2. ORIGIN, CLASSIFICATION AND DISTRIBUTION OF SALT-AFFECTED SOILS

2.1 Origin of salts
2.2 Classification
2.3 Mode of formation
2.4 Distribution

Salt-affected soils occur in all continents and under almost all climatic conditions. Their distribution, however, is relatively more extensive in the arid and semi-arid regions compared to the humid regions. The nature and properties of these soils are also diverse such that they require specific approaches for their reclamation and management to maintain their long term productivity. For any long-term solutions, it is, therefore, necessary to understand the mode of origin of salt-affected soils and to classify them, keeping in view the physico-chemical characteristics, processes leading to their formation and the likely approaches for their reclamation and successful management.

2.1 Origin of salts

The presence of excess salts on the soil surface and in the root zone characterizes all saline soils (Plate 1). The main source of all salts in the soil is the primary minerals in the exposed layer of the earth’s crust. During the process of chemical weathering which involves hydrolysis, hydration, solution, oxidation, carbonation and other processes, the salt constituents are gradually released and made soluble. The released salts are transported away from their source of origin through surface or groundwater streams. The salts in the groundwater stream are gradually concentrated as the water with dissolved salts moves from the more humid to the less humid and relatively arid areas. The predominant ions near the site of weathering in the presence of carbon dioxide will be carbonates and hydrogen-carbonates of calcium, magnesium, potassium and sodium; their concentrations, however, are low. As the water with dissolved solutes moves from the more humid to the arid regions, the salts are concentrated and the concentration may become high enough to result in precipitation of salts of low solubility. Apart from the precipitation, the chemical constituents of water may undergo further changes through processes of exchange, adsorption, differential mobility, etc., and the net result of these processes invariably is to increase the concentration in respect of chloride and sodium ions in the underground water and in the soils. Russian workers (Kovda, 1965) recognize the following sequence of changes in the composition of groundwater in relation to their concentrations (Table 1) as the water moves from humid to arid areas. Similar trends are observed with regard to the chemical composition of groundwater in India.

Plate 1 A typical saline soil in India

Table 1 RELATIONSHIP BETWEEN THE QUANTITY OF SALTS IN NATURAL WATERS IN RELATION TO THEIR COMPOSITION (Kovda, 1965)

Nature of water	Total salt concentration, g/I
Siliceous waters - completely fresh waters containing silica and organic substances	0.01 to 0.1
Fresh calcium-bicarbonate waters	0.2 to 0.3
Sodium bicarbonate waters	0.5 to 0.7
Sodium-bicarbonate and carbonate waters containing sulphate and less often chlorides	0.5 to 3.0
Chloride-sulphate waters	2.5 to 5.0
Chloride waters	>5

Table 2 CHARACTERISTICS OF AN UNIRRIGATED SOIL PROFILE FROM A SEMI-ARID TRACT, HISSAR, INDIA (Abrol, unpublished data)

Depth cm	Clay %	pHs*	ECe (dS/m)
0 - 10	17.3	8.0	1.4
10 - 20	18.5	7.9	0.8
20 - 43	19.0	7.9	0.8
43 - 88	32.5	8.1	1.5
152 - 208	40.8	7.7	4.8
208 - 228	35.8	7.7	11.0

* pH_s - pH measured on soil saturated paste.

Salts released through weathering in the arid regions with limited rainfall are usually deposited at some depth in the soil profile, the depth depending on such factors as the water retention capacity of the soil, seasonal, annual and maximum rainfall, etc. (Yaalon, 1965). If the salts are deposited beyond the rooting zone of most crops, say below 150 cm, they rarely affect the crops adversely unless they are redistributed and accumulate in the surface soil layers (Table 2).

Salt-affected soils generally occur in regions that receive salts from other areas and water is the primary carrier. Although the weathering of rocks and minerals is the source of all salts, rarely are the salt-affected soils formed from the accumulation of salts in situ.

