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Soil Salinity and Soil Alkalinity

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Soil salinity and soil alkalinity:

Soil salinity and soil alkalinity are the results of over irrigation in canal irrigated areas. In canal irrigated areas plenty of the water is available and the farmers indulge in over irrigation of their fields. Under such conditions, the ground water level rises and saline and alkaline efflorescences consisting of salts of sodium, calcium and manganese appear on the surface as a layer of white salt through capillary action.

Salinity means the predominance of chlorides and sulphates of sodium, calcium and magnesium in the soils in sufficient quantity to be able to seriously interfere with the growth of most plants. Alkalinity implies the dominance of sodium salts, specially sodium carbonate.

Although salt of alkali are somewhat different in their chemical properties from the salts of saline soils both soils occur in the same areas. Increasing salinity and alkalinity always indicate extension of water-logging salt encrustation (saline efflorescence) or thur tendencies. Sandy soils are more prone to alkalinity and the loamy soils to salinity-alkalinity.

Salinity and alkalinity have adverse effect on soil and reduce soil fertility. It is estimated that about 80 lakh hectares of land (2.43% of the country’s total area) is affected by the problem of salinity and alkalinity. Vast tracts of canal irrigated areas in Uttar Pradesh, Punjab and Haryana; arid regions of Rajasthan, semi-arid areas of Maharashtra, Gujarat, Andhra Pradesh and Karnataka and coastal areas of Orissa, Gujarat and West Bengal are facing this problem.

The western part of Uttar Pradesh in one of the worst sufferers at the hands of salinity and alkalinity. In some parts of Uttar Pradesh the internal drainage is greatly restricted and the soils are charaterised by alkalinity. Some of the most fertile soils in Punjab and Haryana have been rendered useless by salinity and alkalinity. In Punjab about 6,000 to 8,000 hectares of good land is becoming barren every year due to salinisation.

Although Indira Gandhi canal in Rajasthan has turned the sandy desert into granary, it has given birth to serious problem of salinity and to alkalinity. Alkali soils are met with almost all over the state of Maharashtra. In Gujarat, the area around the Gulf of Khambhat is affected by sea tides carrying silt-laden deposits.

Nearly 173,530 sq km comprising estuaries of the Narmada, the Tapi, the Mahi and the Sabarmati have been damaged in this way. Portions of Dharwar districts and of Bijapur taluk are affected by what is locally known a karl soils which are saline, alkali and fairy deep and clayey. Salt lands of the Nira valley have developed due to excessive irrigation on deep black soils of the locality.

The presence of an excess of sodium salts and the predominance of sodium in the exchangeable complex are divided into the two main groups:

(1) Saline soils and

(2) Alkaline soils.

(1) Saline Soils:

Saline soils contain an excess of sodium salts, but its colloidal material is not yet sodiumised.

(2) Alkali Soils:

In the case of alkali soils, the exchange complex contains appreciable quantities of exchangeable sodium. Such soils may or may not contain excess salts.

Alkali soils may be divided into following groups:

(a) Saline-alkali soils:

When they contain soluble salts in excess they are known as saline-alkali soils.

(b) Non-saline-alkali soils (Alkali soil):

When they do not contain soluble salts, they are called non-saline-alkali soils.

(c) Degraded alkali soils:

Under certain circumstances the clay complex of some alkali soils is broken down to give rise to degraded alkali soils.

The various types of alkaline soils are shown diagrammatically as under:

 

Soil salinity and alkalinity has many adverse effects, some important effects are as under:

(a) Soil fertility is reduced which results in crop failure. Cultivation is not possible on saline soils unless they are flushed out with large quantities of irrigation water to leach out the salts.

0b) Choice of crops is limited because some crops are sensitive to salinity and alkalinity. Only high salt tolerant crops such as cotton, rape, barley etc. and medium salt tolerant crops like wheat, rice, linseed, pulses, millets etc. can be grown.

(c) Quality of fodder becomes poor.

(d) Salinity and alkalinity create difficulties in building and road construction.

(e) It causes floods due to reduced infiltration, leading to crop damage in the adjoining areas.

TABLE 7.2 Salinity Affected Areas in India:

States / Union territories Area(lakh hectares)
1. Uttar Pradesh 12.95
2. Punjab 12.25
3. Gujarat 12.14
4. West Bengal 8.50
5. Rajasthan 7.28
6. Maharashtra 5.34
7. Haryana 5.26
8. Kamataka 4.04
9. Orissa 4.04
10. Madhya Pradesh 2.24
11. Andra Pradesh 0.42
12. Delhi 0.11
13. Bihar 0.04
14. Tamil Nadu 0.04

Characteristics of Saline and Alkaline Soils:

A. Saline Soil:

When the soil contains excess of sodium salts and clay complex still contains exchangeable calcium, the soil is known as saline soil or white alkali or brown alkali soil. The process of accumulation of salts leading to the formation of soils is known as salinization.

