why is soil more saline in summer ?
The distinguishing characteristic of saline soils from the agricultural standpoint, is that they contain sufficient neutral soluble salts to adversely affect the growth of most crop plants. For purposes of definition, saline soils are those which have an electrical conductivity of the saturation soil extract of more than 4 dS/m at 25°C (Richards 1954). This value is generally used the world over although the terminology committee of the Soil Science Society of America has lowered the boundary between saline and non-saline soils to 2 dS/m in the saturation extract. Soluble salts most commonly present are the chlorides and sulphates of sodium, calcium and magnesium. Nitrates may be present in appreciable quantities only rarely. Sodium and chloride are by far the most dominant ions, particularly in highly saline soils, although calcium and magnesium are usually present in sufficient quantities to meet the nutritional needs of crops. Many saline soils contain appreciable quantities of gypsum (CaSO4, 2H2O) in the profile. Soluble carbonates are always absent. The pH value of the saturated soil paste is always less than 8.2 and more often near neutrality (Abrol et al., 1980). Physico-chemical characteristics in respect of a few typical saline soil profiles are presented in Tables 5-8.
Excess salts keep the clay in saline soils in a flocculated state so that these soils generally have good physical properties. Structure is generally good and tillage characteristics and permeability to water are even better than those of non-saline soils. However, when leached with a low salt water, some saline soils tend to disperse resulting in low permeability to water and air, particularly when the soils are heavy clays. Leaching may also result in a slight increase in soil pH due to lowering of salt concentration but saline soils, as will be shown later, rarely become strongly sodic upon leaching if there is an adequate drainage system.
In field conditions, saline soils can be recognized by the spotty growth of crops and often by the presence of white salt crusts on the surface. When the salt problem is only mild, growing plants often have a blue-green tinge. Barren spots and stunted plants may appear in cereal or forage crops growing on saline areas. The extent and frequency of bare spots is often an indication of the concentration of salts in the soil. If the salinity level is not sufficiently high to cause barren spots, the crop appearance may be irregular in vegetative vigour.
Moderate salinity, however, particularly if it tends to be uniform throughout the field, can often go undetected because it causes no apparent injuries other than restricted growth. Leaves of plants growing in salt infested areas may be smaller and darker blue-green in colour than the normal leaves. Increased succulence often results from salinity, particularly if the concentration of chloride ions in the soil solution is high. Plants in salt-affected soils often have the same appearance as plants growing under moisture stress (drought) conditions although the wilting of plants is far less prevalent because the osmotic potential of the soil solution usually changes gradually and plants adjust their internal salt content sufficiently to maintain turgor and avoid wilting.
Symptoms of specific element toxicities, such as marginal or tip burn of leaves, occur as a rule only in woody plants. Chloride and sodium ions and boron are the elements most usually associated with toxic symptoms. Non-woody species may often accumulate as much or more of these elements in their leaves without showing apparent damage as do the woody species.