Question

how does excessive irrigation lead to soil salinity

Answer 1

Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and eventually begin to accumulate.

Answer 2

Most of the salts are water soluble.

Salts are present on the Earth in the form of Rocks and due to the activity of air, water, temperature, animals and natural processes, calamities and etc, disintegration of rock takes place and this disintegrated rock mixes or soluble in water while rain or water streams or river flow and etc. In this way, the salty rocks soluble in rocks

As we irrigate field with this water, the water percolates into the soil and after some days some water utilized by plants, some percolated into deep and some evaporated. In this processes, the salt again precipitated in solid form in the soil. Thus, the irrigation cause salt to accumulate in the Soil.