2.2 Classification

In the course of accumulation of knowledge on the nature, characteristics and plant growth relationships in salt affected soils, two main groups of these soils have been distinguished (Szabolcs, 1974). These are:

i. Saline soils - Soils containing sufficient neutral soluble salts to adversely affect the growth of most crop plants. The soluble salts are chiefly sodium chloride and sodium sulphate. But saline soils also contain appreciable quantities of chlorides and sulphates of calcium and magnesium.

ii. Sodic soils - Soils containing sodium salts capable of alkaline hydrolysis, mainly Na₂CO₃, these soils have also been termed as ‘Alkali’ in older literature.

Although the above two categories account for a very large fraction of salt affected soils the world over, there are transitional or borderline formations which are likely to have properties intermediate between those of the two broad categories. Several local terms in different parts of the world are in vogue to designate such soils. Other categories of salt-affected soils which, though less extensive, are commonly met in different parts of the world are:

i. Acid-sulphate soils

These are soils that have somewhere within a 50 cm depth a pH below 3.5 to 4.0 that is directly or indirectly caused by sulphuric acid formed by the oxidation of pyrite (FeS₂) or, rarely of other reduced sulphur compounds. Potential acid sulphate soils occur in tidal swamps. They have high levels of pyrite, low levels of bases and produce strongly acid sulphate soils when pyrite is oxidized to sulphuric acid after drainage (Pons, 1973). Pyrite formation is favoured in brackish and saline mangrove swamps dissected by tidal creeks where deposition and build up of coastal sediments is slow. Apart from high salinity, the productivity of acid sulphate soils is restricted due to such soil factors as iron and aluminium toxicities, deficiency of phosphorus, etc.

ii. Degraded sodic soils

Degraded sodic soils are usually considered to be an advanced stage of soil development resulting from the washing out of salts. The details of the type of soil developed as the leaching proceeds depends on local conditions, particularly soil texture and type of clay present. As a result of the leaching processes there is a tendency for the dispersed clay and organic matter to move down the profile resulting in the formation of a dark, extremely compact layer having a sharply defined upper surface and merging gradually into the subsoil with increasing depth. The darker colour of the compact layer compared with the layer above may be due to its higher clay content since it does not always have a higher content of organic matter. The upper soil layers have a loose porous, laminar structure due to loss of clay and the upper surfaces of this layer may be paler than the lower, possibly because of silica being deposited on them. The clay pan cracks on drying into well defined vertical columns having a rounded top and smooth, shiny, well defined sides. These can be broken into units about 10 cm high and 5 cm across with a flat base. Below this the column breaks into rather smaller units with flat tops and bottoms which on light crushing break into angular fragments.

As the leaching of these desalinized soils proceeds, the upper horizons deepen and often become slightly acidic in reaction and the amorphous silica content increases. As a further stage of development, it has been suggested that the very characteristic clay pan becomes less pronounced, possibly because of washing down of sandy material from the A horizon in the cracks between the structural units.

There are large areas in western Canada (Toogood and Cairns, 1973; Cairns and Bowsa, 1977), Australia (Northcote and Skene, 1972), USA (Rasmussen et al., 1964) and other countries where soils having profile morphology typical of solonetz/solod soils are found although sodium forms only a minor proportion of the exchangeable ions. It is possible that these soils originally had enough exchangeable sodium for the solonetz-solod morphology to develop in the profile but that most of this sodium has now been lost through leaching.

iii. A large number of sub-categories of salt-affected soils are recognized in different parts of the world depending on the dominance of a particular chemical constituent (e.g. calcium chloride rich soils or soils containing excessive quantities of exchangeable magnesium - magnesium solonetz, etc.) or a particular morphological character of the soil profile, e.g. presence of a structural ‘B’ horizon, etc.

