The following points highlight the three main types of salt affected soils. The types are: 1. Saline Soil 2. Sodic Soils 3. Saline-Sodic Soil.
Type # 1. Saline Soil:
Soil that contains sufficient soluble salts to impair its productivity is known as saline soils, also called as Solonchak. These salts are mainly of chlorides and sulphates of sodium, calcium and magnesium. In arid climatic regions, saline soils may have excessive amount of boron, fluoride as well as nitrates. Sometimes appreciable amount of sparingly soluble gypsum is also observed in some patches.
As per the US Salinity Laboratory Staff (1954), the distinguishing features of saline soils from agricultural stand point, is that they contain sufficient neutral soluble salts to adversely affect the growth of most crop plants. They must have an electrical conductivity of the saturated soil extract (ECe) of more than 4 dSm-1 at 25°C, pHs less than 8.5, and exchangeable sodium percentage (ESP) less than 15. But as per Gupta and Abrol (1990) saline soils are those having pHs less than 8.2, ECe more than 4 dSm -1 and the preponderance of chlorides and sulphates of Na, Ca and Mg. Soil salinity classes mainly based on their conductivity of saturation extract, are given in Table 3.3.
The process of formation of saline soils is known as salinisation. Salinisation is a natural process which occurs from weathering of minerals or from deposits of fossil salts. Low rainfall in arid and semi-arid climates is unable to leach down the soluble weathered products resulting in the deposition of these salts at the soil surface causing salinity hazards.
Some of the human activities accelerating the salinisation process are excessive use of basic fertilisers such as sodium nitrate, basic slag etc., removing native vegetation, growing shallow-rooted annuals and excessive irrigation increases leakage of salts to the groundwater system. Use of saline ground water for irrigation and rise of water table brings salt to the root zone and on the soil surface and have accelerated the process of salt mobilisation and accumulation.
Salinity is often classified into several different types based on the broad cause as described below:
Types of Salinity:
Salinity often classified in different categories like – dryland salinity, irrigation salinity, urban salinity and industrial salinity.
a. Dryland Salinity:
Dryland salinity occurs in all non-irrigated areas due to rising groundwater. It includes both naturally occurring salinity and salinity resulting from increased recharge and/or reduced discharge under dryland farming systems. Dryland salinity may also occur due to soil erosion causing exposure of saline sub-soils.
b. Irrigation Salinity:
Irrigation salinity occurs due to the application of large volumes of irrigation water and leakage from channels and storages.
c. Urban Salinity:
Urban salinity refers to areas of the built environment showing symptoms of salinity. In urban areas the increased recharge and rising groundwater are caused by activities such as clearing of vegetation for development, over-irrigation of gardens and public parks, inappropriate storm water discharge, disruption of natural drainage lines and leakage from water pipes and swimming pools.
d. Industrial Salinity:
Industrial salinity results from processes that accumulate and concentrate salt in industrial waste water. The sources of salt include urban effluent, agricultural chemicals and wastewater from mines and power stations. This saline water can be a pollutant if it is released or leaks into water courses or ground water inappropriately. These sources are often monitored to allow appropriate and timely action to minimise their effects.
e. River Salinity:
River salinity is caused by saline discharges from dryland, irrigation and urban salinity into creeks and rivers. Over time, as salinity within catchments worsens, the quality of river water declines, becoming more and more saline.
Extent of Dryland Salinity:
Dryland salinity is a serious problem faced by arid and semi-arid regions of the world. It is a critical management problem in Western Australia and in Great Plains of North America; extensively occurs in United States (particularly the states of Montana, North Dakota and South Dakota), Canada (the prairie provinces of Manitoba, Saskatchewan and Alberta).
It also occurs in India, Thailand, Afghanistan, Iran and South Africa. In India, dryland saline soils mainly prevail in arid and semi-arid regions of Rajasthan, Gujarat, Haryana, and Punjab states. Dryland salinity has been a threat to the land and water resources in several parts of the world although only in recent years has the seriousness of problem become widely recognised.
Development of Dryland Salinity:
Dryland salinity develops where the salt in the landscape/soil profile is mobilized and redistributed to soil surface by increasing the level of ground water table. The rise in water table levels in dryland areas is due to increase in the rate of leakage and ground water recharge. Native vegetation is effective at using most of the water entering the soil profile from rainfall, allowing only a small proportion to reach the groundwater system.
Dryland salinity occurs due to removal or loss of native vegetation, and its replacement with crops and pastures with shallow root system and introduction of plants with different water use requirements which lead to more water reaching the groundwater system. Current agricultural production systems greatly exceed the capacity of the groundwater systems to discharge the additional water to the rivers and streams. As the input to the groundwater exceeds the output, the water table rise. The groundwater rises to near the ground surface in low-lying areas or on the break of slope.
