affects the soil-water balance may affect the movement and accumulation of salts in the soil: hydrology, climate, irrigation, drainage, plant cover and rooting characteristics, farming practices
Soil Salinity Salinization In some areas (for example the Middle East, Australia) enormous amounts of salts are stored in the soils. Human practices have increased the salinity of top soils by bringing salt to the surface through disrupting natural water cycles (irrigation, land clearing, draining etc. ). Aswan Dam Photo: Hajor (wikimedia commons) One of the best examples of excess salinization was observed in Egypt in 1970 when the Aswan High Dam was built: • Change in the level of ground water before the construction enabled soil erosion • Led to high concentration of salts in the water table. • High level of the water table after construction led to the salinization of the arable land
The Salinization Process Can occur when the water table is between two to three metres from the surface of the soil. The salts from the groundwater are raised by capillary action to the surface of the soil. This occurs when groundwater is saline (which is true in many areas), and is favoured by land use practices, allowing more rainwater to enter the aquifer than it could accommodate. For example, the clearing of trees for agriculture is a major reason for dryland salinity in some areas, since deep rooting of trees has been replaced by shallow rooting of annual crops.
The Salinization Process
Causes of Salinization Primary Salinization: 80% of saltaffected land has a natural origin. Salts formed by weathering of rocks or natural external inputs. Secondary Salinization: 20% of salt -affected lands, and around 15 Mha on the continent of Africa, have a "man-made" origin. Human activity linked to agricultural practices and in particular to irrigation. Flood irrigation
Causes of Salinization on the soil surface occurs where the following conditions occur together: • the presence of soluble salts (sulphates of sodium, calcium, magnesium) in the soil • a high water table • a high rate of evaporation • low annual rainfall Salinization often occurs on the rims of depressions and edges of channels, at the base of hillslopes, and in flat, low-lying areas surrounding shallow bodies of water. These areas receive additional water from below the surface, which evaporates, and the salts are left behind on the soil surface. Human practices that increase the soil moisture content (e. g. irrigation) increase salinization as the water moves through hillslope soil, accumulates in seeps and then evaporates.
Causes of Salinization Irrigation Almost all water (other than natural rainfall) contains some dissolved salts. Water is continually added through irrigation and lost from the soil through evapotranspiration and the salts that are left behind accumulate. Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must be leached out of the plant root zone by applying additional water. This, in turn, can lead to rising water tables, requiring drainage to keep the saline groundwater out of the root zone.
Causes of Salinization Irrigation If the water table rises too high, then natural soil evaporation will begin to draw the salts back upward into the soil profile. The problem is accelerated when too much water is added too quickly due to inefficient water use: • applying more water than is required for leaching, • using bad estimates of evapotranspiration • poor drainage • use of saline water for irrigating agricultural crops. These practices result in the concentration of salts in the soil.
Causes of Salinization Irrigation
Impacts of Salinization Global Extent 4. 5 0. 8 3. 5 Kenya 56. 9 4. 5 8. 2 14. 4 Nigeria 91. 1 32. 9 5. 6 6. 1 88. 4 4. 0 2. 3 99. 5 3. 3 9. 1 Iran 162. 2 19. 4 27. 4 19. 9 Syria 18. 4 5. 2 0. 5 2. 7 Tunisia Near East 22. 8 Egypt Africa Cultivated Area (Mha) Tanzania ( FAO nomenclature) Country Total Area (Mha) Ghana Region 15. 5 4. 9 1. 8 11. 6 Salt-affected Area (Mha) (%) Source : FAO, 2005. Integrated management for sustainable use of salt-affected soils. By A. Mashali, D. L Suarez, H. Nabhan and R. Rabindra, FAO Soils Bulletin.
