Distribution, characteristics and management of calcareous soils

Calcareous soils are widely spread in arid and semi-arid regions and cover almost one-third of the world's land surface area. This chapter gives an overview of the distribution and characteristics of calcareous soils, the limitations of calcareous soils for crop production, and some of the sustainable management practices to overcome crop productivity constraints in calcareous soils. Calcareous soils are characterized by the presence of calcium carbonate (CaCO₃) in the soil parent material and the consequent accumulation of free CaCO₃ in the soil profile. The pH of these soils is usually above 7, and, in the presence of free sodium carbonate, it may exceed 9. In some soils, the free CaCO₃ can concentrate into hard layers, known as ‘caliche’, that are impermeable to water infiltration and plant root penetration. Although calcareous soils may be dominated by free CaCO₃, they may also contain significant amounts of iron (Fe), aluminium (Al), and manganese (Mn) either as discrete and mixed minerals, coatings on soil inorganic size fractions such as clay, sand, and silt, or complexed with soil organic matter. The major crop productivity constraints in calcareous soils include phosphorus and trace element (e.g., iron, zinc, and copper) deficiency, surface crust formation, and an impermeable subsurface compact layer. Soil productivity constraints in calcareous soils can be overcome by proper choice and placement of fertilizers, addition of organic matter, which promotes stable aggregate formation that inhibits dispersion and resists crust formation, and deep ripping to break through the compacted pan layer, thereby allowing root access to soil water and nutrients beneath this layer.