The salinization of the soils or salt-affected soils occurs in almost every place around the globe, in various topographies and various climatic conditions. The soils that are in arid and semi-arid regions are mostly affected by this catastrophic event. This process can be in a form of salinization or sodification which directly contributes to soil degradation and ecosystem disbalancing. In the modern agricultural era, on a global scale, salt-affected soils are one of the major concerns that farmers are facing. Saline or sodic soil, besides its natural occurrence, is also a product of intensive agriculture that implements conventional methods and techniques of food production. Their existence and their rapid creation are one of the major threats to food security and sustainability.
What are types of soil salinity?
Soil salinity in agriculture in a simpler and most correct definition is the high concentrations of salts in the soil profile. The process of salinization can occur either naturally or by anthropogenic activities. The high concentration of salts interrupts the plantβs metabolism making them unable to absorb water, nutrients, and microelements. The plants that are intolerant to high saline or sodic content grown on salt-affected soils are usually wilted or stunted, which will ultimately lead to their death, no matter how good the producers were taking care of them.
Salinity and salt
Salinity in agriculture represents the total amount of salts that are found in the soil or every other medium (rocks, water, etc.) and by the definition of salts in every inorganic mineral that is water-soluble. The most familiar salt is sodium chloride, but in the agricultural sphere, this is not the only salt that can occur in the soil profiles. They can be a combination of a large portion of other elements (ions) such as Na+, Ca+2, K+, Mg+2, Cl-, NO3-, SO4-2, HCO3-, and CO32-. Their origin is usually from the bedrock material that, under the influence of weathering, small amounts from the rocks are dissolved in water, carried away and deposited in the soils, underground waters, and large water bodies (seas and oceans).
Effects of salts on plants
A high concentration of salts in the soil leads to disruption of the osmotic potential in the plantβs cells, especially in the root zone. The osmotic potential is a mechanism with natural flow that allows the transfer of liquid from a medium with high salt content to mediums with lower salt content. More precisely, in the context of plants and salts, means that the plants that have lower concentrations of salts in their root system are becoming unable to retrieve water and nutrients from the soil that have a higher concentration of salts. Additionally, if the soil salt content is extremely high it can lead to the drawback of water from the roots to the soil. The affected plants are usually wilted (no matter how many times the field was irrigated) with typical abiotic stress symptoms such as chlorosis.
Characteristics of saline soil
Based on the type of soil salinity different soils can be formed, i.e., saline, alkaline, and saline-alkali soils. Saline soil is the soil where there is an excess of sodium salts made from chloride, sulfate, bicarbonates and sodium nitrates and the soilβs aggregates contain exchangeable calcium. Alkaline soils usually in their content donβt have an increased level of salts but high quantities of sodium cations (Na+). Saline-alkali soils are a combination of the two types of soil mentioned above where they contain excess salts and clay with high natrium ion content.
What causes soil salinization
Salinization occurs under certain conditions: field topography prone to salinization process under the influence of capillary movement and evaporation of shallow and saline groundwater, in areas with arid and semi-arid continental-sub-Mediterranean climate with strong evapotranspiration. The main sources of salts are saline Paleogene sediments and the chemical decomposition of certain substances. Human influence on the genesis and properties of these soils can be indirect and direct. The indirect impact from human influences are the changes in the pedogenetic factors (hydrographic conditions, vegetation, etc.). The direct impact that intensifies the salinization process is when plants are irrigated with saline water or as a result of the application of mineral fertilizers in soils that have low permeability.
Here are several main situations that lead to salinization of the soils: geological events (excessive losses of organic matter caused by climate change, long-distance winds that carry salts, earthquakes that disrupt the soil profile), natural factors (climate, type of bedrock, land cover and topographic characteristic of the fields) and anthropogenic activities (excessive application of mineral fertilizers, intensive plowing, irrigation with salt-containing water, intensive production) that can directly or indirectly influence in the concentration of salts in the soil.
Indicators that appear during soil salinization:
- Areas in the field where the soil is excessively moist and has water retention
- Bleaching and appearance of crystals on the soil surface
- The increased water level in the rows
- Areas without vegetation (due to the high salt content in that place)
- Light or dark circles on the soil around the places where there is water retention
- Plant wilting
- Plant extinction
- Reduced biodiversity
- Occurrence of weeds that are tolerant of high salt concentrations
How to fix soil salinity
The producers, before making any kind of intervention in order to alleviate this problem of soil salinity in agriculture, need to have specific information regarding the situation on the fields. One of this specific information is the site-specific electrical conductivity map and historical satellite imagery. The specific information can give the producers enough insights about the origin of the problem and where the most degraded parts of the fields are.
Agrotechnical measures that can help fix the soils and prevent salinization:
Improve field drainage
This can be achieved by deep plowing, i.e., plowing at a depth of 60 to 80 centimeters. Deep plowing helps break down the compact soil layer that can be created by the weight of agricultural machinery or by the natural formation of the claypan layer. By breaking the compact layer, water can freely pass into the lower layers and thus wash away the salts that have accumulated over time.
Field rinsing
This operation is performed with the help of large amounts of water that has low levels of salts. Such water allows the salts that are present in the soil to dissolve and later transport them to the lower layers.
Reduction of evaporation
Evaporation only decreases if we have a permanent layer of vegetation in the field or the field has been mulched (whether it is made of organic or artificial material). Reduced evaporation means that the salts that are present are still present but in an aqueous solution and their rinsing in the lower layers is easier.
Application of chemical treatments
Chemical treatments are performed before starting the artificial rinsing described above. Gypsum is usually added so that sodium ions can bind to calcium in gypsum. This reaction helps to eliminate harmful ions that prevent the proper absorption of nutrients by plants. By rinsing the soil, the resulting water-soluble salt can be easily removed from the field.
Sowing of tolerant plant species
In places where we have a high concentration of salts and it is impossible to grow conventional plant species, it is necessary to plant saline soil crops that are salt tolerant. Some of the most tolerant crops are barley, camellia, rye, saffron, sunflower and sugar beet.
Planting of tolerant cover crops
If there are no winter forms of cereals sown, the fields should have a cover crop that will protect the field from wind erosion and evapotranspiration in the autumn-winter period. One of the best methods is to apply a mix of barley, sunflower, and sugar beet after the harvest of the summer crops. This mix will germinate and will enter the winter period in the form of young plants that under the influence of the cold weather will likely die and leave a precious amount of nitrogen and other nutrients. The main reason for salt-tolerant cover crops is to minimize the evapotranspiration during the months where the soil should be bare.
How to prevent soil salinization
Soil salinization can be prevented by implementing several strategies in the production management process. The key to success is to have relevant information from the real situation in the field which can be derived from the implementation of various technologies. Such technologies include site-specific soil analysis (soil maps), electrical conductivity maps, historical satellite/UAV orthophoto imagery, weather sensors and evapotranspiration calculation models. From this data, the producers are able to successfully create detailed and ad hoc crop management plans and decisions. Based on the information, the strategies can involve modified crop rotation, change in irrigation scheduling, application of gypsum and acid, deep plowing, modification of fertilizer content and applied amounts.
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