Chloride in soil: From nutrient to soil pollutant

Chlorine is important for the fertility of soils being a nutrient for crops. However, anthropogenic activities often lead to excessive accumulation of the anion of chlorine, (Cl-) in soils, either directly by applying animal wastes that are usually rich in Cl- or via atmospheric depositions from industrial and municipal sources. An overabundance of Cl- in the environment can impair agricultural land utilization by decreasing soil fertility and by causing crop toxicities. Thus, contamination of soils with Cl- must be avoided and the fertility of already contaminated soils should be restored. Avoidance requires an understanding of the anthropogenic sources of Cl-. In particular, Cl- depositions derived from fertilization, coal burning, or combustion of burnable waste contribute to the contamination of soils. Amelioration requires a range of measures. Suggested are (1) increase of anion exchange capacity in the root zone by incorporation of natural or chemical amendments with high anion exchange capacity, (2) leaching of excess soluble Cl- from the root zone by ponding, (3) phytoremediation using Cl--hyperaccumulators, or (4) providing readily available resources of macronutrient anions for displacing Cl-. It is anticipated that applying these measures could restore the potential for agricultural plant production. Further research is needed into the exploration of crops that can resist high concentrations of Cl- and into approval of novel amendments that can be incorporated into the soil for exchanging Cl-.