Answer: Salt
Explanation: Salination can be defined as the increase in the salt concentration in the soil. Eventually it becomes toxic to the plants when it reaches the concentration of 3,000 - 6,000 ppm.
It decreases the osmotic potential of soil as a result of this plants cannot get water from the soil.
The salt in the soil comes from the irrigation as the water which is used to irrigate the field contains salts in it. As the water evaporates the concentration of salt in the soil increases.