Chlorine and/or chloramine is added to municipal water systems to kill infectious agents, but when the water is used for irrigation, the chlorine/chloramine kills some of the beneficial micro-organisms in the “rhizosphere”, the area surrounding the roots. To protect the micro-life in the soil, remove the chlorine/chloramine from the water.
In the past water was treated with a chlorine compound that had a characteristic odor and taste. This chlorine evaporates when the water sits for a day or two. Now most water systems use “chloramine”, which does not evaporate. The chloramine has to be removed.
Aquarium hobbyist face the same problem because the chlorine and/or chloramine also affects the fish. They use an additive (available at pet supply stores) that removes not just the chlorine and chloramine but the ammonia which is produced by the chemical reaction. Chloramine can also be removed from water by adding 1 gram of vitamin C (ascorbic acid) to 75 gallons of water. Adding the ascorbic acid to the water is safe for both the plants and you.