Saline soils

• Definition

• Further description

• General environment

• Global extent and location

• Land use

• Main production constraints

• Summary

Definition

In the context of agricultural problem soils, saline soils are soils in which a high salt content dominates the problems related to agricultural land use.  They are characterized by an electrical conductivity (EC) of typically more than 15 dS/m. In the World Reference Base (WRB) soil classification system saline soils mainly occur in the Reference Soil Group of Solonchaks.  However, some other Reference Groups may also have a salic horizon such as Histosols, Vertisols and Fluvisols.

Further description

Apart from their high salt content saline soils show a considerable diversity in their hydrological, physical and chemical properties.  Saline soils may be calcium dominated, sodium dominated or magnesium dominated with a subsequent tendency towards structural degradation (depending on the presence or absence of calcium).  Under extreme specific climatic conditions (low rainfall, high evaporation) salts which are present in the soil solution precipitate at the surface in various forms (white efflorescence, salt crusts, non-aggregated brown powder, black salt deposits, evaporative salt crystals, etc.).

General environment

Saline soils are formed when evaporation greatly exceeds precipitation for at least part of the year and where salts are present in moderate to high amounts in the parent material of the soil or with a saline water table at shallow depth.  This combination of environmental conditions occur in inland river basins, former lake bottoms and depressed areas which collect seepage water from surrounding uplands, under a warm to hot climate with a distinct dry season (arid climatic zone and the driest part of the Mediterranean and seasonally dry subtropics).

Global extent and location

Solonchaks cover about 190 million hectares (see table), with vast areas occurring in Chad, Namibia, Australia, along the Gulf and in Paraguay and Uruguay (see figure) where they are often associated with sodic soils.  Saline soils also occur in coastal areas with saline intrusions.

Land use

The salt accumulation may limit plant growth to salt tolerant plants (halophytes) only.  High salt concentrations are toxic or limit plant growth because nutrients are proportionally less available or create physiological drought as a consequence of the high osmotic pressure of the soil solution.  Therefore saline soils are often left in their natural state and used for extensive grazing.  Agricultural use is only possible under careful irrigation management or in the more humid regions, where salt tolerant crops such as rice, millet or palms can be grown as well as fodder crops (alfalfa) and salt-tolerant trees.

Main production constraints

In the more arid regions saline soils cannot be used for normal cropping unless salts are leached out by excess irrigation water. A drainage system that keeps the groundwater table at sufficient depth is normally required.  Lack of salt-free irrigation water is a major problem in this respect.  In fact, in many situations soils may have been made saline by using poor irrigation water (induced salinisation).
In the more humid environments use of saline soils may be possible without major reclamation measures.  However, sustainable use of these soils will be a critical issue.
Avoidance of both salinisation and chemical degradation as well as regeneration of saline soils are very important especially if the expansion of irrigated land is envisaged.

Summary

Saline soils contain considerable amounts of soluble salts which often precipitate on the soil surface or on ped surfaces.  Saline soils have only very limited value for agricultural production if salts cannot be removed.  After reclamation and with careful irrigation management saline soils can be used for sustainable crop production depending on the quality of the irrigation water and functionality of the drainage system.