Calcareous Soils

• Definition

• Further description

• General environment

• Global extent and location

• Land use

• Main production constraints

• Summary

Definition

In the context of agricultural problem soils, calcareous soils are soils in which a high amount of calcium carbonate dominates the problems related to agricultural land use.  They are characterized by the presence of calcium carbonate in the parent material and by a calcic horizon, a layer of secondary accumulation of carbonates (usually Ca or Mg) in excess of 15% calcium carbonate equivalent and at least 5% more carbonate than an underlying layer. In the World Reference Base (WRB) soil classification system calcareous soils may mainly occur in the Reference Soil Group of Calcisols.

Further description

The secondary calcium carbonates are formed under arid or semi-arid climatic conditions when the carbonate concentration in the soil solution remains high. Accumulation starts in the fine and medium-sized pores at the surface of contact between the soil particles.  This accumulation may be rather concentrated in a narrow zone of the solum or more dispersed, depending upon the quantity and frequency of rainfall, topography, soil texture and vegetation.  In some soils the calcium carbonate deposits are concentrated into layers that may be very hard and impermeable to water (also called "Caliche").  These caliche layers are formed by rainfall (at nearly constant annual rates) leaching the salts to a particular depth in the soil at which the content is so low that the carbonates precipitate.  They are also formed by salts moving upward from a water table (caused by irrigation) and precipitating near the top of the capillary fringe.

General environment

Calcareous soils are typical soils of semi-arid and arid climates, especially where calcareous parent material occurs in level to hilly land and with a sparse natural vegetation of xerophytic shrubs and ephemeral grasses.

Global extent and location

The total extent of Calcisols is estimated at 800 million hectares worldwide (see table), mainly concentrated in arid or mediterranean climates.  However, the total area of calcareous soils is difficult to estimate because many Calcisols occur together with Solonchaks (see salt-affected soils) that are actually salinized Calcisols and/or with other soils of carbonate enrichment that do not key out as Calcisols.  The figure gives an indication of the regions where Calcisols occur.

Land use

The land use of calcareous soils is highly variable: it ranges from non-used wastelands (deserts) to intensively cultivated irrigation areas.

Main production constraints

Calcareous soils develop in regions of low rainfall and must be irrigated to be productive.  Therefore one of the main production constraints is the availability of water for irrigation.  The quality of the irrigation water is of crucial importance for sustainable agricultural production on calcareous soils.  Frequently, the irrigation water is the cause of many management problems.  Almost all waters used for irrigation contain inorganic salts in solution.  These salts may accumulate within the soil profile to such concentrations that they modify the soil structure, decrease the soil permeability to water, and seriously injure plant growth.
Crusting of the surface may affect not only infiltration and soil aeration but also the emergence of seedlings. Cemented conditions of the subsoil layers may hamper root development and water movement characteristics.
Calcareous soils tend to be low in organic matter and available nitrogen.  The high pH level results in unavailability of phosphate (formation of unavailable calcium phosphates as apatite) and sometimes reduced micronutrient availability, e.g. zinc and iron (lime induced chlorosis). There may be also problems of potassium and magnesium nutrition as a result of the nutritional imbalance between these elements and calcium.

Summary

Calcareous soils are soils rich of calcium carbonate which occur mainly in the semi-arid subtropics of both hemispheres.  Potential productivity of calcareous soils is high where adequate water and nutrients can be supplied.  The management of water and nutrients is the main production challenge.  Optimal amounts of water for plant growth have to be provided without wastage, and salts which may affect plant growth have to be controlled.