Excessive irrigation can lead to soil salinity in a few ways. First, if water evaporates from the soil surface faster than it can percolate down to the roots of plants, then dissolved salts are left behind and accumulate at the soil surface. Second, if irrigation water contains high concentrations of dissolved salts, these can be transported into the root zone where they can accumulate and lead to soil salinity. Finally, if irrigation water is applied more frequently than necessary, it can leach soluble nutrients from the soil, leaving behind salts that can accumulate and lead to soil salinity.