Soil Salinization

Salinisation is the process that leads to an excessive increase of water-soluble salts in the soil. The accumulated salts include sodium, potassium, magnesium and calcium, chloride, sulphate, carbonate and bicarbonate (mainly sodium chloride and sodium sulphate). A distinction can be made between primary and secondary salinisation processes. Primary salinisation involves salt accumulation through natural processes due to a high salt content of the parent material or in groundwater. Secondary salinisation is caused by human interventions such as inappropriate irrigation practices, e.g. with salt-rich irrigation water and/or insufficient drainage. 

Sodification is the process by which the exchangeable sodium (Na) content of the soil is increased. Na+ accumulates in the solid and/or liquid phases of the soil as crystallised NaHCO3 or Na2CO3 salts (salt “effloresces”), as ions in the highly alkaline soil solution (alkalisation), or as exchangeable ions in the soil absorption complex (ESP). 

In coastal areas, salinisation can be associated with the over exploitation of groundwater caused by the demands of growing urbanisation, industry and agriculture. Over extraction of groundwater can lower the normal water table and lead to the intrusion of marine water. Natural disasters in coastal areas, such as tsunamis, can cause severe salinisation problems with several years of low fertility of the affected soil before recovery. In Nordic countries, the de-icing of roads with salts can lead to localised salinisation.

Salinity is one of the most widespread soil degradation processes on the Earth. According to some estimates, the total area of salt affected soil is about one billion hectares. They occur mainly in the arid–semiarid regions of Asia, Australia and South America. In Europe, salt affected soil occurs in the Caspian Basin, the Ukraine, the Carpathian Basin and the on the Iberian Peninsula. Soil salinity affects an estimated 1 million hectares in the European Union, mainly in the Mediterranean countries, and is a major cause of desertification. In Spain 3% of the 3.5 million hectares of irrigated land is severely affected, reducing markedly its agricultural potential while another 15 % is under serious risk. Salt affected soil can be divided into five main groups:

• Saline soil (Solonchak) with high amount of water soluble soils.
• Alkaline soil (Solonetz), high alkalinity and high exchangeable sodium percentage (ESP).
• Magnesium soil: high magnesium content in the soil solution.
• Gypsiferous soil: strong gypsum or calcium sulphate (CaSO4) accumulation.
• Acid sulphate soil: highly acidic iron or aluminium sulphate accumulation.

• source of salt (local weathering, surface and subsurface waters, human activities);
• transporting agents accumulating salts from large areas to smaller deposits as well as from thick geological strata to thinner horizons (usually water, wind);
• limited vertical or horizontal drainage conditions;
• driving force for movement of solution, usually relief (surface runoff), hydraulic gradient (groundwater flow), suction (capillary transport) or concentration gradient (diffusion);
• negative water balance (evapotranspiration greater than precipitation).

Two main types of salt accumulation in soil can be distinguished in Europe:

• Continental salt accumulation due to intense weathering and arid climate or due to hydro-geological conditions (e.g. closed evaporative basins).
• Human induced salt accumulation due to improper land use (e.g. irrigation, fertilizer application).

The Carpathian Basin in Hungary is a good example of the first case. Surface runoff, seepage and groundwater transport soluble weathering products from a large water catchment area to the lowest part of the basin where subsurface waters, enriched with sodium, calcium and magnesium carbonate (salts), accumulate in a thick continuous aquifer. In poorly drained, low lying areas, capillary flow transports high amounts of water soluble salts from the shallow, stagnant groundwater to the overlying soil horizons. Due to the chemistry of the soil solution (strongly alkaline), the sodium is the dominant element in the migrating waters. High sodium saturation of heavy-textured soil with large amount of expanding clay minerals results in unfavourable soil properties and limits their fertility, productivity and agricultural utility.

Salinization processes are near to irreversible in the case of heavy-textured soils with high levels of swelling clay. Although a combination of efficient drainage and flushing of the soil by water is often used, the leaching of salts from the profile is rarely effective. 

Because the reclamation, improvement and management of salt affected soils necessitates complex and expensive technologies, all efforts must be taken for the efficient prevention of these harmful processes. Permanent care and proper control actions are required. Adequate soil and water conservation practices, based on a comprehensive soil or land degradation assessment, can provide an “early warning system” that provides possibilities for efficient salinity (or alkalinity) control, the prevention of these environmental stresses and their undesirable ecological, economical and social consequences.

The 9 magnitude earthquake that occurred at 00.58 UTC on 26th December 2004 at the interface between the India and Burma plates off the west coast of Northern Sumatra, Indonesia, triggered massive tsunamis that affected several countries throughout south and south east Asia (India, Bangladesh, Mynamar, Sri Lanka, Indonesia, Maldives and Thailand) as well as in East Africa (Somalia, Kenya, and Tanzania). The total inundated zone is estimated at ca. 60 000 sq. km. Soils of these areas have been affected by erosion and scouring that modifies the topography, land leveling and the elimination of bunds (for paddy fields), soil fertility losses when upper layer is washed away, deposition of salted sediment, salt infiltration and trash and debris accumulation. Recovery of the affected areas will require several years in some areas lacking sufficient rainfall for rapid outwash of accumulated salts and will be an additional burden to the local population.