"If you have a peat based container mix, why add dolomite lime to it as some recommend, if your water pH is high."

When we measure pH, we are measuring the concentration of hydrogen (H+) ions in the irrigation water or soil (nutrient) solution. As a guideline, water for irrigation purposes is usually best if its pH lies between 5.0 - 7.0. The measure of alkalinity gives us the water's ability to neutralize acidity. The level of bicarbonates, carbonates, and hydroxides in water determine its alkalinity. The desirable alkalinity range for irrigation water is 0 to 100 ppm carbonates, with 30 - 60 ppm optimum for most plants.

Whenever you test the water you'll be using for irrigation, you should always include the measure of both pH and alkalinity. A pH test on its own, does not give any indication of alkalinity. Water with high levels of bicarbonates or carbonates (high alkalinity) always has a pH value >7, but water with a high pH doesn't necessarily have high alkalinity. This is a very important concept because high alkalinity exerts more significant effect on plant nutrition and the fertility of growing media than pH.

High pH irrigation water generally causes no problems as long as the alkalinity is low. Since high pH water with low alkalinity has little ability to neutralize acidity, it's effect on media pH will be minimal. Cause for more concern are situations where you must utilize water having both high pH and high alkalinity for irrigation, which will cause the pH of the growing medium to increase substantially as media ages. If your water analysis is known to be alkaline, you may need to significantly reduce the addition of compounds containing Ca or Mg because of the dilute solution of limestone in the water. The smaller the container - the more serious the issue because small volumes of soil offer less buffering to pH change. In high alkaline situations, you can often expect/experience Ca and Mg deficiencies along with micronutrient deficiencies (both real and antagonistic).

Al

Well said. For example, the rain water I collected has a pH of 8.5 to 9, but the EC is very low, about 7us, of course this is not alkalinity. My tap water has a high pH too, but the alkalinity is around 40, which is considered soft water. I can safely use this water to water my peat based container plants without worrying they will change the pH too much.

Justaguy,
You stated awhile ago that you add vinegar to your watering. Do you also add Dolomite to your container mix?

I used to until I realized what was happening in my containers due to the irrigation water being high in alkalinity (ph only 7.5).

Now I split the total amount between lime and gypsum and in containers that drain onto soil where I have plants growing nearby I skip lime entirely and just use gypsum for the Ca and then use Foliage Pro which has Ca and Mg in it.

I still use vinegar most every time I water though after seeing what soil tests indicated the tap water did to the native (ground) soil's pH over a few years and my own testing of container pH after a season.

I will test things again after this season and see what's what. If necessary I will omit lime and just use gypsum + Foliage Pro. As it is I have 3 wine barrel containers that no longer have lime added as they drain onto the area my in ground blueberries grow.

Your rain water cannot have a pH that high. It simply isn't possible.

Could it be because of my asphalt shingle? I calibrated my Hanna meter against the pH 7.01 standard solution. This meter is very reliable. Now I am concerned, will find it out.

Could it be because of my asphalt shingle?

No. If it was due to asphalt shingles most folks with rain barrels would have similarly high pH rain water. The pH of rainwater over the US tends to range from 4.2-5.5ish.

You can google 'rain pH map' and see maps with average rain pH values for the nation. Rain is simply never anything but acidic, the only question is how acidic.

JaG, just to let you know. I live in Ottawa, Canada. But I don't think this will change the rain water pH.

I will find it out tomorrow. Maybe my calibration solution is in question. But after adding 2ml of vinegar into a gallon of rain water, the pH drops to below 5.

Justaguy,

I measured the fresh rain water and the rainwater from my rain gutter. I used a glass put on the ground to collect the rain water, this will guarantee minimum contamination. The fresh rain water has a pH about 5.4 and EC about 7us. The water from rain gutter has a pH about 7.2 and EC about 17us. I am sure there is something in my shingle that contaminated the rainwater. My shingle is probably the organic Asphalt Shingle. But I am not too concerned about the pH value of the water from rain barrel anyway, since its alkalinity is very low which is unlikely to have any noticeable effect on my container pH. Also given the container culture, the mix and fertilizer we are using, the pH is not as important as in the ground.

Thanks

I agree with your analysis, Yellowthumb. It is very interesting that the water merely passing over your shingles would have it's pH changed so much.

Still, as you noted the alkalinity is pretty much zilch so a non issue.

Thanks Justaguy.
Yes, I am surprised by the change too. I think it also depends on the way it rains. We have the ongoing shower like drizzle, which will probably have more contact time with the Shingle.
The rainwater from the rain barrel has a very slight tan color as well if compared to the tap water. Otherwise it's not noticeable. I have been using this water on my ornamental plants and sometimes vegetables with very good results.

Thanks all for your responses!

Will go out and measure alkalinity tomorrow.