Substances of Concern in Drainage Salts Salt management is an inevitable chore of irrigated agriculture. Salts occur naturally as rock weathers and forms soil. While California's rainfall is enough to dissolve bits of rock into salts, it's sometimes not enough to remove them, so they build up in the soil. Irrigation causes still more salt accumulation, because surface water diverted to irrigate fields already contains salts in solution. The CVP and SWP import about 1,600,000 tons of dissolved salts into the San Joaquin Valley every year.
Salts accumulating in the root zone make it harder for a plant to absorb the water it needs.
Beyond a certain point, which varies for different species, the plant wilts and dies. To prevent this, farmers must apply enough extra water to carry the salt in solution down below the root zone. It is estimated that a million acres of waterlogged high salinity soils will soon require drainage. Fertilizers Agricultural drainage water contains some nitrogen compounds from manmade fertilizers. Crops rarely absorb more than half these nutrients.