Saline Arid Zone: Definition and Formation

After reading this article you will learn about the definition and formation of saline arid zone.

Definitions of Saline Arid Zone:

1. The soils containing excess soluble salts, generally chlorides and sulphates of sodium, magnesium and calcium (excluding gypsum) that adversely affect the crop growth are called saline.

The main causes of saline soil development can be attributed to the:

i. Presence of native saline ground water.

ii. Saline water irrigation.

iii. Rise of water table as a result of blocked drainage.

iv. Limited rain water reaches in saline zones.

v. Shortage of fresh quality water to meet the leaching requirements.

2. Soils having excessive accumulation of natural electrolytes (mainly chlorides and sulphates of sodium, magnesium, calcium and potassium excluding gypsum) enough to interfere with the normal growth of majority of crop plants are called saline.

Formation of Saline Arid Zone:

Saline desert may be formed by the individual or combined action of lie following processes:

1. By Irrigation:

During irrigation the well continue to work, he ground-water that is brought to the surface is likely to be saltier than the surface water. It is a perverse fact of nature that water will evaporate while .alt will not. The result is that by irrigation of this saltier ground-water the soil become increasingly saline. Because in desert regions the potential evaporation is always greater than the precipitation.

The more spectacular manifestations of salinity usually arise when a saline groundwater approaches the surface, and this condition is often ­consequence of irrigation.

The extent of salt additions due to irrigation, apart I from the consequences of rising the water table, can be illustrated by considering a model area irrigated for 100 years with water containing 300 parts per million (ppm) of soluble salts, the annual application being one meter of water.

In this case irrigation would add 2-7 per cent of salt to the upper one meter of soil. It might well be that the precipitation in the soil of insoluble salts would reduce the addition to half this figure, but long continued irrigation with almost any irrigation water will clearly lead to salinization if there is no removal.

Such additions are sufficient to account for much of the salinity of ancient irrigated areas without it being necessary to postulate pre-existing accumulations or weathering within the profile.

2. By Rain-Water:

The amount of salt which may be added to soil is through rain-water. There are good reasons for believing that the origin of salt in rain is from the sea. In Europe the chloride content of rain water decreased rapidly away from the coast.

Drischel’ observed 6 to 13 mg of chloride per litre at a distance of 5 to 10 km from the coast. This is equivalent to an annual addition of 0 025 per cent of sodium chloride to the upper 10 cm of soil with a rainfall of 370 mm.

3. By Aeolian processes:

Saline soil may also form by Aeolian processes when salt dust develops at the seashore and is blown into the- interior of the land. Aerial movement of salts also takes place when wind crosses salt-encrusted desert surfaces.

It has been calculated that some 100,000 tons of salt were transported annually by wind from salty exposures of the Rann of Kutch near the Indus delta to Rajasthan. Other examples of saline soils formation by aeoline process are outer region of the Namif desert and salt pans in the arid part of Western Australia.

Ecological factors effecting the amount of salt spray along the coasts of are:

1. Foam-formation on the tops of the waves;

2. The strength and frequency of winds sweeping inland from the sea;

3. The topography of the coastal area;

4. Dune areas-main dunes, the dune pattern, etc.

5. The conditions for evaporation-wind velocity, humidity, air, water, temperatures, size of drops, insolation exposure, etc.

6. The amount, the frequency and the intensity of precipitation;

7. The physical structure of coastal soils;

8. The sail content of sea-water.

4. By Murine sedimentary rocks:

The brakishness of soils cur where marine sedimentary rocks (Jurassic, Cretaceous or Tertiary) posed. Often efflorescence of salt is seen in certain layers of these. This salt is originated from the sea water and became included into the luring sedimentation of the sea bed.

During weathering these salts are exposed at the soil surface and the small amount of precipitation wash the into un-drained depressions. The evaporation over an extended period of causes a considerable accumulation of salt and thus saline pans are formed.

5. By Capillary action:

The brackish soil can develop in desert with time when the salt content of the ground water is low. This may med, if the ground water level is high enough to moisture the soil e by capillary action and if water evaporates continuously. This type of capillarity is often climate dependent.

6. From previous sea bed or big lake:

Saline soil can also develop by slowly drying out of a previous sea bed or big lake, when finally Dune large or several small salt lakes remain, for example, Casparian depression.

7. Chloride-sulphate brackening:

In the deserts and semi- s where sedimentary rocks occur, highly soluble chlorides and sulphates accumulate in the ground water of the un-drained area. In all depressions where the ground-water reaches the soil surface by capillary action, a white crust develops. This process is known as chloride-sulphate brackening.

With regard to saline arid zone formation there are areas where salt accumulation occurs (geological speaking), temporarily on the soil or more frequently – in the soil with a greater speed that the global or coastal salt circulation could equilibrate.

Kovda (1961) distinguished the following types in the alien of saline soils:

1. Continental cycles – in inland regions that have no run off.

2. Delta cycles – where accumulation of salt carried by rivers from the inland, alternates with accumulation of salts carried in from the sea by the tides.

3. Marine cycles – on coastal plains of dry lands and along the shores of shallow bays.

4. Artesian cycles – Carrying the salts upwards from great depths.

5. Anthropogenic cycles – salinization by irrigation etc.

These types of saline soil formation may be temporarily restricted but they too are integral parts of Global salt circulation (Fig. 24-1).

The Global salt circulation (or to be more exact, global NaCl and MgCl₂ circulation) constitutes a specific and distinct part of the well-known general mineral cycle. It is best comparable in its movement with the global water circulation and not with that of the other minerals because a major part of the circulation of the other minerals depends on the movement of the earth crust (elevation of sediments, mountain building etc.) and the subsequent erosion.

The speed of the movement of NaCl or MgCl₂ in solution back to the ocean is approximately the same as that of water and certainly not of the same order of time that is required to level mountains by wind and water erosion.

The formation of inland saline arid zone areas is dependent upon a number of factors. Some of these also operate in maritime regions. These factors may be following:

i. Precipitation:

The magnitude of this factor will determine the degree of leaching, the depth location of the layer of salt accumulation and the need or otherwise for irrigation.

ii. The Vegetation:

With decrease of vegetation cover, surface evaporation will tend to increase, and in the summer very high salt concentrations will be reached in the soil surface layers and frequently the phenomenon of salt efflorescence can be observed.

iii. Slope of ground:

This will determine the drainage pattern and the soil will be more saline towards the lower levels where the water accumulates.

iv. Depth of soil water-table:

In general the nearer this is to the surface, the more constant will be the soil salinity.

v. The depth of the salt deposit:

The nearer this is to the surface, the more saline will be the surface layers unless there are periods of heavy precipitation.

vi. Temperature:

This is a factor of profound important in inland regions. High summer temperatures common in the centres of continents bring about excessive summer salinities due to high rates of evaporation.