Saline soils represent a group of soils in which percentages of soluble salts, usually chlorides and sulphates of the alkali bases are very high.
The pH of saline soils are always high. In India saline soils occur in many provinces, as U.P., West Bengal, Punjab, Bihar, Orissa, Maharashtra Tamil Nadu, M.P., Andhra Pradesh, Gujarat, Delhi and Rajasthan covering an area of about 7 million hectares.
There are 4 major tracts in India where salinity problem is acute.
These are:
(a) The arid tract of Rajasthan and Gujarat,
(b) Semi-arid alluvial tracts of Punjab, Haryana and Uttar Pradesh,
(c) The arid and semi-arid tracts of Southern States, and
(d) The coastal alluvium.
In U.P, the saline (usar) soils are distributed in Kanpur, Lucknow, Hardoi, Unnao, Allahabad, Rai Bareh, Azamgarh and many other districts covering about 1.29 million hectares. In Punjab alone saline soil covers about 2 million hectare area. In U.P. and Punjab the saline soils are gradually increasing in area. These lands are known by a variety of names in local agricultural parlance. By far the most common of them is mar derived from the Sanskrit word Ushtra meaning sterile or barren. Other terms like reh, char, lone, thur or shora are also popular. The word alkali is of Arabic origin meaning ash-like and is used to designate hard and intractable soils generally known by the names rakkar, kallar, bara and bari.
The salty soils are of three types:
(i) Saline or solonchak or white alkali soils:
In these, salinity is caused by soluble salts other than alkali salts. They have high soluble salts and low exchangeable sodium.
(ii) Alkali or sodic or solonetz or black alkali soils:
These are formed by accumulation of alkalies, such as Na, K etc. in excess. Such soils have low salt content but high exchangeable sodium.
(iii) Saline-alkali soils:
In these, alkali and other soluble salts have combined effects. They are also called saline sodic as they have high salt content and high exchangeable sodium. The United States Salinity Laboratory recently designated these soils scientifically on the basis of soil analysis following the ideas of Sigmond and Gedroiz (1954) (Table 23.4).
Three classes of salty soil:
Soils in which salinity is mainly due to accumulation of alkali salts are called alkali soils or usar soils. High alkalinity in the soil adversely affects the plant growth, thereby reduces the crop yield. Such sterile or unproductive soils are called barren soils. The main salts present in the alkali soils are Na2SO4, K2SO4, NaCl, and KCl.
Types of alkali soils:
These soils are of two types:
(i) Black alkali soils—In these soils, Na2CO3 is found in excess.
(ii) White alkali soils—In this group, NaCl is present in excess.
Russians call such soils as solonchack. Bertholet suggested that Na2CO3 was formed in black alkali soil by interaction of NaCl and CaCO3.
2NaCl + CaCO3 = Na2CO3, + CaCl2
Factors Which Make the Soils Alkaline:
These are:
1. Poor drainage in arid region,
2. Rapid evaporation of alkaline soil solution, and
3. Excess uptake of alkaline salts and little percolation.
In arid and semi-arid regions, the rainfall is too low to leach or remove the saline matter from the top soils. Besides this, water along with dissolved alkali salts moves upward by capillary action which on reaching to the soil surface evaporates and the salts accumulate in the form of a hard layer or pan in the subsoil. This hard layer is responsible for impermeability of such soils. Miller is of the opinion that many plants absorb excess acidic ions, e.g., NO–3, than the basic ions. This excessive removal of acidic ions results in the accumulation of basic ions which make the soil alkaline.
According to a chemical hypothesis, alkali soils may result in the following steps:
(a) Reaction between NaCl or KCl and soil (S):
NaCl + S (Soil) → Na (S) + CI– ion
(b) Then the soluble products are leached away from the soil surface by drainage water, and
(c) Finally, reaction between insoluble Na (S) complex and carbonates.
