Sodic soils are having high proportion of sodium at exchange complex. The sodium ion at exchange complex usually exceeds 15 percentage in this soils. These soils also have high proportion of carbonates and bicarbonates and hence the pH always more than 8.5. precipitated CaCO3 present in this soil insoluble in nature. One contrary degraded sodic soils have low pH at surface but exchangeable sodium percentage is more than 15. these soils do not have precipitated CaCO3. Sodic soils are formed due to the soil forming process of alkalization (accumulation sodium in soils) which solodi solids / degraded alkali (sodic) formed by the process called solidization.
Effect of soil sodicity
Since these soils have high amount of CO3 and HCO3 and high pH. Carbonate, bicarbonate and OH (hydroxy) ion injuries on plants are observed. High sodium in clay becomes reason for dispersed nature of clay under wet moisture regions. Dispersed nature of clay leads to soapy feeling of soils, stagnation of water, poor infiltration/ percolation and poor aeration. During dry periods these soils become hard mass. These soils have poor workability both under wet and dry seasons, further hazardous effects of Na on plants are also seem. Sodium carbonate with water releases Na+, HCO-3 and OH- ions which are harmful to growing plants in these soils.
2 Na+ + CO32+ + H2O → 2 Na+ + HCO3- + OH-
Higher pH also unfavorable for the growth of microorganisms. Low microbial activity causes slow decomposition of organic matter and hence nutrient availability is also affected specifically nitrogen, sulphur etc. Since these soils enriched with high Na at exchange complex Ca and Mg availability are also less. High pH becomes the reason for non availability of Fe and Zn, particularly Zn availability is less and deficiency of Zn is N common in this soils. Phosphorus availability is also less due to conversion phosphors into insoluble calcium and Magnesium phosphates. All the above effects on plants result in drying of plants in patches in a field. Under extreme conditions no plants are sum in these soils.
Reclamation
Physical
This is not actually removes sodium from exchange complex but improve physical condition of soil through improvement in infiltration and aeration.
a. Deep ploughing is adopted to break the hard pan developed at subsurface due to sodium and improving free-movement water. This also help in improvement of aeration.
b. Providing drainage is also practiced to improve aeration and to remove further accumulation of salts at not gone.
c. Sand filling which reduces heaviness of the soil and increases capillary movements of water.
d. Profile inversion – Inverting the soil benefits in improvement of physical condition of soil as that of deep ploughing.
Biological
Biological reclamation aims at improvement in physical condition of soils through addition of organic matter and to some extent solubilized native Ca for reclamation. Biological reclamation is also not a complete reclamation which not reduces sodium from soil below 15 percentages.
a. Addition of organic matter (Farm yard manure, press mud, Green manures, Green leaf manures) improves general physical condition of these soils. Further decomposition of organic matter releases organic acids and inorganic acids which are contracts the ill effects of high pH of soils.
b. Growing grasses like Cynodon dactylon, Brachiaria mulica, Chlon’s gayana, pai korai etc which are become sodium from the soils. But the removal is slow process.
c. Afforestation i.e growing trees which are also behaves as grasses and add organic matter through leaf fall.
E.g. AZadirachta indica, prosopis juliglora, Tamarix, rticulata, Albizia procera, Zizyphus sp., Acaciasp., Cassia sp. etc.
Chemical Reclamation
This aims at removal of sodium from exchange complex by introducing calcium.
Materials
Gypsum, Calcium Chloride, Calcium Carbonates, phosphor gypsum etc are used for reclamation which are directly supply calcium as they have calcium in their composition. Among them gypsum is most commonly used. CaCO3 is insoluble in nature which of no use in sodic soils (have already precipitated CaCO3) but can be used in degraded sodic soils (do not have precipitated CaCO3) since pH of this soils are low and favoring solubilization of CaCO3.
Some of indirect suppliers of Ca Viz. Elemental sulphur, sulphuric acid, Iron sulphate are also used for sodic soils. These materials on application solubilized precipitated
CaCO3 in sodic soils and releases Ca for reclamation (for exchange reaction). Since degraded sodic soils do not have precipitated CaCO3, use of this materials is not beneficiate. Lime sulphur supply Ca both directly as well as indirectly.
Reactions: (Direct sources)
Na-Micelle-Na + CaSO4 → Micelle-Ca + Na2SO4 (leachable)
Na-Micelle-Na + CaCl2 → Micelle-Ca + 2NaCl (leachable)
Indirect sources
Sulphur and iron sulphate are converted into sulphuric acid which dissolvers native CaCO3. Thus Ca is made available for exchange reactions.
Management of Sodic soils
Reclamation of sodic soils requires good quality water to leach the sodium salts that are released during reclamation process. In the absence of good quality water these soils can be managed by following physical and biological methods as explained above which are nto true reclamation though they are called as reclamation. In addition to physical and biological methods, choosing crops suitable to sodic soil also benefits.
Crops tolerant to high sodicity are paddy (Py 1, CO 43,Paiyur 1, TRY 1 and TRY 2 etc) and ragi ( CO 11, CO 12 and CO 13). Crop tolerant to medium sodicity are sorghum (CO 24 and CO 25) sugarcane (COC 771), Cumbu CO 6, X 5), Cotton MCU 7, MCU 10), Wheat, sunflower, subabol, Lucerne, varagu, guava, etc. Crops sensitive to sodicity are beans, green gram, black gram etc.