In humid areas, salinity from subsurface intrusion may occur near bodies of saltwater or where brackish waters are used for supplemental irrigation. Soluble salts present in treated sewage effluent are also a potential salinity hazard. Any remaining soil water that percolates downward to the groundwater eventually reaches a stream or reappears at the soil surface, depending upon topography and geology. With each succeeding use of streamwater for irrigation, the water becomes more saline. Percolating groundwater may dissolve additional salts from the substrata as it slowly moves toward a stream.
Salts accumulate in the plant root zone as soil water is depleted by evapotranspiration.
Drainage, either natural or man-made, must be adequate to allow leaching of excess salts from the root zone. If salinity is higher at the soil surface, that means either the water table is very shallow or the crop is severely under-irrigated. Where corn is grown with only natural precipitation, the water entering the soil profile is essentially salt-free. As that water moves down through the profile, it dissolves the soluble salts that are present in the soil as a result of natural weathering processes, and thus the downward-moving water becomes more salty with depth. For all crops, including corn, roots extract water and leave most of the salts behind, increasing the salt concentration in the remaining soil solution.