2 Na (S) + CaCO3 → Ca (S) (alkaline) + Na2CO3 (alkaline)
Effects of alkali salts on vegetation:
The alkali salts show the following effects on plants:
(i) Due to excessive accumulation of salts, concentration of soil solution becomes high. This decreases absorption of nutrients by plants and causes plasmolysis of cell cytoplasm in the plants which may be fatal sometimes. All these effects are responsible for stunted growth of plants.
(ii) If sodium is absorbed by the plants in excess, it shows toxic effects. Chloride salt of alkaline elements causes the death of trees. BaCO3 and BaCl are toxic to all plants.
(iii) Presence of excess salts in the soil retards the germination of seeds and growth of seedlings. Plants die before bearing fruits.
(iv) Alkali salts in the soil also affect the plant growth by reducing the size of leaves in alkali soil, plant roots remain superficial, bark of stem turns brown or black, green tissues are less developed.
Alkali tolerance in plant:
Some plants are resistant to alkali salts. Barley, wheat oats sorghum, sugar-beet, berseem are best suited to grow in alkaline soil. Cotton and grapes are also alkah tolerants. Uppal et al (1961) prepared a list of crops that can be grown at various stages of reclamation. They can be categorized as high, medium and low salt tolerant crops. High salt tolerant crops are Dhaincha, paddy, sugarcane in kharif and oat, berseem, lucerne, sanji (Trigonella spp), and barley in rabi. Medium salt tolerant crops to be tried during second stage of reclamation are castor, cotton, jowar, bajra, and maize for kharif and mustard and wheat for rabi.
Low salt tolerant crops are sesamum, moong, urd, arhar, and sannhemp in kharif and gram peas, linseed during rabi. Uppal et al also listed babul, dhak, jhand, khair, chokra, neem, Lasora, Sisham, siris, bahera and reonjha as trees that can be planted on saline alkali soils.
The alkali tolerance of plants depends upon:
(i) Physiological constitution of cell cytoplasm of the plant.
(ii) Length of roots. Shallow roots are more affected by alkalinity than the deeper roots.
(iii) Alkali salts reduce and correct the soil acidity and improve the physical conditions of the soils.
(iv) Calcium salts provide calcium to the plants.
(v) Many alkali salts change toxic elements, such as aluminum and Mn. into their harmless compounds.
Reclamation of Alkali, Saline and Saline-Alkali Soils:
The excessive accumulation of alkali salts in the soils is injurious for plants growth It is necessary, therefore, to reduce the percentage of salts to optimum or normal level so that plants may grow luxuriantly in such soils.
There are several methods of reclamation which can be grouped as follows:
(A) Chemical method in which some chemicals are added to the soil in order to brine the alkalinity to desired level.
(B) Mechanical practices such as improving drainage and leaching, mechanical shattering of clay pans, and scrapping.
(C) Cultural method (growing salt tolerant plants).
Since fundamental causes in various groups of salty soils are different, their reclaiming techniques are different. Hence, these are discussed separately.
1. Reclamation of Alkali Soils:
Alkali soils are best reclaimed by the following methods:
(A) Chemical method:
(1) By cationic exchange (replacement of alkali from soil colloids by calcium ions). Application of calcium sulphate (gypsum) in the soil reduces alkalinity to a great extent and makes the soil fertile.
The reaction proceeds in the following way:
2Na-clay + CaSO4 → Na2SO4 + Calcium-clay
Na2CO3 + CaSO4 + CaCO3, + Na2SO4
Good drainage leaches away Na2SO4.
(2) Alkali salt percentage can also be reduced in the soil by the use of acid forming chemical amendments such as sulphur, ferrous sulphate and limestone. Sulphur, when applied to the soil, oxidises and forms sulphuric acid which converts carbonates of sodium and potassium to Na2SO4 and K2SO4 respectively that may be removed from top soil by drainage water. The amount of gypsum and sulphur required to reclaim the alkali soils will be different depending upon the degree of alkalinity, drainage and buffering capacity of soils.
