i) The difference between saline and sodic soils

In saline soils, sodium joins with chlorine to form a salt, NaCl. The presence of salt in the soils reduces the availability of water to plants and, at high enough concentrations, can kill them.
In sodic soils, much of the chlorine has been washed away, leaving behind the sodium ions attached to clay particles in the soil.
Soils are considered sodic when their exchangeable sodium percentage (ESP) is more than 6%. When wet, the clays in sodic soils lose their ability to stick together, leading to unstable soils which readily erode and become impermeable to both water and plant roots. They often have a severe surface crust.
Soils can be saline and sodic at the same time. The inherent sodicity problem will usually be masked by the obvious visual salinity symptoms.