This is the level of natrium in a sample of water measured in mmol/l of mg/l

Sodium is commonly present in water, but only small quantities are taken up by plants. An excess of sodium causes salinity problems. If the sodium concentration in the root zone is too high, it will be detrimental to the crop. If Na accumulates above the maximum acceptable concentration in the root zone, growers must discharge a fraction of the recirculated nutrient solution in order to prevent yield reduction or a decline in product quality.

Sodium

Na