The types of reaction which occur when an amendment is applied to an alkali soil are given below:
(1) With Sulphur:
(i) 2S + 3O2 = 2SO3
(ii) SO3 + H2O = H2SO4
In the next step, if soil is calcareous—
(iii) H2SO4 + CaCO3 = CaSO4 + CO2 + H2O
(iv) 2Na-Clay + CaSO4= Ca-Clay + Na2SO4
But if the soil is non-calcareous—
(v) 2Na-Clay + H2S04= 2H-clay + Na2S04
(2) With lime-sulphur:
(i) CaS5 + 8O2 + 4H2O = CaSO4 + 4H2SO4
Now if the soil is calcareous—
(ii) H2SO4 + CaCO3 = CaSO4 + CO2 + H2O
(iii) 2Na-Clay + CaSO4 = Ca-Clay + Na2SO4
But if the soil is non-calcareous—
(ii) 10Na-Clay + 4H2SO4 + CaSO4 = 8H-Clay + Ca-Clay + 5 Na2SO4
(3) With ferrous sulphate:
(i) FeSO4 + H2O = H2SO4 + FeO
Now if the soil is calcareous—
(ii) H2SO4 + CaCO3 = CaSO4 + CO2 + H2O
(iii) 2Na-Clay + CaSO4 = Ca-Clay + Na2SO4
But if the soil is non-calcareous—
(ii) 2Na-Clay + H2SO4 = 2H-Clay + Na2SO4
(4) With limestone on non-calcareous soils:
(i) Na-Clay + H2O = H-Clay + NaOH
(ii) 2H-Clay + 2NaOH + CaCO3 = Ca-Clay + Na2CO3 + 2H2O
(5) With any H-Clay:
(i) 2H-Clay + CaCO3 = Ca-Clay + CO2 + H2O
The use of pyrite (FeS2) as an amendment is a recent development in the chemical amelioration and reclamation of alkali soils. In presence of moisture and air, pyrite is converted into sulphuric acid which then replaces exchangeable sodium by hydrogen or calcium released from insoluble calcium present in the soil. In addition it is said to correct iron deficiency and lime induced iron chlorosis in alkali soils. It is important to mention that the formation of H2SO4 in the soil by the application of pyrite may take place through chemical and microbiological actions. Pyrite is oxidised according to the following equation suggested by Bloomfield (1973).
FeS2 + 2Fe+3 = 3Fe+2 + 2s (Chemical)
Sulphur thus formed could be the substrate for thioxidants which convert it into H2SO4.
S + 3(0) + H2O = H2SO4
Temple and Kochler (1954) explained the action of ferroxidans on the formation of H2SO4 as follows:
FeS2 + H2O + 7O = FeSO4 + H2SO4
2FeSO4 + O + H2SO4 = Fe2(SO4)3 + H2O
FeSO4 formed in the above reaction may be converted into H2SO4 by hydrolysis.
FeSO4 + H2O → H2SO4 + FeO
In brief, the pyrite is oxidized in soils to ferrous sulphate and sulphuric acid as depicted in the following equation:
2FeS2 + 2H2O + 702 = 2FeSO4 + 2H2SO4
Both sulphuric acid and ferrous sulphate help in reclamation of calcareous as well as non- calcareous salt affected soils by lowering the pH and solubilising free calcium from calcium carbonate present.
The reactions are given below:
In salt affected calcareous soils:
la. CaCO3 + H2SO4 = CaSO4 + CO2 + H2O
lb. 2Na-Clay + CaSO4 = Ca-Clay + Na2SO4
II. FeSO4 + H2O → FeO + H2SO4
H2SO4 formed in reaction II reacts as per equations la and lb
III. H2SO4 also neutralizes NaHCO3 and Na2CO3 present in these soils.
2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2CO2
Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2
But if the soil is non-calcareous:
(I) 2Na-Clay + H2SO4 = 2H-Clay + Na2SO4
(II) FeSO4 + H2O = FeO + H2SO4
H2SO4 formed in reaction II acts in similar manner as in reaction I.
