Sodic 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 sodium content dominates the problems related to agricultural land use.  They are characterized by a natric horizon associated with humus-rich surface horizons and saline subsoils.  A bleached layer (an albic horizon) may be present between the surface and the natric horizon.  In the World Reference Base (WRB) soil classification system sodic soils mainly occur in the Reference Soil Group of Solonetz.  However, Solonetz may be associated with Histosols, Gleysols, Chernozems, Kastonozems, Vertisols and Solonchaks.

Further description

Dry conditions and inherent salinity of parent material and groundwater are conducive to the formation of sodic soils.  Na2CO3 may be formed in these soils by evaporation of water containing sodium bicarbonate or by biological reduction of sodium sulphate.  Clay is dispersed and worked into the subsoil, forming a dense accumulation horizon, the natric horizon.  Clay in the topsoil is also decomposed in the high pH conditions resulting from the presence of Na2CO3 and of periodically low salt concentration combined with high exchangeable sodium, leading to the formation of the columnar or prismatic structures typical for the subsoil of sodic soils.  Soil reaction is strongly alkaline with a pH of more than 8.5.

General environment

Sodic soils are formed in environments with a pronounced dry season and where sodium is present in excess over calcium, due to saline groundwater or sodium-containing minerals in the parent material.  They carry a specific grass and herb vegetation composed of halophytes and grasses.  Sodic soils occur in environments similar to those of saline soils but are also important in colder climates with a pronounced dry season, such as in Canada and the former USSR.

Global extent

The total area is estimated at about 135 million hectares (see table), but as the distribution of sodic soils is very scattered worldwide (see figure) the individual areas are often too small for inclusion in overview soil maps at a world scale.  Occurrence of sodic soils is important in Ukraine, Russia, Kazakhstan, Hungary, Bulgaria, Romania, China, USA, Canada, South Africa and Australia.

Land use

Most sodic soils are left idle or they are used for extensive grazing only.  Some salt-resistant crops as mustard and sorghum may be cultivated on sodic soils with a humus-rich surface horizon without amelioration.  After reclamation, sodic soils can be productive for arable crop production especially in cooler climates, for instant for wheat production.

Main production constraints

Plant growth is directly affected, as sodium in high concentrations is toxic for most crops, while the dense subsoil and unfavourable physical properties of these soils hinder downward water percolation and the growth of roots.
Sodic soils also dispose of poor aggregate stability, impermeability under wet conditions and hardness of the natric horizon when dry.
Reclamation of sodic soils depends much on the thickness of the humus-rich surface layers and the presence of carbonates close to the surface.  Deep ploughing and mixing the carbonate or gypsum containing horizon with the A horizon can be used to improve sodic soils. Furthermore, sodic soils can be reclaimed (at high cost) by deep ripping and leaching with calcium-rich water.  But ameliorants such as gypsum or pyrite applications are found to be the most effective on sodic soils under irrigation.  However, sodic soils are very difficult to cultivate due to the dispersed state of clay and organic matter.

Summary

Sodic soils are soils of arid and semi-arid areas.  They have a natric subsurface horizon, characterized by a sharp increase in clay, columnar or prismatic structure and high alkalinity.  Sodic soils are of very limited value for agricultural production because of their alkalinity and high exchangeable sodium.  Sodic soils can be improved by deep ploughing and the addition of gypsum.  Then, in cooler climates, they can support sustainable agricultural use, for instant for wheat production.