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How to interpret a municipal water report

Posted by Oscarmatic CA z10/24 (My Page) on
Fri, Nov 22, 13 at 13:11

There are a few threads here mentioning the impact of tap water minerals, hardness, and ph on container plants grown in the gritty or 5:1:1 mix. I've done some reading on how to intepret water reports, but the available information is oriented toward safe drinking water, rather than adapting soil and fetilizers.

I'm just starting on my first container of 5:1:1, so doing all the research to understand both *what* to do and *why*. In that spirit, I wonder if some experienced grower might be willing to post a short guide to intepreting city water reports. I'm based in San Diego, so trying to intepret a report on Physical, Mineral, and Metal Characteristics or the overall annual drinking water quality report.

I see sentences like this one, which look very informative, but I don't have enough understanding to know how I would adjust my soil or fertilizer accordingly:

"Typically, drinking water in San Diego averages about 15.4 grains per gallon (gr/gal) or 263 parts per million (ppm), and depending upon water demand and area of the City you live can range from 13.4 to 19.0 gr/gal or 229 to 325 ppm."

Thanks in advance for your insight!


Follow-Up Postings:

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RE: How to interpret a municipal water report

The most important things to identify are pH, alkalinity, salinity and nutrient content. Look for calcium, magnesium and sulfur levels to determine if their levels are significant enough to contribute to plant nutrition.

If your alkalinity is above 100ppm, you'll need to flush your pot fairly frequently with clean water and/or add vinegar to your irrigation water until the pH is around 6.0 or lower.


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RE: How to interpret a municipal water report

A quick glance tells me the pH is way too high, in average around 8.5. In most places city water has a pH of less than 7, meaning slightly acidic. That is perfect for garden veggies. 7 is fine too, as it will not do anything, since it is neutral. I would consider 7.5 be ok too. But 8.5 is probably too alkaline.

If you were gardening in ground, you could live with that by keeping your soil acidic.(pH ~=6). But with a potting mix like 511, how do you do it? The only way is to add a lot more calcium/lime/gypsum to it.


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RE: How to interpret a municipal water report

  • Posted by nil13 z21 L.A., CA (Mt. Wa (My Page) on
    Sat, Nov 23, 13 at 11:03

If you were gardening in ground, you could live with that by keeping your soil acidic.(pH ~=6). But with a potting mix like 511, how do you do it? The only way is to add a lot more calcium/lime/gypsum to it.

what? I think you mean that the only way is to add an acid to the water in order to bring the pH down.


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Reading the San Diego water report

Thanks for the tips on interpreting the water report. Here's my take on what I'm reading at the PDF linked below.

pH: avg 8.11, max 8.64
- From the tap, my aquarium test kit tells me I get water closer to the high end of the reported pH range: 8.4. I understand this to mean that I should add vinegar to the water when I water the plants, to bring the pH closer to 6 or 7. I will mix up some samples at various concentrations and use the test kit to determine how much vinegar to use.

Alkalinity (Total as CaCO3): avg 94 ppm, max 107 ppm
- Based on Oxboy555's recommendation above, with alkalinity around 100 ppm I plan to flush the pots regularly using Al (Tapla)'s techniques, to wash salts out of the 5-1-1 potting mix I will use.

Using the indices described on the Wikipedia page for Hard water, I can determine:

Ryznar Aggressive Index: avg 7.58, max 8.29
-- "For 6.5 < RSI < 7 water is considered to be approximately at saturation equilibrium with calcium carbonate
For RSI > 8 water is under saturated and, therefore, would tend to dissolve any existing solid CaCO3
For RSI < 6,5 water tends to be scale forming"
Langlier Index: avg 0.27, max 0.76
-- "In practice, water with an LSI between -0.5 and +0.5 will not display enhanced mineral dissolving or scale forming properties. Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving abilities while water with an LSI above +0.5 tends to exhibit noticeably increased scale forming properties."
Calcium Hardness: avg 104 ppm, max 125 ppm
-- Moderately hard to Hard water

Calcium: avg 41.7 ppm, max 50 ppm
Magnesium: avg 16.6 ppm, max 19.6 ppm
Potassium: avg 3.86 ppm, max 4.26 ppm
Sulphur: not listed
-- This is where I'm lacking any useful context. Is 41 ppm a lot? A little? What target should I be comparing these numbers to? I would appreciate some input from our brilliant experienced growers, soil scientists, and chemists in the group.

Thanks!

Here is a link that might be useful: Physical, Mineral, and Metal Characteristics


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RE: How to interpret a municipal water report

DWD2 posted a link to a PDF in another thread, and I found it very useful on this topic:

Here is a link that might be useful: Substrate pH and Water Quality, from The North Carolina Cooperative Extension Service


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RE: How to interpret a municipal water report

You could likely save yourself some money and hassle and just avoid the acid inclusion altogether. 100ppm alkalinity is far from terrible and it will help keep your soil pH stable.

Keep in mind water pH doesn't matter much. It's mainly an indicator. Alkalinity is the KEY thing you need to know about your irrigation water and yours is right on the border between acceptable and high-ish. As long as you flush liberally every couple of months, you'll be fine without acid/vinegar.

Your calcium and mag levels are fine/average.


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RE: How to interpret a municipal water report

I think your alkalinity is too high, Flushing alone is like flushing gasoline out of you tank with gasoline. I would use vinegar myself. At least when you flush.
It depends a lot on what you are growing. If cacti or blueberries, they would not grow well or not at all. Most plants don't care, some even like it like lilacs.


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RE: How to interpret a municipal water report

In a perfect world, I think I'd take an alkalinity of around 50ppm with each watering - enough to add some buffering to the substrate to help offset acidity from the bark/peat/fert/nitrification, but not enough to spike up my pH dramatically over time.


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