This is one of those things every knows but no one can tell you why. Everyone will tell you rain makes a garden grow. But I had to find out why.
There was no information to be found on rain water in Houston. If you are from another city you may or may not find some useful information online. I purchased an aquarium water test kit at the local per store and used it on the water.
Here in The Woodlands, and I imagine most of Houston, our tap water has a high kH. kH is the carbonate hardness of water. The higher the kH the more difficult it is to change the pH of the water and the more difficult it is to absorb or neutralize acid. Water kH can be increased by adding baking soda or reduced by adding CO2 ( you can purchase tabs to drop in water at the pet store ).
For your aquarium and pond plants a high kH is good. Over time this releases CO2 to the water which benefits the plants. When I switched from tap water to bottled in the aquarium the plants all died.
The pH of the tap water was 8.5 when I measured it, and the rain water was 6.5, which is where the bottled water I tested came in. pH is very important to plants. pH is a measure of the hydrogen in your soil. The scale runs from 0 to 14, 0-7 being acidic, 7 is neutral, and 7-14 is basic. You really want your garden soil and water pH to be between 5 and 7. More or less than that and most plants will have trouble getting the nutrients that are in the ground. This is why everything greens up after a rain. It is much easier for the plant to use the nutrients that are in the ground at that pH. At the pH of our tap water it is difficult for plants to get there nutrients. The nutrients may be there, but they can’t get in.
So is rain or tap water better for your plants? Both are, at least locally.
Some plants may be extremely sensitive to the fluoride and chlorine in tap water. You’ll know by the edges of the leaves or tips turning brown. Usually just a very thin brown edge and usually just on houseplants. If you have a plant that is sensitive to chemicals, cut your tap water with some bottled water when you water it.
You can help your soil pH by adding peat moss to your soil each year ( see article below ), but you are not likely to change it long term. You’ll see your plants leaves turn yellow while the veins stay green when the plant is unable to take up nutrients. Adding fertilizer, especially one you can spray onto the plant will help. Iron added to the soil around the plant also will help.
Though I am sure there are many other interesting differences between rain and tap water here in Houston, I’m still looking for information. Everything else I’ve been able to test came out the same for both.
pH soil and plants