Soil salinity refers to presence of salts in root zone of the soil. It adversely affects the growth of plants. General symptoms of salinity in plants are retarded growth and plants with fewer and smaller leaves that are darker green than normal leaves.
The soils in which concentration of salts is very high are called salt-affected soils or thur soils. Salt-affected is a collective term which includes saline, sodic and saline-sodic soils.
Salt affected soils occur commonly in arid and semiarid regions where annual rainfall is less than evapotranspiration. However, these also occur in sub-humid and humid regions.
The main soluble salts that occur in salt-affected soils are sodium, calcium, magnesium, chloride and sulphate ions. In addition, potassium, carbonate, nitrate, borate and lithium salts are also present in smaller amounts.
The salt-affected soils may be:
Primary Salt-affected Soils: The soils that develop as a result of causes (imperfect drainage of surface and ground waters) are caned primary salt-affected Solis or fossil saline soils.
Secondary Salt-affected Soils: The soils that develop due to accumulation of salts as a result of rise in ground water table or under faulty systems of irrigation are called Secondary salt-affected soils.
The salt-affected soils are formed by the following processes:
Salination: Salination is the process of accumulation of soluble salts by which saline soils are formed. The process of leaching of soluble salts out of the root zone is called desalination.
Sodication: Sodication is the process of accumulation of exchangeable sodium in the soils which results in the formation of sodic soils. The process of removal of exchangeable sodium from the soil is known as desodication.
About 573 million hectares (mha) in Pakistan are salt-affected. About 80% of the salt-affected soils of the Punjab and 56% of those of Pakistan are as a whole saline-sodic.
Causes of Salinity in Pakistan
The major causes of salinity in Pakistan is poor leaching of salts in arid and semiarid regions, irrigation water, ground water and other factors.
Poor Leaching of Salts
The original source of salts found in salt-affected soils is exposed rocks and minerals of earth’s crust. During weathering of rocks and minerals, large quantities of salts are brought into solution. These salts are generally carried downward by rain-water into the ground water and ultimately reach the ocean. In arid and semiarid climates, however, evapotranspiration exceeds rainfall in most years, and the rainfall is not enough to leach the salts out of the root zone. Thus, these salts concentrate in the soil and ground water causing salinity.
Irrigation water is an important source of salts and therefore salinity. Although concentration of salts in river waters of Pakistan is generally low, however they make a significant contribution to the amount of salts in irrigated soils, especially when drainage is poor. It has been observed that in areas where ground water table is not shallow, irrigation water is the main source of salts in the formation of secondary salt-affected soils. Saline-sodic ground waters used for irrigation in SCARP (Salinity Control and Reclamation Project) areas are a source of soluble salts and exchangeable sodium.
Insufficient or unequal application of irrigation water also results in accumulation of salts in the soil, since it causes rise in water table.
Under water logged and shallow water-table conditions in arid regions, ground water may be a significant source of salts in the development of secondary salt-affee•ed s quality of ground water in Pakistan is quite variable from place to place depend»eg rainfall history, distance from rivers, and nearness to sources of salts like the Sat Punjab and ocean in Sindh and Bafochistan. In general, the concentration Of s’ats water increases as the distance from a nver increases, Similarly the concentrat•n c the ground weter increases down slope within each doab.
Other relatively minor sources of safts are flood water, inland deposits, sea sprays, sea-water intrusion, salt-loving vegetation, industrial sewage water, Sea-water Intrusion is important along the coastal areas.
Types of Saline Soils
The salt-affected soils in Pakistan are classified on the basis •of:
the appearance of soluble salt incrustation on the soil surface,
on the percentage of water-soluble salts in the soil,
by determining the electrical conductivity of saturation extract (saturation extract is a measure of quantity of soluble salts present), and
ESP (exchangeable sodium percentage) or SAR (sodium adsorption ratio),
The last two are most common basis of classification of salt-affected soils.
on the basis of above mentioned criteria three types of salt-affected soils are recognized. These are: saline soils, sodic soils and saline-sodic soils.
The soils containing sufficient amounts of soluble salts that hinder the germination and growth of most crop plants are called saline soils. Locally such soils are called thur.
The saline soils have following characteristics:
white salt encrustation is present on the surface, therefore these are also called white alkali soils.
ese soils have electrical conductivity of the saturation extract (ECe) of 4 or greater than 4 decisiemens per meter a 25 “C, and an ESP (exchangeable sodium ercentage) of less than@The p o saturated paste (pHs) is usually less then 8.5.
Chlorides and sulphates are the principal anions.
Sodium is not the dominant soluble cation in these soils and seldom comprise more than 50% of the soluble cations.
The saline soils are flocculated and their permeability is greater than or equal to that of normal soils.
The saline soils are highly hygroscopic because of presence of Calcium chloride and Magnesium chloride, therefore they absorb atmospheric moisture and their soil surface is moist.
The saline soils are usually devoid of vegetation or crop plants growing in it exhib:t stunted growth.
The saline soils can be reclaimed b establishing adequate drainage and leaching with water.
The soils that contain sufficient exchangeable sodium ions which affect the;r Properties and plant growth are called sodic soils.
The sodic soils exhibit following characteristics:
The ESP (exchangeab\e sodium percentage) of sodtc sot(s is uctivity of saturation effect is less thon •l dSm- pH ECe electricai
The sods are poor in drainage and aeration because of their dispersed nature
The orgarnc matter is dispersed and dissolved in highly alkaline soils and deposited on the sod surface after the evaporation of soil solution. This causes the darkening of the sod surface, therefore these soils are also called
It is difficult to maintain soil tilth favourable for seed germination, seedling, and plant growth in sodic soils. It is also difficult to replenish water supply the root zone by irrigation.
The soils can be reclaimed by application of amendments (chemicals).
The soils that contain both soluble salts and exchangeable sodium in quantity are called saline-sodic soils. Locally such soils are called thur bara. Such soils result of both the processes of saiination and sodication.
The saline-sodic soils possess the following characteristics:
They have an ESP (exchangeable sodiu electrical conductivity of saturation effect f 4dS m-l. The pH of the soil dependSon the dominance of either soluble salts ess an 8.5) or exchangeable sodium (more than 8.5).
Soluble salts in the soils affect plant growth directly while exchangeable sodium affects it both indirectly by degrading soil properties and directly through the adverse effects of exchangeable sodium on plants.
The saline-sodic soils can be reclaimed by application of amendments like gypsum