Characteristics	Saline soils	Sodic soils
1. Chemical	a. Dominated by neutral soluble salts consisting of chlorides and sulphates of sodium, calcium and magnesium.	a. Appreciable quantities of neutral soluble salts generally absent. Measurable to appreciable quantities of salts capable of alkaline hydrolysis, e.g. Na2CO3, present.
	b. pH of saturated soil paste is less than 8.2.	b. pH of the saturated soil paste is more than 8.2.
	c. An electrical conductivity of the saturated soil extract of more than 4 dS/m at 25 °C is the generally accepted limit above which soils are classed as ‘saline’.	c. An exchangeable sodium percentage (ESP) of 15 or more is the generally accepted limit above which soils are classed as ‘sodic’. Electrical conductivity of the saturated soil extract is generally less than 4 dS/m at 25 °C but may be more if appreciable quantities of Na2CO3 etc. are present.
	d. There is generally no well-defined relationship between pH of the saturated soil paste and exchangeable sodium percentage (ESP) of the soil or the sodium adsorption ratio (SAR) of the saturation extract.	d. There is a well defined relationship between pH of the saturated soil paste and the exchangeable sodium percentage (ESP) of the soil or the SAR of the saturation extract for an otherwise similar group of soils such that the pH can serve as an approximate index of soil sodicity (alkali) status.
	e. Although Na is generally the dominant soluble cation, the soil solution also contains appreciable quantities of divalent cations, e.g. Ca and Mg.	e. Sodium is the dominant soluble cation. High pH of the soils results in precipitation of soluble Ca and Mg such that their concentration in the soil solution is very low.
	f. Soils may contain significant quantities of sparingly soluble calcium compounds, e.g. gypsum.	f. Gypsum is nearly always absent in such soils.
2. Physical	a. In the presence of excess neutral soluble salts the clay fraction is flocculated and the soils have a stable structure.	a. Excess exchangeable sodium and high pH result in the dispersion of clay and the soils have an unstable structure.
	b. Permeability of soils to water and air and other physical characteristics are generally comparable to normal soils.	b. Permeability of soils to water and air is restricted. Physical properties of the soils become worse with increasing levels of exchangeable sodium/pH.
3. Effect on plant growth	In saline soils plant growth is adversely affected:	In sodic soils plant growth is adversely affected:
	a. chiefly through the effect of excess salts on the osmotic pressure of soil solution resulting in reduced availability of water;	a. chiefly through the dispersive effect of excess exchangeable sodium resulting in poor physical properties;
	b. through toxicity of specific ions, e.g. Na, Cl, B, etc.;	b. through the effect of high soil pH on nutritional imbalances including a deficiency of calcium;
		c. through toxicity of specific ions, e.g. Na, CO3, Mo, etc.
4. Soil improvement	Improvement of saline soils essentially requires removal of soluble salts in the root zone through leaching and drainage. Application of amendments may generally not be required.	Improvement of sodic soils essentially requires the replacement of sodium in the soil exchange complex by calcium through use of soil amendments and leaching and drainage of salts resulting from reaction of amendments with exchangeable sodium.
5. Geographic distribution	Saline soils tend to dominate in arid and semi-arid regions.	Sodic soils tend to dominate in semi-arid and sub-humid regions.
6. Ground-water quality	Groundwater in areas dominated by saline soils has generally high electrolyte concentration and a potential salinity hazard.	Groundwater in areas dominated by sodic soils has generally low to medium electrolyte concentration and some of it may have residual sodicity so has a potential sodicity hazard.

2.3 Mode of formation

2.3.1 Saline soils
2.3.2 Sodic soils

Although weathering of rocks and primary minerals is the chief source of all salts, salt-affected soils rarely form through accumulation of salts in situ. The major factors responsible for the formation of two principal categories of salt-affected soils are discussed below:

2.3.1 Saline soils

i. Use of saline groundwater: When groundwater is the only source available for irrigation, high salinity of the irrigation water can cause a build up of salts in the root zone, particularly if the internal drainage of the soils is restricted and leaching, either due to rainfall or applied irrigation, is inadequate.

