Salinity is a term used to describe a condition where there are high concentrations of salt in the soil. This condition can occur as a result of over watering which causes the water table (also referred to as the ground water) to rise and with it the natural salts in the soil.
During the summer months when the water table recedes the salts are left behind creating salinity.
It should be noted that often the salts accumulate at the base of a plant’s root system creating a toxic growing environment. The salts accumulate at the base of the plants’ root systems because its the roots of the plants that draw the water to feed the plant. And it is the water that is carrying the salts. The plant’s root system only takes the water while the salts remain in the soil.
Toxic Land Check's Blog 
February 13, 2010 at 3:28 pm
[…] Some plants are susceptible to certain chemicals. Substances that are poisonous to plants are called Phytotoxic. But what is phytotoxic to one plant may not necessarily affect another. For example small amounts of salt can adversely effect citrus trees and have minimal affect on say, eucalyptus trees (this can be a huge issue for fruit growers in arid countries such as Spain, Italy and Australia and is referred to as salinity). […]