Question of the Week
I've heard rainwater has a lower pH than municipal water. Would this be an advantage in a rainwater-collection system?
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Answer: Rainwater starts out close to a neutral pH of 7. However, even the amounts of naturally occurring carbon dioxide (CO2) in the atmosphere lower the pH of rainwater. So rainwater has a natural pH of 5.4 to 5.7. With the increased amounts of manmade CO2 in the atmosphere, rainwater, especially in the industrial eastern United States, can have an even lower pH (referred to as acid rain). Rainwater can also pick up other pollutants like sulfur dioxide and nitrogen oxide as it falls. The amounts of contaminates and ranges of pH in your rainwater depend on location, but the rainwater you collect will almost certainly have a lower pH than the city-supplied water.
There is no definitive answer to the question of whether rainwater is better for the plants than city water. City water contains chlorine and sometimes fluoride. Chlorine will dissipate but may have an effect on soil microbes before it does, which would affect the nutrient availability to plants. Fluoride can build up in the soil and can be taken up by plants in small amounts. This last point is more of a concern about safe levels of fluoride for human consumption than about plant nutrient uptake.
Nutrient uptake in plants is mostly determined by organic matter content of the soil, microbial life in the soil, and the pH of the soil. Since acid rainwater can lower the pH and, in turn, the microbial life of the soil, it can have a negative effect on plant nutrient uptake. On the other hand, urban soils tend to have a high pH because of all the concrete used in urban development, so acid rainwater may help balance the pH of your urban soil. Also, rainwater may contain dissolved nitrogen that can help with increased nitrogen availability to plants. Good-quality handheld pH testers are available to take the guesswork out of soil pH. You can and should test your pH levels and adjust with lime for acid soils and with organic matter such as peat moss for a soil pH that is too high.
More important than the question of what type of water to irrigate with is whether you have enough water to irrigate with. Soil moisture is the biggest factor in plant nutrient uptake. If you irrigate with rainwater, do you have enough surface area and storage capacity to collect and store a sufficient amount to do the job? You can fix a pH problem, but if soil is too dry the microbial life shuts down, nutrients become immobile, plant roots die and plant growth stops. If you decide to go with a rainwater system, design and capacity is important, as well as a city water backup if it doesn't rain enough.
For more information, see ATTRA's Starting a Farm in the City at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=21.