I want to ask :why salinization is usually find in desert ?

Salination is a natural process that results from:
* high levels of salt in the soils
* landscape features that allow salts to become mobile (movement of water table)
* climatic trends that favor accumulation

In some areas (for example in Australia), salinity is an inherent situation.
I think this is the case of why deserts have salinization. The preliminary requirement is the amount of salt IN the UNDERGROUND WATER and when times goes by, water evaporates and the concentraion of salt will be higher.

The concentration of salts, due to repeated capillary rise and evaporation followed by repeated IRRIGATION and LEACHING, is bound to increase downstream. If irrigation and leaching continues, it will increase soil errosion rate and desertification rate. (e.g.South Great Plans of North America)

One of the best examples of excess salination was observed in Egypt in 1970 when the Aswan High Dam was built. The change in the level of ground water before the construction had enabled soil erosion, which led to high concentration of salts in the water table. After the construction, the continuous high level of the water table led to the salination of the arable land.

I think the links below can clearly show how and where's salinization. http://www.edwardgoldsmith.com/page158.html
http://en.wikipedia.org/wiki/Salinization