Soil, Salts and Salinity
All of the soil on earth began as rocks. The rocks broke down through the natural process of erosion by wind and water, and the sand they formed became fertile as carbon-based life forms decayed upon it. The rocks that formed the earth contained the raw materials for salts---sodium, calcium, magnesium and other elements. They combined with water and airborne elements---chlorides, phosphates, sulfates and other compounds---as earth evolved from a nitrogen-based to an oxygen-based atmosphere. Depending on the geologic history of an area, these salts settled in the water tables, rock layers and topsoil that covered them all.
Soils that contain high levels of salt or sodium can slow the germination and growth of plants. They provide "overdoses" of nitrogen, causing discoloration or even the death of vegetation.
Salts were first transported by the Ice Age glaciers as they ground their way across the continents. Rocks were ground into sand but, more importantly, were moved by the tumultuous glaciers as they scooped up rock, soil and water and deposited them thousands of miles from their origin. Many salts were washed from the soil by rain and runoff to concentrate in the oceans, where the lighter water molecules evaporated to form clouds. Salt from the oceans was periodically carried inland by storms and changing ocean levels over millennia, forming "salt marshes" and changing the course of rivers, depositing salts along their banks.
More Time, More Salts
As the earth aged, more salt compounds formed and were carried by the filtration of water through the "aquifer," an area of soil and rock, downward into water tables, where it was held until it returned to the surface via springs, seepage or wells. Humans consumed some of this water but also used it for crop irrigation, adding to the movement of salts through waterways and soils. High concentrations of salts without enough precipitation to flush them through the soil form salt flats in desert areas. In river water used to irrigate, though, the salts created soil salinization and the formation of new salts in the aquifer, including formation of limestone, a sedimentary rock composed of calcium carbonate.
Fertilizers containing nitrate and phosphate salts added to soil salinization and poor drainage or seepage from water tables can cause "sodic" soils---soils with high concentrations of sodium. High concentrations of salts or sodium in farming areas can destroy an agricultural economy.