(i) Saline soils contain usually chloride, sulphate, bicarbonates and sometime nitrates of sodium. The presence of chloride and sulphate of sodium gives a white colour on the soil surface. When nitrates are in excess they give a brown colour to the soil.

(ii) Exchangeable sodium percentage (ESP) is very low, being less than 15% of the total cation exchange capacity (C.E.C.).

(iii) As a consequence of low ESP, generally pH varies between 7.5 and 8.5.

(iv) Total soluble salt content is more than 0.1%. it is high enough to interfere with normal growth of most plant species.

(v) Electrical conductivity (E.C.) of solution extract (saturated soil) is 4 or more m mhos/cm.

(vi) Saline soils remain in a flocculated condition (granulated). It is permeable to water and air.

(vii) Saline soils usually have a surface crust of white salts, especially in the season when the net movement of soil moisture is upward. Salts dissolved in the soil water move up to the surface, where they are left as a crust when the water evaporates.

B. Alkaline Soil (Sodic Soil):

(a) Non-saline-alkali soils:

The characteristic features are the presence of collodial complex that is saturated with exchangeable sodium, and the absence of appreciable quantities of soluble salts. These soils are often called ‘black alkali’ soils, because they are black, owing to the effect of the high sodium content which causes the dispersion of the organic matter. These soils are also called typical usarsoils. These soils contain sodium carbonates (Na2 CO3) in abundance.

(i) Exchangeable sodium percentage is greater than 15%.

(ii) Consequently pH ranges from 8.5 to 10 (strongly alkaline).

(iii) Total soluble salt (sodium) content is less than 0.15.

(iv) Electrical conductivity (EC) is usually less than 4 mmhos/cm.

(v) Colloidal complex is deflocculated and dispersed. The clay swells and chokes the soil pores. Hence, permeability to water and air is poor (or infiltration and aeration is slow).

(vi) The presence of free sodium carbonate has a toxic effect on plant roots. Also, the high pH and poor physical condition of soil adversely affect plant growth.

(vii) Sodium carbonate absorbs organic matter, so there is great depletion of organic matter. Therefore, these soils are almost barren (Usar).

(b) Saline-alkali soils:

These soils are both saline and alkali. There can be all stages in transition with varying degree of dominance of salt content and pH. According to movement of soluble salts, formation of saline-alkali and non-saline alkali soils depends. Soil contains Na-clay as well as excess soluble salts.

If the soluble sodium salts are not leached out due to the insufficiency of rain water, they remain in the soil. The soil thus contains Na-clay and excess soluble, salts in solution. Such soils are known as saline-alkali soils. They are thus, developed as a result of the combined process of salinization and alkalization. In spite of the presence of sodium clay (Na-clay) the soil remains friable and possesses aggregate (flocculated). This is because the presence of sodium salts does not allow the sodium clay to get dispersed and keeps it flocculated.

Thus, this soil behaves more or less like saline soils. If due to much water soluble salts are leached down, and soil contains Na-clay only. Thus, this soil behaves more or less as non-saline-alkali soil. Therefore, the soil structure becomes un-favourable for the entry and movement of air and water.

Usually these soils have the following characteristics:

(i) Exchangeable sodium is more than 15%.

(ii) A variable pH, usually above 8.5, depending upon the relative amounts of exchangeable sodium and soluble salts. When soluble salts are leached downward, the pH will rise above 8.5, but when the soluble salts again accumulated, the pH again falls to 8.5.

(iii) Generally soluble salts content is more than 0.1%.

(iv) Electrical conductivity is greater than 4 mmhos/cm.

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C. Degraded Alkali Soils:

The soil does not contain free calcium carbonate (CaCO3). As a result of prolong leaching under this condition,Na-clay hydrolyses NaOH which combines with CO2 or soil air and forms sodium carbonate (Alkaline condition).

Sodium carbonate (Na2 CO3) dissolves humus. Humus (organic matter) is deposited in the lower layer. The lower layer thus, acquires a black colour. At the same time, a part of exchangeable sodium of the surface layer is replaced by hydrogen. H-clay (acid soil) formed in this way does not remain stable. The process of break-down of H-clay under alkaline condition is known as solodization and the soil as formed is called Solod, Soloth or degraded alkali soil.

