Sodic soils contain a large amount of exchangeable sodium and low levels of soluble salts.
Excess exchangeable sodium has an adverse effect on plant growth, soil structure and results in reduction in crop yields.
In sodic soils, clay particles tend to move far apart from each other in a process called dispersion. The forces that hold clay particles together are disrupted by the large sodium ions. The soil aggregates break down, and the dispersed clay particles move throughout the soil and clog its pores.
As a result, water infiltration through the soil, as well as its hydraulic conductivity, is reduced (the hydraulic conductivity is a measure of the soil's ability to transmit water). Therefore, soil might become waterlogged upon wetting.
Sodium affected soils tend to swell when wet and to harden and crack when dry. When dry, sodic soil typically develops a hard, dry crust on its surface.
The damage to soil structure reduces the oxygen availability in the root zone, restricting plant growth. The hard crust restricts root growth and seed emergence.
In addition, sodic soils are also subject to erosion, which results in soil and nutrient loss. The run-off, containing the nutrients and pesticides which are adsorbed on the clay particles, might reach drinking water sources and contaminate them.
Salinity and sodium have the opposite effect on soil structure. While sodium enhances soil dispersion, salinity causes soil particles to bind together (to aggregate). The aggregation of soil particles improves the physical properties of the soil. Soil becomes more permeable, aeration is better and root growth improves. Although salts prevent the destructive effects of sodium on soil structure, excessive salinity is nevertheless detrimental to plant growth.
At high soil pH (>8), carbonates become the dominant form of alkalinity. As the soil dries, calcium and magnesium precipitate out of the soil solution and adsorbed calcium and magnesium leave the clay particles to maintain equilibrium. Sodium then becomes more dominant in the solid phase of the soil. The conclusion is that the effects of sodium on soil structure are more significant at high soil pH values.
The Exchangeable Sodium Percentage is defined as the amount of sodium adsorbed to soil particles, expressed in percent of Cation Exchange Capacity (CEC):
ESP= Exchangeable Na (meq/100 g soil)/CEC(meq/100 g soil).
Technically speaking, soils that have a threshold value or exchangeable sodium percentage (ESP) of over 15 are categorized as sodic soils.
Since measuring ESP is not always feasible, many laboratories use the SAR as an index for soil salinity. SAR is calculated as follows: Na/((Ca+Mg)/2 )^2.
Options for correcting Sodic soils-related problems can range from changing the type of crops to be grown to more tolerant species, to utilizing soil amendments.
Sodic soils can be improved or reclaimed by replacing the exchangeable sodium with calcium ions. The common approach is to incorporate effective soil amendments that directly or indirectly facilitate the replacement of the soil's exchangeable sodium.