pH stability of rainwater

I have a rainwater collection barrel that I have been using and the pH is around 7.2. I have been pHing the water and adjusting to around 6 every watering and feeding. My question is if I pH the whole rain barrel how long will it remain stable? It would be nice to just go and fill my watering utensil and water all 6 of my plants without having to pH everytime. For feedings I would have to pH because my nutes always lower my pH.


The high PH is not an issue for the plants , the issue is the Alkalinity of the water - the ability of the water to neutralize acid( carbonates,bicarbonates and etc … ) , and on the long run to mess with the medium PH . High PH can be indicator of high Alkalinity , but it doesnt need to be . You can test your water for alkalinity , if the levels are not high , a high PH will not affect the medium .
A lot people are not adjusting the PH if they are low on Alkalinity and you will not have problem . I will still suggest to adjust the PH if you use dry nutes for better mixing .

a) The PH has no to negligible effect to the medium PH , the ALKALINITY DOES
b) You need to worry about medium PH , the solution PH is just maintaining the medium PH
c) Mediums with High Cation Exchange Capacity are taking care of the PH for a long time even if the water is high in alkalinity
d) The right choice of fertilizer can fix the problem , N from ammonium and urea is lowering the PH
e) It will not hurt the plants to not adjust the PH if you have solid medium and low alkalinity as long the medium PH is fine , but in ideal scenario i will adjust the PH on every watering and feeding … dont be lazy :smiley:

1 Like

Sorry I just read that… and I feel I was walked in a circle that got me no where but back to the question @mayakid was asking… when you say Ph is not an issue it’s alkalinity then to say…

confuses me…


@Ladithief I’ll try to explain.

The alkalinity of your source water, how much CaCO3 is already dissolved in it, has an affect on how easy it is to adjust pH and how long the pH level will be maintained.

Water with more CaCO3 will require more acid to lower the pH and will change it’s pH back to it’s starting level more quickly.

Water with less CaCO3 will require less acid to lower the pH and its pH level will be more stable once adjusted.

Fertilizer with ammonium will drop the pH in the root zone. Most hydroponic nutrients use calcium nitrate with a small amount of ammonium nitrate to keep pH stable.

Here’s a nice shot writeup about it. Water Alkalinity vs pH in your Growing Medium - What's the Difference? | PRO-MIX Greenhouse Growing

The short answer to your question @mayakid is that rainwater is very pure, so has very little CaCO3, and will hold the pH level you adjust it to very easily. CaCO3 also helps to buffer pH and keep it stable, so additional CaCO3 from tap or well water can help keep it stable longer.


We are opening very complex thing . The rainwater is not buffered , doesnt contain bicarbonates , meaning it takes the PH of the environment ( CO2) , if the rain water is applied on medium with 6 PH it will take that value of 6 PH .


Applying pure water with low/no alkalinity is not good idea either , it will leak easy most of the ions , leading to decalcifying and acidification with time .

Option is constant feeding alternating basic fertilizer containing cal-mag with a low acidic fertilizer .

Lowering PH on Rain Water doesnt make any sense , we use acids to neutralize alkalinity , i bet he has hard time to PH rainwater with low or no alkalinity .

1 Like

@devjyarn thatnk you for the easy to understand version… I am not one for doctorate/deep science or legalese lingo… it is what I thought without using names and chemical formulas (CaCO3) is that calcium carbonate???


You got it, yes. Calcium carbonate. Sometimes called “hardness” in your municipal water tests.


And my hot tub…lol… I don’t have hardness in my municipal…lol… again thanks… learned more this morning in the 5 minutes before bed then I did I all day yesterday…lol…