Question

why does the salinity of soil increases

Answer 1

Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salination can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean.

Answer 2

because Na+ increases in the soil
it occurs when enough salts accumulate in the root zone