sodic

Sodic soils are soils whose exchange complex is dominated by sodium ions, giving them a high exchangeable sodium percentage (ESP). This is commonly defined as ESP exceeding 15%, with higher values indicating stronger sodicity. Unlike saline soils, sodic soils typically have low soil solution salinity (low EC) but elevated pH, often above 8.5.

Causes and occurrence: Sodicity develops when sodium accumulates on clay minerals due to insufficient leaching, particularly in arid and semi-arid regions or in irrigated lands with poor drainage. Problems arise when irrigation water supplies sodium and drainage is inadequate, allowing Na to replace other cations on the exchange complex.

Soil properties and impacts: High Na on exchange sites reduces clay flocculation, causing dispersion of soil particles, crust formation, reduced infiltration, poor aeration, and strong soil crusting. Water movement is hindered, leading to perched water and degraded root growth.

Management and reclamation: Reclamation typically uses calcium sources, notably gypsum, to exchange Ca2+ for Na+ on the soil matrix, promoting flocculation and improved structure. Leaching with sufficient drainage and relatively low-SAR irrigation water helps remove sodium from the root zone. Other measures include improving organic matter, deep drainage, and crop selection.

Relation to salinity and alkalinity: Sodic soils may be low in soluble salts yet highly alkaline; the term alkali soil is sometimes used historically. Some soils become saline-sodic with both high soluble salts and high ESP.