(i) The soil reaction of the surface layer is acidic (pH 6.0). This layer is usually very thin, hardly a few inches in depth.

(ii) The lower layer which constitutes the main soil body has a high pH (more than 8.5).

(iii) ESP is greater than 15%.

(iv) EC less than 4 mmhos/cm.

(v) The lower layer has black colour.

(vi) It develops columnar (prism-like) structure.

(vii) Soils become compact and has low infiltration, and permeability.

Formation of Saline and Alkaline Soil:

Origin or development of saline and alkaline soil depends upon following factors:

(i) Arid and Semi-Arid Climate:

Alkaline soils are those that have an alkaline reaction or whose pH is greater than 7.0. Alkalinity is due to sodium salts in soil solution or the presence of sodium clay or both. They are formed in arid and semi-arid regions which have very low rainfall and high evaporation.

The low rainfall in these regions is not sufficient to leach out the soluble products of weathering and hence, the salts accumulate in the soil. During rain, the salts dissolve in rain water and move down in the lower layers. However, due to the 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.

(ii) Poor Drainage of Soil:

During the periods of high rainfall, the salts are leached from the upper layer and, if the drainage is impeded, they accumulate in the lower layer. When water evaporates, the salt is left in the soil. Such soils are generally developed in low-lying areas or in basin shaped areas.

(iii) High Water Table:

The ground waters of arid regions usually contain considerable quantities of soluble salts. If the water table is high, large amounts of water move to the surface by capillary action and the evaporated, leaving soluble salts on the surface.

(iv) Overflow of Sea Water over Lands:

Low lying areas near the sea which get sea water during tides. Salt water accumulates and enrich the soils with salts.

(v) Introduction of Irrigation Water:

The ground water of arid regions are generally saline in nature. With injudicious irrigation the percolating water may get linked with the saline ground water. During dry weather the soluble salts of the ground water may, thus, get carried to the surface and increase the salinity of the land. The irrigation water may be itself rich in soluble salts and add to the salinity of the soils.

(vi) Salts Blown by Wind:

In arid regions near the sea, lot of salt is blown by wind year after year and get deposited on the lands. Due to low rainfall they are not washed back to sea and thus, add salinity to the land. The salinity of Rajasthan has developed to a great extent, due to this reason.

(vii) Saline Nature of Parent Rock Materials:

If soils develop from saline nature of parent rock materials, soil would be saline.

(viii) Excessive Use of Basic Fertilizers:

Use of alkaline fertilizers like sodium nitrate, basic slag etc., may develop alkalinity in soil.

(ix) Humid and Semi-Humid Regions:

Alkaline soils develop in other areas also, e.g., in semi-humid and temperate regions, especially in depressions where drainage is defective and where the underground water table is high or close to the surface. There are three distinct stage in the evolution of saline and alkali soils.

They are as follows:

1. Saline soils (Salinization):

Soil contains excess of sodium salts while the clay- complex (soil-colloid) still contains exchangeable calcium and magnesium. In these soils the colloids are not damaged by sodium.

2. Saline-alkali soils:

When soluble sodium salts accumulate in a soil over a prolong period, form sodium clay (sodium becomes the predominant cation in soil solution). If the soluble salts (sodium) are not leached out due to the insufficiency of rain, they remain in the soil. They are thus, developed as a result of the combined process of salinization and alkalization. Sodium salts keep soils in flocculated conditions.

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3. Alkalinization (non-saline-alkali soils):

When soluble salts (from saline-alkali soils) are removed by leaching as a result of the increase in rainfall, it gives rise to non- saline-alkali soil (only Na-clay in the soil colloids). Calcium carbonate (CaCO3) reacts with Na-clay and give rise to Ca-clay and sodium carbonate (Na2CO3). Due to low CaCO3,Na2 CO3 converts Ca-clay into Na-clay. The clay is thus sodium saturated.

If CaCO3 is absent, it forms degraded alkali soils. Na-clay hydrolyses (during leaching) and liberates NaOH which combines with the CO2 and forms sodium carbonate.

Detrimental Effects of Soil Salinity and Alkalinity:

Saline Soils:

(i) Absorption of water and nutrients:

Excessive salts in the soil solution increase the osmotic pressure of soil solution in comparison to cell sap. This prevents absorption of moisture and nutrients in adequate amounts by the roots.

(ii) Salt toxicity:

When the concentration of soluble salts increase to high level then it produces toxic effect directly to plants. Saline soils are usually barren but potentially productive soils.

