Salinity and sodicity
Saline soils are those in which there is an accumulation of salts more soluble than gypsum, sufficient to interfere with the growth of most crops and other non-specialized plants. They correspond to the SOLONCHAK of the FAO classification of soils.
Index Salinity and sodicity Generation cycles of saline soils Continental cycles Marine cycles Deltaic cycles of salt accumulation Artesian cycles Anthropogenic cycles General damage caused by saline soils Particular damage and toxicity in saline soils Damage to the soil due to sodium Damage to the soil due to chlorides Damage to the soil by sulphates Soil damage by carbonates and bicarbonates Soil damage by fluorides Soil damage by boron Solutions to saline soils Crops sensitive to salinity Crops moderately sensitive to salinity Crops moderately tolerant to salinity Crops tolerant to salinity
Sodic soils are those that contain enough exchangeable sodium to negatively affect both plant production and soil structure. They are SOLONETZ soils of the FAO classification.
Saline-sodic soils have more soluble salts than gypsum and, in addition, a high percentage of exchangeable sodium.
To evaluate the response of both plants and the behavior of a soil in relation to salinity and sodicity, the use of two basic parameters has become widespread: the electrical conductivity of a saturated paste extract that measures the salinity of a sample. of soil in conditions of water saturation; and the Sodium Absorption Ratio (RAS or SAR in English) which measures the degree of modification that irrigation water can cause. The RAS is measured in (meq/l) and its formula is:
The RAS denotes the relative proportion in which the proportion of the sodium cation is found with respect to the Calcium and Magnesium ions (the divalent cations Ca+2 and Mg+2) that compete with sodium for exchange places in the soil.
If the Na+ ion predominates in water, it will induce changes in the positions of Ca+2 and Mg+2 due to sodium. This increase in sodium in the soil can lead to soil degradation, loss of its structure and permeability.
When the SAR concentration exceeds 10 meq/l, the water is considered alkalizing with consequences for the soil.
To explain the formation of saline soils, salinization cycles have been established.
Salinization cycles can be of various continental, marine, delicia, artesian and anthropogenic types.
Saline soil generation cycles
The formation of saline soils in inland lands is due to cycles of mobilization, redistribution and accumulation of chlorides, sulfates, carbonates and sodium bicarbonate. They are phenomena that are conditioned by the climate, the soil moisture regime, geomorphological position and the type of drainage that exists in the area.
The soils of the plains along the coasts, bays and marshes may show accumulation of sea salts, mainly sodium chloride. The salts in these saline soils come from shallow saline water tables, from tidal flooding water or from contributions of salts transported by the wind.
Deltaic cycles of salt accumulation
In river deltas, generally fertile and with great agricultural activity, it is necessary to take into account the interactions between sea water, fresh water from the river and water from the water table that can generate saline soils. The balance of this system is very fragile and easy to alter by both natural and anthropic phenomena (dams, canals, transfers and irrigation).
Artesian cycles are due to the emergence of deep phreatic waters that rise through microfaults and fractures and that can pass through materials that lead to salinization.
The processes described above can be modified by human actions. The main actions are: transformation from dry land to irrigated land, which is forced to use poor quality irrigation water and industrial and mining activities.
For example, acid rains are salinizing and increase the rate of weathering and decomposition of soil rocks, and mining (especially road aggregate mining) can leave saline or sodic materials in the soil.
General damage caused by saline soils
The high concentration of salts or sodium increases the osmotic pressure of the soil solution, making plants have greater difficulty absorbing water.
Saline crusts form on the surface of the soil, which hinders the exchange of energy, water, gases and fertilizers.
In the case of sprinkler irrigation, the accumulation of salts and their crystallization on the leaves causes burns on them.