Sodic Soil: Effects, Properties, and Treatment

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Sodic soils are formed when sodium ions are relatively higher than the other cations, impacting the soil structure.

The amount of sodium in the soil is the key measure of sodicity and is referred to as the Exchangeable Sodium Percentage (ESP). Sodicity in soil degrades soil properties by weakening the bond between soil particles. An ESP of 6% is considered the threshold, where the cation sodium in soil has an adverse impact on soil structure when in contact with fresh water, causing clay dispersion.

Effects of Sodicity on Soil Structure

1. Sodicity leads to reduced water flow through soil, limiting leaching, causing salt to accumulate over time, and developing saline sub-soils.
2. Wetting of Sodic soil leads to dispersion in the soil surface, causing crusting, waterlogging, low hydraulic conductivity rates, excessive runoff, and erosion.
3. Due to dispersion in the subsoil, the process of erosion accelerates, which can cause the appearance of gullies and tunnels.
4. In irrigation, the sodic soil prevents water storage due to swelling and dispersion, which block the pores and reduce the internal drainage of the soil. This effect has an adverse effect on soil tilling, poor seed germination, limited root growth, and vulnerability to wind and water erosion.

Properties of Sodic Soil

The general properties of sodic soil are listed below:

1.  Sodic soils have a pH value greater than 8.5.
2. The Sodium Absorption Rate (SAR) of sodic soil is higher than 13.
3. It has an Exchangeable Sodium Percentage (ESR) of more than 15. 
4. Electrical conductivity (EC) of sodic soil is lesser than 4.0 dS m^-1.
5. Sodic soil shows poor water infiltration properties.

Treatment for Sodic Soil 

The reclamation and improvement of sodic soil require the removal or replacement of exchangeable sodium. This can be carried out in many ways, including physical, hydro-technical, chemical, biological, and phytoremediation techniques.

1. Physical Method

1. Deep Tillage

The physical method of improving sodic soil is by the process of deep tillage. It helps to recover sodic soil by effectively breaking down the soil plows, increasing total porosity (especially large porosity), reducing soil bulk density, and promoting root extension into the deep soil. It also significantly improves microbial diversity, rhizosphere microbes, and soil water storage capacity. 

The cement-like hard soil layer is destroyed by deep plowing (1-2 m) or ripping, which typically results in a permanent improvement of the soil structure and physical qualities. However, due to deep tillage, sodic subsoil is brought to the surface, leading to logging and infiltration problems. Therefore, it has been reported that deep tilling and rice-wheat crop rotations are most effective in the recovery of sodic soils. Deep tilling can increase grain yield and planting density by 1.08-5.21%.

2. Diluting with high-salt water

In locations where water is not a limiting issue, sodic soil regeneration is possible. It can be done by continually diluting the soil with divalent cation-rich high-salt water. The use of high EC in water limits soil dispersion and causes soil colloids to flocculate. At the same time, by displacing the exchangeable sodium, the Ca ions in the water diminish sodicity. The water depth should be at least 9-10 times the depth of the recovered soil to guarantee good soil recovery.

Chemical Method

1. The sodic soil with a pH below 7 is suitable for ground limestone and agricultural lime (CaCO3) amendment. When the pH is higher than 7.0, the effectiveness of limestone as a corrector is significantly reduced as its solubility decreases with increasing soil pH.
   Some sodic soils which contain an excess of exchangeable sodium also contain significant amounts of exchangeable hydrogen. In the presence of exchangeable hydrogen, an acidic reaction occurs which reduces the soil's pH.
2. Most of the sodic soil has high pH as it contains a measurable amount of free sodium carbonate, which gives these soils a high pH, mostly greater than 8.2. In this case, limestone does not work as an effective amendment. Therefore, in these soils, only soluble calcium salts like gypsum and calcium chloride substances are beneficial. 
3. Sulfuric acid is an oily, caustic liquid and usually has a purity of about 95%. When applied to a sodic soil containing calcium carbonate, it immediately reacts to form calcium sulfate, thereby providing indirectly soluble calcium. 
4. Iron sulfate and aluminum sulfate (alum) are usually highly pure and soluble in water. When applied to the soil, it dissolves in the presence of water, and a hydrolysis reaction occurs. As a result, sulfuric acid is formed, and it reacts with the calcium carbonate present in the sodic soil and provides soluble calcium. 

Biological Method

1. The use of organic matter, compost, and plant roots help dissolve insoluble calcium compounds present in sodic soil. Although this technique has been extensively tried, it is commonly accepted that choosing a suitable recovery strategy relies on soil geography and physicochemical properties. 
2. It has been reported that sodic soil improvement by agroforestry systems can also improve the biological production of sodic soil.
3. Plant-microbe interaction is a beneficial link between plants and microorganisms, and it is an efficient method for the reclamation of soil. Rhizosphere bacteria are found to be responsible for positively increasing the absorption of nutrients by plants or producing those compounds, which help to promote plant growth and thus regenerate sodic soil quality.
4. However, biological methods are a little slower than chemical methods to make positive changes and depend on the presence of calcite in the soil. In addition, its range is limited in highly sodic soils because the growth of bioremediation crops may vary, and the use of chemical modifiers such as gypsum is unavoidable.
5. Geotextile fabrics and mattings that provide sodic soil protection, shrouding, and assist with plant establishment.

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