Reclamation of Saline and Alkali Soils: 3 Methods

This article throws light upon the top three methods used for reclamation of saline and alkali soils. The methods are: 1. Eradication 2. Conversion 3. Other Methods (Salt Precipitation Theory).

Method # 1. Eradication:

The most common methods generally used to saline soils are:

(i) Under drainage and

(ii) Leaching or flushing.

A combination of the two, flooding after tile drains is the most effective. Leaching method is very much effective in ameliorating saline soil whose soluble salts are largely neutral and high in Ca²⁺ and Mg²⁺ and very little exchangeable Na⁺. Leaching saline-alkali or sodic soils with water high in soluble salt but low in exchangeable Na⁺ may be effective.

Leaching Requirement (LR):

The leaching requirement may be defined as the fraction of the irrigation water that must be leached through the root zone to control soil salinity at any specified level. The leaching requirement (LR) is simply the ratio of the equivalent depth of the drainage water to the depth of irrigation water and may be expressed as a fraction or as per cent.

Under the assumed conditions (uniform aerial application of irrigation water, no rainfall, no removal of salt in the harvested crop and no precipitation of soluble constituents in the soil), this ratio is equal to the inverse ratio of the corresponding electrical conductivities as follows:

LR = D_dw × 100/D_iw = EC_iw × 100/EC_dw

where LR = Leaching requirement expressed in percentage

D_dw = Depth of drainage water in inches

D_iW = Depth of irrigation water in inches

EC_iw = Electrical conductivity of the irrigation water in dSm^-1

EC_dw = Electrical conductivity of the drainage water in dSm^-1

In applying this equation, a value is usually assumed for EC_dw (8 dSm^-1 for most of the field crops) to represent the maximum soil salinity that can be tolerated. For irrigation waters with conductivities of 1, 2 and 3 dSm^-1 respectively, the leaching requirement will be 13, 25 and 38 per cent (taking the value of EC_dWas 8 dSm^-1). Leaching requirement as related to crop salt tolerance is shown in Fig 15.2.

Method # 2. Conversion:

In this conversion method different chemical amendments are used for changing part of the caustic alkali carbonates into sulphates and ultimately lost by leaching. AH chemical amendments are not suitable for all soil conditions.

The amendments suitable for different soil conditions are indicated below:

Chemical reactions involving reclamation of salt affected soils:

(i) Gypsum (CaSO₄.2H₂O):

When gypsum is applied to ameliorate salt affected soils, the following reaction will take place and loss of exchangeable sodium (Na⁺) occurs and calcium will take the place of sodium on the exchange complex. Gypsum reacts with both Na₂CO₃ and the adsorbed sodium as follows:

(ii) Sulphur:

When sulphur is applied to salt affected soils (alkali and saline alkali) the following reaction takes place.

On calcareous alkali soil

The production of H₂SO₄ is common for all soils as mentioned above.

(iii) Iron Pyrite (FeS₂):

When iron pyrite is applied to sodic soils, the following reaction takes place:

(iv) Iron Sulphate (FeSO₄):

When iron sulphate is applied to the soil, the following chemical reaction will occur.

(v) Lime Sulphur (CaS₅):

When lime sulphur is applied to soils, the following reaction takes place:

CaS₅ + 😯₂ + 4H₂ODCaSO₄ + H₂SO₄.

If the soil is calcareous then the production of gypsum will take place from H₂SO₄ as follow:

If the soil is non-calcareous then the following reaction will take place.

Control:

The retardation of evaporation is an important feature of the control of salty soils. This will not only save moisture but will also retard the upward translocation of soluble salts into the root zone. Salt free irrigation water should be used. The use of salt resistant crops is an another important feature of the successful management of saline and alkali soils.

Highly salt resistant crops — Barley, sugar beet, cotton etc.

Moderately salt resistant crops — Wheat, rice, maize etc.

Low salt resistant crops — Beans, raddish etc.

Method # 3. Other Methods (Salt Precipitation Theory):

Besides these, recently salt precipitation theory is employed satisfactorily for the reclamation of sodic soils. The elimination of salts and exchangeable sodium from soils by leaching is presently practising, but the leached salts have been washed into groundwater’s or streams, making those waters more salty and again that too much salty water is used for irrigation purpose.

Due to such use the soils are further subjected to salt problems. With this view, a new concept in managing salty soils has been developed and that is known as precipitation of salts.

This idea suggests that instead of leaching salts completely away, they can be leached to only 0.9-1.8 m deep (3-6 ft.) where much of the salt would form slightly soluble gypsum (CaSO₄.2H₂O) or carbonates (CaCO₃, MgCO₃) during dry periods and not react any longer as soluble salts.

The amount of salt precipitating out will vary with the cation and anion composition of those salts. The ions precipitating will be mostly those of calcium, magnesium, carbonate, bicarbonate and sulphate. Estimates are that 30 per cent of the total salts may eventually precipitate.

The rest two-third of salts cause very little effect on the yields of corn and tomatoes. The management technique is simply to apply less water, but to do it more carefully to ensure uniform depth of wetting.