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Accumulating salts are not just sodium (Na+) and chloride (Cl-) ions, but are also essential plant nutrients... calcium (Ca2+), magnesium (Mg2+), potassium (K+), ammonium (NH4+), hydrogen (H+), phosphate (HPO42-), sulfate (SO42-), and nitrate (NO3-) are all salts. Nitrate accumulation is particularly frequent in high tunnels as high-value crops are well fertilized, and since there is no rainfall to leach the nutrients, they accumulate year after year.
To learn more about soil salinity, how it happens, and what you can do about it, please read Perennia’s factsheet: http://www.perennia.ca/wp-content/uploads/2015/12/Soil-Salinity-NS-Hi-Tunnels.pdf