Soluble Salts: Nature and Sources | Soil

After reading this article you will learn about the nature and sources of soluble salts found in soil.

Nature of Soluble Salts:

Soluble salts, as referred to in soil science, are those inorganic chemicals that are more soluble than gypsum (CaSO₄.2H₂O), which has a solubility of 0.241 gm. per 100 ml of water at 0°C. Common table salt (NaCl) has solubility nearly 150 times greater than gypsum (35.7 gm. per 100 ml).

Most soluble salts in saline soils are composed of the cations sodium (Na⁺), Calcium (Ca²⁺) and magnesium (Mg²⁺) and the anions chloride (CI^–), sulphate (SO₄^2-) and bicarbonate (HCO3). Usually smaller quantities of potassium (K⁺), ammonium (NH₄⁺), nitrate (NO₃^–), and carbonate (CO₃^2-) also occur, as do many other ions.

Among all these salts, the dominant anions are chloride (CI^–) and sulphate (SO₄^2-) and cations are Na⁺, Ca²⁺ and Mg²⁺.

Boron may also be present in saline soils which in small concentration is also toxic to plant.

Sources of Soluble Salts:

There are various sources from which soluble salts are accumulated in the soil.

(1) Primary Minerals:

It is the original and important source of all the salt constituents. During the process of chemical weathering, which involves hydrolysis, hydration, solution, oxidation and carbonation, various constituents like Ca²⁺, Mg²⁺ and Na⁺ are gradually released and made soluble.

(ii) Arid and Semi-Arid Climate:

Salt affected soils are mostly formed in arid and semi-arid regions where low rainfall and high evaporation prevails. The low rain fall or precipitation in these regions is not sufficient to leach out the soluble weathered products and hence the salts accumulate in the soil.

During rain, the salts dissolve in rainwater and migrate downward. However, due to limited rainfall, the downward movement is restricted to a short distance only. In dry weather, the salts move up with the water and are brought up to the surface where they are deposited as the water evaporates.

(iii) Ground Water:

Ground water contains large amounts of water soluble salts which depends upon the nature and properties of the geological material with which water remains in contact where water table and evapotranspiration rate is high, salts along with water move upward through capillary activity and the salts accumulate on the soil surface in the form of crystallization.

(iv) Ocean or Sea Water:

Sea water enters into the land by inundation and deposited on the soil surface as salts.

(v) Irrigation Water:

The application of irrigation water without proper management (i.e. lack of drainage and leaching facilities) increases the water table and surface salt content in the soil.

(vi) Salts Blown by Wind:

In arid regions near the sea, appreciable amount of salts are blown by wind year after year and get deposited on the surface soil. Due to low rainfall the deposited salts are not washed back to the sea or leached to the lower soil horizon and thereby develop salinity in the soil. The salinity of Rajasthan is mostly developed through this source.

(vii) Excessive use of Basic Fertilizers:

Use of basic fertilizers like sodium nitrate (NaNO₃), basic slag etc. may develop soil alkalinity.