ii. Saline seeps, common in North America, Australia and other countries, are the result of excessive leaching that results from reduced evapotranspiration after a change in land use from a natural forest vegetation to a cereal grain crop or a shift in cropping pattern such as the introduction of a fallow season in a grain farming system. The percolating water passing through saline sediments is intercepted by impermeable horizontal layers and conducted laterally to landscape depressions causing extensive soil salinization (Doering and Sandoval, 1976).

iii. Salinity problems are also caused by the ingress of sea water through tidal waves, underground aquifers or through wind transport of salt spray. Soluble salts have also been continually exchanged between land and sea - most transfer of salts from the sea taking place through the uplift of marine sediments and exposure on the earth’s surface. For soils of semi-arid regions where rainfed agriculture is practised, serious salinity problems can arise if the rainfall is only approximately equal to the evapotranspiration and soluble salts are present in the root zone from either marine deposits or other sources.

iv. Salinity problems are most extensive in the irrigated arid and semi-arid areas. In every river basin, prior to the introduction of irrigation, there exists a water balance between the rainfall on the one hand and stream flow, groundwater level and evaporation and transpiration on the other. This balance is disturbed when large additional quantities of water are artificially spread on the land for agriculture. An important new contribution to groundwater is introduced in the form of seepage from irrigation channels, from irrigation water added over and above the quantities actually utilized for meeting the evapotranspirational needs of crops, and obstructions in the natural drainage brought about by new developments in the area. These new additions to the groundwater will raise the subsoil water level or may form a perched water table. Studies (Gardner and Fireman, 1958; Sharma and Prihar, 1973) have shown that once the water table is within 1 to 2 m of the soil surface, it can contribute significantly to evaporation from the soil surface and therefore to the root zone salinization. Salinization problems can be more severe when the salinity of groundwater is high, as is usually the case in arid regions.

v. Localized redistribution of salts can often cause salinity problems of a significant magnitude. Soluble salts move from areas of higher to lower elevations, from relatively wet to dry areas, from irrigated fields to adjacent unirrigated fields, etc. Salts may also accumulate in areas with restricted natural drainage caused by the construction of roads and rail lines or other developmental activities. Evaporation of stagnant waters may leave considerable amounts of salts on the soil surface.

2.3.2 Sodic soils

The mechanisms responsible for the formation of sodium carbonate in soils which characterize sodic (alkali) soils have been discussed in several standard works (Kelly, 1951; Bazilevich, 1965). Groundwater containing carbonate and bicarbonate is one of the chief contributing factors in the formation of sodic soils in many regions. Sodic soils occur in Egypt in Wadi Tumilat, Ferhash and Wadi-El-Natroun. The soils are reported to have formed by desalinization in the absence of enough divalent cations in some parts of the Nile Delta, by high carbonate and bicarbonate water in Wadi Tumilat and by denitrification and sulphate reduction under anaerobic conditions in Wadi-El-Natroun (Elgabaly, 1971).

Reduction of sulphate ions under anaerobic conditions and in the presence of organic matter was reported to result in the formation of sodium carbonate (Whittig and Janitzky, 1963). According to Bhargava et al. (1980) the alternate wet and dry seasons and the topographic (drainage) conditions appeared to be the contributing factors in the formation of vast areas of sodic soils in the Indo-Gangetic plains of India (Plate 2). During the wet season water containing products of alumino-silicate weathering accumulated in the low lying areas. In the ensuing dry season, as a result of evaporation, the soil solution is concentrated resulting in some precipitation of the divalent cations, causing an increase in the proportion of sodium ions in the soil solution and on the exchange complex with simultaneous increase in pH. This process repeated over years resulted in the formation of sodic soils.

Plate 2 Extensive areas of sodic lands lying barren in Northern India

Beek and Breemen (1973) pointed out that highly sodic soils could be developed in a closed basin with an excess of evaporation over precipitation if the inflowing water has a positive residual sodicity. Similarly, groundwater containing residual sodicity could result in the formation of sodic soils when the groundwater table is near the surface and contributes substantially to evaporation.