Impacts of Salinization Global Extent • 10 hectares of land suitable for cultivation is lost every minute, 3 hectares of which is as a result of salinization (equivalent to 1. 5 Mha per year). • Today almost 400 Mha are affected by salinization. • In Africa almost 40 Mha are affected by salinization, around 2% of the total surface area. • In the Near East almost 92 Mha are affected by salinization, around 5% of the total surface area. International Commission on Water Resources Systems (2006)
Impacts of Salinization Urban Areas Salinity in urban areas often results from the combination of irrigation and groundwater processes. Cities are often located on drylands, leaving the rich soils for agriculture. Irrigation is also now common in cities (gardens and recreation areas). Can cause yield damage to infrastructure (roads, bricks, corrosion of pipes and cables)
Impacts of Salinization Vegetation Salts in the soil increase the efforts by plant roots to take in water. High levels of salt in the soil have a similar effect as drought - making water less available for uptake by plant roots. Few plants grow well on saline soils; therefore, salinisation often restricts options for cropping in a given land area. Salinization degrades the quality of shallow ground water and surface water resources, such as ponds, sloughs, and dugouts.
Impacts of Salinization The Aral Sea
Managing Salinization Prevention of salinization by using humic acids - especially in regions where too much irrigation was practiced. In soils with excess salts, humic acids can fix anions and cations and eliminate them from the root regions of the plants. Saltbush: A type of plant that is able to tolerate saline conditions and draws salt up into its leaves. Saltbush plants nr Port Augusta
Managing Salinization Identifying the Problem Early signs: • Increased soil wetness in semiarid and arid areas • The growth of salt-tolerant weeds • Irregular patterns of crop growth and lack of plant vigour Advanced signs: • White crusting on the surface • A broken ring pattern of salts adjacent to a body of water • White spots and streaks in the soil • The presence of naturally growing, salt-tolerant vegetation Soil salinity can be estimated by measuring the electrical conductivity of the soil solution. Electrical conductivity increases in a solution in direct proportion to the total concentration of dissolved salts. USDA Natural Resources Conservation Service (1998)
Managing Salinization Reducing the severity and extent of soil salinity is primarily a problem of water management. Water management can be addressed in two ways: (1) by managing the area contributing excess water to the soil (recharge area) or (2) by managing the area where the excess water comes to the surface (discharge area)
Managing Salinization Recharge management: • Plant crops that use the available soil moisture. Shallow rooted crops may not extract excess subsoil moisture that can lead to salinity. • Remove excess water from recharge areas of seeps by using actively growing, deep-rooted plants. Perennial plants and forages, especially alfalfa, are useful for this purpose because they have a longer growing season and take up more water from a greater depth in the soil than annual plants. Forages may also increase organic matter in the soil and improve soil structure. • Return manure and crop residue to the soil to increase soil water retention. • Reduce summer fallow by continuous cropping. USDA Natural Resources Conservation Service (1998)
Managing Salinization Recharge management (cont. ): • Manage snow so that it is evenly distributed and does not pond on thawing. • Divert surface water to down slope ponds. • Use cropping and tillage systems that promote infiltration and permeability e. g. building organic matter for soil aggregation and avoiding compaction. USDA Natural Resources Conservation Service (1998)
Managing Salinization Discharge management: • Grow salt-tolerant crops. • Reduce deep tillage, which may bring up salts from deeper soil horizons. • Plant forage crops or trees next to bodies of water to increase water use. • Install artificial drainage systems in severely affected areas only. • Eliminate seepage from irrigation canals, dugouts, and ponds. Generally, control measures should take an integrated approach involving cropping, structural methods, and tillage systems. USDA Natural Resources Conservation Service (1998)
Summary • Salinization is a problem of global significance and one that is rapidly growing as a consequence of human activity • Better agricultural and land management practices are necessary to reduce the impact of salinization • Management efforts should focus both on recharge (the amount of water infiltrating into the soil) and discharge (the uptake/removal of water from the soil).
References A. Mashali, D. L. Suarez, H. Nabhan and R. Rabindra, Integrated Management for Sustainable Use of Salt-affected Soils, FAO Soils Bulletin, Rome (2005) USDA Natural Resources Conservation Service (1998) ‘Soil Quality Information Sheet. Soil Quality Resource Concerns: Salinization’ Available at: soils. usda. gov/sqi/publications/files/salinzation. pdf
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