Alkaline Soil:

(i) Dispersion of soil particles:

Under alkali soil conditions, the damage is not due to salt concentration. The sodium adsorbed by clay and colloids causes dispersion of clay which results in a loss of desirable structure and development of compact soil.

(ii) Physical properties affected:

Due to compactness of soil, aeration, permeability, drainage and microbiological activity are reduced.

(iii) Availability of plant nutrients reduced:

The high pH in alkali soil causes a reduction in the availability of plant nutrients such as phosphorus, calcium, nitrogen, iron, copper, manganese and zinc. Under saline-alkali conditions there may be actually transitional stages, from high salinity-low alkalinity to low salinity-high alkalinity. Under such conditions, the crops may suffer due to high salinity as well as to un-favourable effects of alkalinity.

Reclamation of Saline and Alkali Soils:

Schoonover (1959) in his study of soil problems in India, has listed the following technical requirements for reclamation of saline and alkali soils:

1. Adequate drainage.

2. Availability of sufficient water to meet crop use and also leach the salt below the root zone in the soil.

3. Better than average soil management to include perfect land leveling, good bunding for irrigation and advanced agronomic practices.

4. Protection and reclamation to be taken in large blocks.

5. Irrigation water should be of good quality.

I. Saline Soil Reclamation and Management:

Saline soils in which the soluble salts contain appreciable amounts of calcium and magnesium do not develop into alkali soils by the action of leaching water. The reclamation is comparatively easy in such soils. The main problem is to leach the salts downward below the root zone and out of contact with subsequent irrigation water.

Following methods may be used for removal of salts:

(A) Mechanical Methods:

(i) Flooding and leaching down of the soluble salts:

The leaching can be done by first ponding the water on the land and lowering it to stand there for a week. Most of the soluble salts would leach down below the root zone. After a week, standing water (dissolved with soluble salts) is allowed to escape. Such, 2 to 3 treatments are given to reclaim highly saline soils. Sometimes gypsum is also added to flood water when the soluble salts are low in calcium to check development of alkalinity.

(ii) Scrapping of the surface soil:

When the soluble salts accumulate on the soil surface, scrapping helps to remove salts. This is a temporary cure and salinity again develops on such lands.

(B) Cultural Methods (Crop, Soil and Water Management):

(i) Providing proper drainage:

If the soil is not free draining, artificial, drains are opened or tile drains laid underground to help wash out the salts.

(ii) Use of salt free irrigation water:

Salt free good quality of irrigation water should be used.

(iii) Proper use of irrigation water:

It is known that as the amount of water in the soil decreases the concentration of salts in the soil solution increases, thus, moisture should be kept at optimum field capacity.

(iv) Planting or sowing of seeds in the furrow:

The salt concentration even in smaller amounts is most harmful to the germinating seedlings. Water generally evaporates from the highest surface by capillarity and hence, these points have maximum salt concentrations. If the seeds or seedlings are planted inside the furrows, they escape the zone of maximum salt concentrations and thus, can germinate and develop properly during their early growth stage.

(v) Use of Acidic Fertilizer:

In saline soil, acidic nature of fertilizers (e.g., Ammonium sulphate) should be used.

(vi) Use of organic manures:

The organic manures have very high water-holding capacity. When sufficient amount of these manures are added the water-holding capacity of soil increases and as a result the conductivity of the soil solution decreases.

(vii) Ploughing and leveling of the land:

Ploughing and leveling of the land increases the infiltration and percolation rate. Therefore, salts leach down to the lower levels.

(viii) Retardation of water evaporation from soil surface:

Water may be conserved in the soil retarding the water evaporation. Thus, salts may remain in the lower level with the water.

(ix) Growing of salt tolerant crops:

(a) High salt tolerant crops: Para grass, barley, sugar beet, etc.

(b) Moderately salt tolerant crops: Wheat, rice, sorghum, maize, flax etc.

(c) Low salt tolerant crops: Beans, radish, white clover etc.

(d) Sensitive crops: Tomato, potato, onion, carrot etc.

II. Reclamation and Management of Alkali (Saline-alkali and non-saline-alkali) Soils:

Alkali soils cannot be reclaimed by mere flooding the land. In the case of saline-alkali soils, flooding is likely to do more harm. Leaching (flooding) down of soluble salts make the soil alkaline (only Na-clay remain in the soil). Soils get dispersed and become compact (impervious).

In alkali (non-saline-alkali) soils, exchangeable sodium Na-clay is so great as to make the soil almost impervious to water. But even if water could move downward freely in alkali soils, the water alone would not leach out the excess exchangeable sodium. The sodium-cation must be replaced by calcium-cation and then leached downward.

