It seems strange to say there is a difference between one water source and another. But environmental factors and the source of each water supply influence the chemical makeup.
For the home gardener, it’s very important to know exactly what is in your water. The water you use, like all other inputs, should be as well suited to your plants’ needs as possible.
This begs the question, which water source is best? Tap, rain, or some other source?
Tap water is treated with chlorine or chloramine. Seasoned gardeners will tell you that tap water can be used after 24 hours because the chlorine will dissipate through evaporation. However, chloramine does not. Tap water can also contain fluoride, salts, and minerals that are harmful to the soil in heavy concentrations. This is described as “hard” water and it usually carries a higher pH than rainwater which can cause nutrient lockout, a condition where plants are unable to assimilate the nutrients that are present.
Rainwater, on the other hand, is usually “soft” water. That is to say that it does not naturally contain the minerals that are typically blamed for the hard water marks seen in bathroom sinks and bathtubs. The pH is typically more acidic, usually between 5 and 7, than that of tap water and much better suited for nutrient uptake by plants and better for the soil. Additionally, rainwater also contains dissolved nitrogen that plants love.
Rainwater is better suited to plant growth and development because of the ideal pH, lack of “hard” water minerals, additives, and conditioners, and the presence of beneficial nutrients. That’s not to say that you can’t use tap water in a pinch, but who doesn’t love a good rain shower every once in awhile!