You may not realise it but chlorine is classed as a plant micronutrient meaning that it is essential for the proper growth of plants. It is important to note though, that it is the ion (Cl‑) rather than the gas (Cl2) that is used by plants. In particular, chlorine is important for plant photosynthesis as it is involved in the opening and closing of stomata (pores in leaves that enable plants to take in and release carbon dioxide, oxygen and other gases as required. It also helps ensure leaves are firm.
It has only been fairly recently that the effects of chlorine deficiency have begun to be documented. Soil is unlikely to be deficient in chlorine in coastal areas (due to the large amounts of sodium chloride (salt) in the soil) but sandy, inland soil may become deficient in chlorine if rainfall is high or plants are irrigated too frequently.
Symptoms of Chlorine Deficiency
Chlorine deficiency results in blotchy leaf chlorosis and necrosis. Chlorine deficiency can be distinguished from other nutrient imbalances because leaves will exhibit characteristic distinct and abrupt boundaries between the affected and healthy tissue. Severe cases of chlorine deficiency may result in bronzing and/or wilting of leaves though these are not reliable symptoms.
Treating Chlorine Deficiency
As with any suspected nutrient imbalance, if you think your plants may be deficient in chlorine you should check that the soil is neutral with a pH of around 7 (unless the affected plant requires special pH conditions ‑ azaleas and blueberries require acidic soil for instance). It's always a good idea to also check that the soil isn't getting waterlogged or too dry, that it isn't compacted and that there is plenty of organic matter in it.
If symptoms persist, see if you can find a laboratory that will test your soil to determine the amount of chlorine in it. Not all labs will do this but it's worth hunting around for one that will so that you can be sure to correctly diagnose the problem. If the soil test shows that your soil is lacking chlorine, you can then use a chlorine containing fertiliser to amend the soil. Specialist fertiliser suppliers should be able to produce a custom fertiliser to meet your needs but if your soil has low to moderate levels of potassium, you may be able to use muriate of potash (potassium chloride or KCl) to increase the amount of both chlorine and potassium in the soil ‑ you just need to be careful that you don't end up with excess levels of potassium in the soil. Do not use table salt (sodium chloride or NaCl) to increase soil chlorine levels as excess sodium levels in soil can be detrimental to plant health.
Chlorine toxicity can occur naturally when plants are grow in coastal soils and near chlorinated pools (though much of the damage associated with chlorinated pools is due to the vapours coming into contact with leaves rather than because the plant has uptake too much chlorine).
Symptoms of Chlorine Toxicity
Chlorine toxicity will usually result in necrosis along leaf margins and smaller than usual leaves and plants. Symptoms are usually seen on older leaves first. In some species, chlorosis may also occur.
Treating Chlorine Toxicity
As always, your first port of call is the soil pH. You should ensure the pH of your soil is around 7. It's also a good idea to make sure your soil isn't compacted and that it drains freely. Plants may struggle to uptake some other
If your soil contains high levels of chlorine because you live on the coast, you may need to grow chlorine tolerant plant species. You could also grow plants in containers and regularly replace the potting mix when chlorine levels rise.