Following chemical methods are used for reclaiming the alkali soils:

(A) Chemical Methods:

(i) Application of gypsum:

By cationic exchange, calcium is often used to replace sodium in alkali soil. If the soil has no reserve of calcium carbonate, the addition of gypsum (calcium sulphate) is necessary. When gypsum is used as a reclaiming agent, calcium replaces the exchangeable sodium and converts the clay back into calcium-clay (Ca-clay).

Sodium sulphate goes into solution and is then removed by washing it out with water or leaching down with water with the help of artificial drains. Addition of gypsum improves physical conditions of soil. Soils become flocculated and drainage improves. pH is lowered down to a desirable level.

Gypsum requirement is alkaline soil:

For reasonable crop production on a sodic soil, the lowering of the ESP to the level of 10 is considered sufficient. The amount of gypsum required to be added to a sodic soil to lower the ESP to a desired value is known as gypsum requirement. It is expressed in milliequivalent of Ca++ per 100 gm. of soil. Gypsum requirement can be calculated from the data on CEC and ESP of the soil.

For a sodic soil, suppose, CEC = 30 and ESP = 60, gypsum requirement to lower the ESP to 10, will be:

or = 10 m.e. of Ca++ per 100 gm. soil.

Besides gypsum that is best soil amendment for sodic soil, several other materials may be used for reclaiming alkaline soils.

Gypsum equivalents of some such materials are given below:

(ii) Use of sulphur:

In the case of alkali soil that contains free calcium carbonate, addition of sulphur, sulphuric acid, iron and aluminium sulphate, green manure (produce acidity) etc. reclaim the soil very effectively. The acidity developed during the course of their decomposition of soil, neutralizes alkalinity. At the same time brings calcium carbonate into solution which then reacts with the sodium clay and converts it into calcium clay.

When sulphur is spread on the soil, it is oxidised to sulphuric acid, which converts sodium carbonate into sodium sulphate. If calcium carbonate is not present in the soil, it should be added artificially when sulphur is used for reclamation.

Reactions are as follows:

In above mentioned both cases, it is necessary to leach out the sodium salts, formed as a result of bases exchanges with the help of artificial drains.

(iii) Addition of organic matter:

The addition of organic matter increases acidity, thus, helping in lowering the pH. Organic matter is especially helpful where sulphur is added to correct the alkalinity. The organic matter supplies food for the bacteria that stimulates the oxidation of sulphur to the sulphate form. The combination of sulphur, organic matter and gypsum has also been used with success.

(iv) Use of sulphuric acid:

Sulphuric acid changes the sodium carbonate to the less harmful sulphate and also tends to reduce the intense alkalinity. It should be used in the presence of calcium carbonate.

(v) Addition of molasses:

Addition of molasses in the soil provide the source of energy for microorganism which on fermentation produce organic acids. The organic acids reduce alkalinity.

(vi) Use of Pyrite:

Pyrite is a mineral containing iron and sulphur and generally it has a chemical composition of FeS2. Pyrite is found all over the world in igneous and metamorphic rocks and at some places as sedimentary deposits as well.

Pyrite is pyrophoric in nature, produces sulphuric acid and iron sulphate on coming in contact with air and water. The sulphuric acid so produced reacts with the native CaCO3 of these soils to produce soluble calcium which then replaces sodium from the exchange complex.

2 FeS2 + 2 H2O + 7 O2 â†’ 2 FeSO4 + 2 H2SO4

Pyrite application in non-calcareous alkali soil is not affective because they lack free CaCO3 to be dissolved by H2SO4 to produce Ca needed for the replacement of Na from exchangeable complex of sodic soils.

Pyrite application is recommended in the summer season because oxidation of Pyrite is rapid in the temperature range of 25° to 40°C. The activities of microorganism (Thiobacilli) are high in the above temperature range. Low temperature in winter season retards oxidation.

Pyrite should not be applied in the rainy season or in Paddy field. The activity of microorganism (Thiobacilli) decreases at very low at anaerobic (water logged) condition. The activity of microorganism is high in moist soil with good aeration and moderate temperature.

Dose of Gypsum and Sulphur:

On an average for every one milliequivalent of sodium to be replaced, 1.7 tons of gypsum or 3.2 tons of sulphur is required. The amount of gypsum and sulphur required to replace different amount of exchangeable sodium are given in the Table 9.2.

Amount of gypsum and sulphur required to replace indicated amount of exchange sodium

(B) Cultural Method:

Same cultural practices are followed as described in the reclamation of saline soils.


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