WATER LOGGING

It is a situation when water become stagnant in fields....Water logged areas have very few spaces in which air is present...It means there is a lack of oxygen....This situation occurs (as is common for salinization) in poorly drained soils where water can't penetrate deeply. For example, there may be an impermeable clay layer below the soil. It also occurs on areas that are poorly drained topographically. What happens is that the irrigation water (and/or seepage from canals) eventually raises the water table in the ground -- the upper level of the groundwater -- from beneath. Growers don't generally realize that waterlogging is happening until it is too late -- tests for water in soil are apparently very expensive.
Both waterlogging and salinization could be reduced if the efficiency of irrigation systems could be improved, and more appropriate crops (less water hungry) could be grown in arid and semi-arid regions.


WATER SALINITY
Water salinity is the amount of salt contained in the water. It is also called the "salt concentration" and may be expressed in grams of salt per litre of water (grams/litre or g/l) (see Fig. 104), or in milligrams per litre (which is the same as parts per million, p.p.m). However, the salinity of both water and soil is easily measured by means of an electrical device. It is then expressed in terms of electrical conductivity: millimhos/cm or micromhos/cm.
Soil salinity

The salt concentration in the water extracted from a saturated soil (called saturation extract) defines the salinity of this soil. If this water contains less than 3 grams of salt per litre, the soil is said to be non saline.
Most crops do not grow well on soils that contain salts.

One reason is that salt causes a reduction in the rate and amount of water that the plant roots can take up from the soil. Also, some salts are toxic to plants when present in high concentration.


SODICITY
Salty soils usually contain several types of salt. One of these is sodium salt. Where the concentration of sodium salts is high relative to other types of salt, a sodic soil may develop. Sodic soils are characterized by a poor soil structure: they have a low infiltration rate, they are poorly aerated and difficult to cultivate. Thus, sodic soils adversely affect the plants' growth.

IMPROVEMENT OF SALINE SOILS
mprovement of a saline soil implies the reduction of the salt concentration of the soil to a level that is not harmful to the crops.

To that end, more water is applied to the field than is required for crop growth. This additional water infiltrates into the soil and percolates through the rootzone. During percolation, it takes up part of the salts in the soil and takes these along to deeper soil layers. In fact, the water washes the salts out of the rootzone. This washing process is called leaching.
The additional water required for leaching must be removed from the rootzone by means of a subsurface drainage system (Chapter 6). If not removed, it could cause a rise of the groundwater table which would bring the salts back into the rootzone. Thus, improvement of saline soils includes, essentially, leaching and sub-surface drainage.

IMPROVEMENT OF SODIC SOILS

Improvement of sodic soils implies the reduction of the amount of sodium present in the soil. This is done in two stages. Firstly, chemicals (such as gypsum), which are rich in calcium, are mixed with the soil; the calcium replaces the sodium. Then, the replaced sodium is leached from the rootzone by irrigation water.
Link: http://www.fao.org/docrep/r4082e/r4082e08.htm