The soil in which salt crust is left due to evaporation of water is called saline soil and this problem is called salinity. A saline soil has electrical conductivity of its saturation equal to or greater than 4m.mho per centimeter and its exchangeable sodium percentage is less than 15. Its pH is less than 8.5. Saline soil has a large amount of salts. But saline soil does not contain enough exchangeable sodium ions. These salts interfere in the germination of plants. Saline soil has white incursion on the surface of soil. It is called white alkali. The soil is well flocculated. It contains chlorides and sulphate of sodium, potassium and magnesium. Causes of Salinity
- The main reason which caused the salinity is geological ages. Most of the area now included in Pakistan was a vast sea in the past. It was called Tethys Sea. The sea changed its path. It left large quantity of salts. Then canal system was established in this area. The water table rises due to the seepage of water from canals. It brings out dissolved salts from the bed rocks. These salts caused salinity.
- Water seeps through the unlined canal banks and raises the water table. This water also contains dissolved salts of carbonate, sodium and potassium. Water is evaporated but salts are deposited on it. It causes salinity:
- Salinity can occur when the water table is between two to three meters from the surface of the soil. The salts from the groundwater are raised by capillary action to the surface of the soil. This occurs when groundwater is saline.
- The dissolved salts are also present in irrigated water. They also spread over the soil. Almost all water (other than natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil. They accumulate in the soil. Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must lie leached out of the plant root zone by applying additional water.
- Large amount of salts are already present in the alluvial soils. These slats dissolve in the rising water table. It has increased the problem of salinity.
Effects of salinity
There are following affects of water logging and salinity:
- Salinity destroys the soil structure. Different layers of the soil are badly affected.
- The growth of plants is very poor in saline soil. Fligh salt contents adversely affect the plant. Salinity reduces the seed germination and growth in many plants. It reduces yield. Saline soil prevents uptake of CC and mg– ions by plant. It causes deficiency of these ions in plants.
Reclamation of saline soil
1. Saline soil contains chlorides and sulphate of sodium and potassium. These salts are soluble in water. These salts can be leached out with the help of water. This water can be passed into drains. This process is called flushing. Flushing of soil after planting rice crop is very helpful in controlling salinity.
2. Chemicals like gypsum, sulphuric acid, sulphur and iron sulphate also help in control of salinity. Gypsum (CaSO4) reacts with sodium (Nat) to form Sodium sulphate (NaSO4). Sodium sulphate is leached out.
CaSO4 + Na p NaSO4 + Ca++
Sulphuric acid reacts with calcium carbonate to form calcium sulphate. It is leached out.
H2SO4 + CaCO3 CaSO4 + CO2
- The tube wells draw water from the land. They lower the water table. Thus salts are also flush down from the surface into the earth.
- Farm manure should be used at the rate of 10-20 tons per acre. This manure is composed garbage from the towns. It improves the soil structure porosity and nutrition.
- Soybean, spinach, cabbage, cauliflower, sweet potato, ginger, cucumber, pea sugar beet etc. can be grown in moderate saline soils.
- Some grasses like Sporobolus arabicus, Cynodon dactylon, Chloris barbata and herbs like Melilotus albus, Trifolium fagelliferum are grown in saline soils. Seshania hispinosa and Sesbania sesban (Jantar) is also beneficial in improving the soil. It adds nitrogen in it. Moreover, the deep roots of Sesbania bispinosa can also break the hard soil. The respiration of roots of Sesbania produces CO,. It reacts with calcium carbonate and produce calcium bicarbonate (CaHC01)2. It can exchange its calcium ion with sodium ions.
- Some botanists have put forward the biotic treatment of salinity. Itincludes growing salt accumulating plants. These plants are Suaeda fruticosa, Suseda monoica etc. These plants accumulate salts in crystalline and solution from in the cells of fleshy leaves. Thus it improves the soil.