As the water table rises, it dissolves and moves salts in the soil profile towards the soil surface by capillary force. Capillary rise allows ground water to be drawn into the dry soil along with salts above the water table like water in a sponge. When ground water reaches at surface it may evaporate, leaving the salts behind. The salts that left behind concentrate and can be moved sideways by ground water in a process called lateral flow or up and down the soil profile. This salinisation process depends on many factors like rainfall, evaporation and vegetation cover etc.
Salinity Tolerance of Plants:
The relative tolerance of plants to salinity varies from crop to crop even variety to variety. It depends upon the physiology of the plants, rooting habit, growth stage, its ability to regulate the ionic imbalances, etc., also depends on external factors like climate, salt concentration in soil, nature of salts, their relative proportion, and their distribution in the profile. A list of crops according to their level of tolerance to salinity is presented in Table 3.4.
Type # 2. Sodic Soils:
Sodic soils are non-saline soil containing sufficient exchangeable sodium (Na) to adversely affect crop production and soil structure under most conditions of soil and plant type. Carbonates and bicarbonates of sodium are the dominant salts and the concentration of neutral salts is very low. Sparingly soluble gypsum is nearly absent in such soils.
The pH of the soils usually varies in between 8.5-10. 0, ECe is less than 4 dSm1, exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR) more than 15 and 13, respectively. However, under Indian conditions, alkali soils are distinguished as those soils having pH less than 8.2 and the ratio of [Na+]/([Cl– ]+[SO42- ]) in soil solution more than 1.0.
Earlier these soils are popularly known as black alkali soils. Under high pH, organic matter get dissolved and deposited on the soil surface forming a thin film over the soil particles. This happens when water evaporates through capillary action. These soils are mainly distributed in the arid and semi-arid regions of Punjab, Haryana, Uttar Pradesh, Bihar and Rajasthan.
Sodic soils are generally found in low lying areas with annual rainfall of 550 to 900 mm. As this amount of precipitation is insufficient to leach down the weathering products, sodium will become dominant cation which increases SAR (>13). These sodium ions replace calcium and magnesium ions on clay surfaces, hence, the solubility of calcium carbonate, calcium sulphate and magnesium carbonates limits and precipitates. As a result ESR increases (>15).
Problems of Soil Alkalinity:
Dominance of exchangeable sodium in the soil solution affects both the physical and chemical properties of soils.
a. Soil Structure Collapse:
Sodium dominated in alkali soils raises zeta potential and as a result fine clay particles get dispersed in soil solution. It causes collapse in soil structure and closing-off soil pores. As a result soil aggregates weaken, permeability of soil reduces, soil crusting and hardening of soil occurs upon drying.
These soils have poor aggregate stability, poor organic matter content and low microbial growth. Due to the presence of sodium carbonate and bicarbonate sodicity effect is more. Such soils also suffer from reduced availability of N, K, Zn, Fe etc.
b. Specific ion Effect:
Cations like calcium and magnesium are sometimes found to be deficient due to the presence of excess amount sodium in soil solution. This is termed as ‘specific ion effect’. It is believed to happen due to the interaction of soil exchange complex and the exchangeable cations in the soil solution.
In general sodic soils have poor physical properties due to high ESP.
Some of the major constraints of alkali soils are highlighted as follows:
(i) Toxic Effect to Plant Growth:
High sodium content of soils is often toxic to many plants which exhibit poor growth and yield in such types of soils. Toxic concentrations of CO32- and HCO3- have adverse effect on seedling emergence.
(ii) Concentration of Hydroxyl (OH–) Ions:
Hydroxyl ions have toxic effects on plant growth when present in excessive amount and usually such effects are noticed at soil pH more than 10.5.
Soil Alkalinity and Crop Growth:
With the increase in sodicity, soil physical properties are adversely affected and cause various problems to plant growth and development.
Such adverse effect on plants in sodic soils may be due to the following factors:
a. Dispersion of soil colloids result in formation of dense, impermeable and compact surface crusts that hinder the seedling emergence.
b. Restricted entry of air and water movement in such dispersed soils result in poor root growth.
c. Restricted internal drainage leads to water logging for temporary periods.
d. Exchangeable sodium affects the soil pH which in turn influences the plant growth. Although high pH of sodic soils has no direct adverse effect on plant growth per se, it frequently results in lowering the availability of some of the essential plant nutrients (P, Ca, N, Mg, Fe, Cu, and Zn).
e. Compaction of top soils and sub soils create problems for root penetration and soil tillage operation.
f. Sodium, molybdenum and boron present in toxic amounts in sodic soils causing injury to plants.
g. It will be better to mention that the adverse effects may be the result of interaction of more than one factor.
Type # 3. Saline-Sodic Soil:
Saline-sodic soil is defined as a soil having a conductivity of the saturation extract greater than 4 dSm-1 and an exchangeable sodium percentage (ESP) greater than 15. The pH is variable and usually above 8.5 depending on the relative amounts of exchangeable sodium and soluble salts. These soils form as a result of the combined processes of salinisation and alkalisation. If the excess soluble salts of these soils are leached downward, the properties of these soils may change markedly and become similar to those of sodic soil.