The pH of tap water and rain water

Somehow, it just seems instinctively ‘right’ that rain water is the best water for the garden. And, by and large, that is completely true. Keeping to an instinctual level, it’s easy to comprehend that the soil and all its microorganisms are ‘used’ to rain water. Plants are used to it too, and provided nothing gets badly out of kilter things go along just fine.

One of the major differences between rain and tap water is (usually) their pH. Now, despite having studied soil science for years, I still find pH, acidity, alkalinity, basicity, conductivity and the various other terms relating to a soil’s chemistry pretty confusing. But sticking to pH, I was interested to know how the rain water here in north Wales compared to the tap water. So I checked them out with some pH papers.

pH paper test of rain water and tap water

It’s not proper science, but it’s still interesting to see that while the tap water is showing somewhere between pH7 and pH8*, the rain water is somewhere between pH5 and pH6. In a way, that seems like hardly any difference at all, but because the pH scale is logarithmic, each one point difference in the pH value is the same as a ten fold increase (or decrease) in the amount of hydrogen ions (which could be called acidity or alkalinity for simplicity, but that would upset a lot of people). Two points difference in pH is 100 times more (or fewer) H ions and so on. And to plants and soil organisms, that matters a lot.

During the recent dry spell, my collected rain water soon ran out and for nearly three weeks I’ve been watering with tap water. Mainly that hasn’t affected the plants (not visibly anyway) but some, particularly the blueberries (which love a low pH) show yellowing of the leaves very quickly when watered with tap water (not least because they’re really not at all well suited to the soil and weather here – but that’s a whole other story). That chlorosis can be  quickly remedied with a liquid feed aimed at ericaceous (low pH loving) plants applied to the leaves as well as the soil around the bushes: that will tide them over until the rains come back again.

pH strips can be bought for a few pennies on the internet – they’re great fun to play with and you can soon find yourself marvelling at the pH values of all kinds of household liquids:)

*Dwr Cymru – the local water supply company – provides full analyses for the water they supply: they’re freely available on their website. The actual average value for the water’s pH here is 7.622 – so that seems to indicate that the cheap little pH papers are giving a fair estimation.



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