2.4 Distribution

There are extensive areas of salt-affected soils on all the continents but their extent and distribution has not been studied in detail. In some countries even the existence of these soils was discovered only through a survey or the pressing demand for agricultural utilization of a region. A first attempt to compile information on the extent of salt-affected soils on a worldwide basis was made by F. Massoud based on the FAO/Unesco Soil Map of the World; information in Table 4 is based on this study. Information in respect of countries in Europe is based on publications by Szabolcs (1974, 1979). Szabolcs (1979) has also presented maps showing the distribution of salt-affected soils in most continents. Any attempt to increase food production in coming years must pay adequate attention to the improvement of existing salt affected soils with little or no production and to prevent further deterioration of productive soils through these degradation processes.

Table 4 WORLD DISTRIBUTION OF SALT-AFFECTED AREAS

Continent	Country	Area, 1 000 ha		Total
		Saline/Solonchaks	Sodic/Solonetz
North America	Canada	264	6 974	7 238
	USA	5 927	2 590	8 517
Mexico and Central America	Cuba	316	-	316
	Mexico	1 649	-	1 649
South America	Argentina	32 473	53 139	85 612
	Bolivia	5 233	716	5 949
	Brazil	4 141	362	4 503
	Chile	5 000	3 642	8 642
	Colombia	907	-	907
	Ecuador	387	-	387
	Paraguay	20 008	1 894	21 902
	Peru	21	-	21
	Venezuela	1 240	-	1 240
Africa	Afars and Issas	1 741	-	1 741
	Algeria	3 021	129	3 150
	Angola	440	86	526
	Botswana	5 009	670	5 679
	Chad	2 417	5 850	8 267
	Egypt	7 360	-	7 360
	Ethiopia	10 608	425	11 033
	Gambia	150	-	150
	Ghana	200	118	318
	Guinea	525	-	525
	Guinea-Bissau	194	-	194
	Kenya	4 410	448	4 858
	Liberia	362	44	406
	Libyan Arab Jamahiriya	2 457	-	2 457
	Madagascar	37	1 287	1 324
	Mali	2 770	-	2 770
	Mauritania	640	-	640
	Morocco	1 148	-	1 148
	Namibia	562	1 751	2 313
	Niger	-	1 389	1 389
	Nigeria	665	5 837	6 502
	Rhodesia	-	26	26
	Senegal	765	-	765
	Sierra Leone	307	-	307
	Somalia	1 569	4 033	5 602
	Sudan	2 138	2 736	4 874
	Tunisia	990	-	990
	United Rep. of Cameroon	-	671	671
	United Rep. of Tanzania	2 954	583	3 537
	Zaire	53	-	53
	Zambia	-	863	863
South Asia	Afghanistan	3 103	-	3 101
	Bangladesh	2 479	538	3 017
	Burma	634	-	634
	India	23 222	574	23 796
	Iran	26 399	686	27 085
	Iraq	6 726	-	6 726
	Israel	28	-	28
	Jordan	180	-	180
	Kuwait	209	-	209
	Muscat and Oman	290	-	290
	Pakistan	10 456	-	10 456
	Qatar	225	-	225
	Sarawak	1 538	-	1 538
	Saudi Arabia	6 002	-	6 002
	Sri Lanka	200	-	200
	Syrian Arab Rep.	532	-	532
	United Arab Emirates	1 089	-	1 089
North and Central Asia	China	36 221	437	36 658
	Mongolia	4 070	-	4 070
	USSR	51 092	119 628	170 720
South-East Asia	Democratic Kampuchea	1 291	-	1 291
	Indonesia	13 213	-	13 213
	Malaysia	3 040	-	3 040
	Socialist Rep. of Vietnam	983	-	983
	Thailand	1 456	-	1 456
Australasia	Australia	17 269	339 971	357 240
	Fiji	90	-	90
	Solomon Islands	238	-	238

Source: Massoud, 1977

Source: Szabolcs, 1974