(3) Dhar’s method. In India, Dr. Neel Ratan Dhar (1935) succeeded in reducing the alkalinity and salinity of the soil by the use of molasses and press-mud.
For one acre land he recommended the mixture of the following substances:
(i) 2 tons of molasses, (ii) 1-2 tons of press-mud (a waste product of sugar industry) and (iii) 50-100 pounds of P2O5 in the form of basic slag.
The molasses is fermented by soil microbes and as a result of fermentation organic acids are produced which lower the alkalinity and increase the availability of phosphates. The press- mud contains Ca which forms calcium salts that reduce the content of exchangeable sodium. Phosphate helps in the microbial fixation of nitrogen into nitrogenous compounds in the soil.
(B) Mechanical methods:
The alkali salts are removed by:
(1) Scraper or by rapidly moving streams of water,
(2) Deep ploughing of the land which reduces the alkalinity and makes the soil more permeable.
(3) Application of green manures of Dhaincha, guar, jantar (Sesbania aculeata) has been found most successful in reclamation of alkali and saline soils.
(4) Spreading of straw and dried grasses and leaves on the alkaline soil.
(C) Cultural method:
Growing of alkali tolerant crops and plants, such as sugar-beet, rice, patsann (Hibiscus cannabinus), wild indigo and babul in such soils successfully reduces alkalinity. Rice is commonly the first crop grown on salty lands to be reclaimed. In Punjab the usual practice of reclamation of salty lands involves growing of paddy after first initial leaching followed by berseem or senji which has higher water requirement than Dhaincha as green manure which IS followed by sugarcane and then wheat or cotton.
Introduction of leguminous crops helps in building up of nitrogen supply and opens the soils. Dhaincha-paddy-berseem rotation has been found to be the best cropping pattern on mild type of alkali soils in Punjab region. In U.P. also, paddy or dhaincha-paddy are the usual crops taken during first stage of reclamation of salty soils. This is followed by berseem or barley in winter. Pulse crops like gram or peas show poor performance.
II. Reclamation of saline soil:
Saline soil can be reclaimed by the following methods:
(1) By lowering the water table 5-6 feet below the surface. In sloppy area, it can be done by making network of 5-6 feet deep trenches at right angles to the slopes. In course of 2 or 3 Successive leaching, harmful salts are removed. A deep ploughing is also helpful in reclamation of saline soil. This also makes the soil loose and thus facilitates the downward movement of salty water in the soil.
(2) Salt tolerant crops, e.g., rice, sugar cane, barley and castor gradually remove salts from the soil.
(3) In case of saline soils which do not contain calcium salts, the addition of CaSO4 (gypsum) is beneficial. Supply of calcium in the soil can indirectly be maintained by of organic matters which on decomposition produce CO2. The CO2 gas, so produced, combines with insoluble calcium carbonate in moist condition to form soluble calcium bicarbonate. This also reduces alkalinity.
(4) Application of green manure, organic manures, organic residues, acids or acid formers is yet another good way to reduce salinity.
III. Reclamation of saline-alkaline soil:
Here the problem of reclamation is two-fold because of:
(a) Heavy accumulation of different types of salts,
(b) Poor percolation due to the presence of hard clay pan and highly dispersed sodium clay.
Such soil can be reclaimed by:
(i) Mechanical shattering of clay pans. This helps in downward movement of water.
(ii) Application of gypsum in the soil. This is followed by flushing with plenty of water.
(iii) Green maturing with Dhaincha (Sesbenia aculeata).
(iv) Growing of salt tolerant plants, e.g., paddy in kharif and oat and barley in rabi seasons are recommended for such soils. ,
Schoonover (1959) worked on the soils of India and enlisted the following technical requirements for reclamation of saline and alkaline soils:
(1) Necessity of good drainage.
(2) Availability of sufficient water to wash the excess salts from the top soils.
(3) Good soil management including land leveling, good bonding for irrigation and recent and advanced agronomic practices.
(4) Protection of soil from erosions.
(5) Good quality of irrigation water.