Water pH

[ChipMolly]

Given my variable success with growing orchids at home in the past 2 years on my apartment balcony in Sydney, I thought I might test my water pH. I use a triple filtered system rather than tap water. I bought one of those cheap pH meters which come with calibration powder sachets you make up. Seems to work quite well. To my surprise my filtered water is around pH7.8. Tap is pH8.0.

I have read somewhere including here I think, that nutrient availability (Ca, Mg etc) is optimal at a slightly more acidic pH range, 4-6. Is this true?

If so what's the best way to adjust my irrigation water pH?

[Optimist]

I think it may be-- what little I know is that ph is determined by the amount of hydrogen and hydroxil molecules. Acidic water has more hydrogen molecules. Rain water is slightly acidic. Not sure about negative ions or positive. Hydrogen molecules are used in the production of simple sugar in plants.

Hydroxil molecules interfere with the calcium, mg etc. Uptake.

How to stop? Use rainwater?

Wrote it down here somewhere but it is or was theorized that velamin on roots has the job of catching suspended molecules of nutrients in rain water. The nutrients exist in rainwater only for the first few seconds? That is how orchids are fed.

You are in luck, I found it! Wendt's hypothisis, velamin takes up heavilt charged nutrients.

Oecologia, 2013 March. A paper by ZotzG and Winkler, U. This is because of negitive and positive charge in the cell wall of the velamin.

So it goes together. Charged hydrogen atoms, acidic water, rain water.

[Orchid Whisperer]

You can adjust pH with acidic materials from the kitchen. Experienced growers in my orchid club have used vinegar or grapefruit juice as the acid.

Here is what I would do. Measure the pH of the water before adding acid. Use an eye dropper, add a few drops of your acid to a gallon of water, mix thoroughly, measure again. Keep doing this, write down each step the number of drops added and the pH. You just need to be in that range from pH 5.5 - 6.5. Do this once, you will know the amount of acid to add to get it close to the right pH for the next time. You can still use your meter for a final pH check.

[Optimist]

Does this method lower the alkalinity? His ph was at 7.8, which is over acidic, moving into the alkaline range. If it is all a matter of a dropper of vinegar, cool.

[tucker85]

My water is pH 7.5. I use a quarter teaspoon of vinegar per gallon and it brings the pH down to 6.5.

[Ray]

pH and alkalinity are totally different creatures.

Taking a clip from one of Bill Argo's articles about plant nutrition:

The term pH is a direct measurement of the balance between acidic hydrogen ions (H+) and basic hydroxide ions (OH-), and can be measured with a pH meter.

Alkalinity is a measure of how much acid it takes to lower the pH below a certain level, also called acid-buffering capacity. Alkalinity is usually measured with a test kit where dilute acid is added until a color change occurs at a specific pH. Alkalinity is not a specific ion, but rather includes the concentration of several ions that affect acid-buffering capacity. Under most conditions, the ions that have the greatest effect on alkalinity are bicarbonates like calcium, magnesium, or sodium bicarbonate and, to a lesser extent, carbonates like calcium or sodium. Several other ions including hydroxides, phosphates, ammonium, silicates, sulfides, borates, and arsenate also can contribute to alkalinity, but their concentration is usually so low that they can be ignored.

You can adjust the pH easily through additives, but that's going to do nothing about the alkalinity. To affect that, you'll need to use a pure water source.

ChipMolly - contact your municipal water supplier. They ought to be able to give you a water analysis.

[Orchid Whisperer]

Thanks Ray for the clarification of pH vs. alkalinity.

Optimist, as the Bill Argo quote indicates, what you are thinking of as alkaline really should be referred to as basic.

Some alkalinity in water is a good thing for orchids. Moderate alkalinity can buffer the effects of additions to water supply (e.g., fertilizer) on pH. Very low alkalinty water (pure rain water, distilled/de-ionized water, RO water) can be just as problematic as high alkalinity water. I personally prefer to use a mix of rain water and tap water when adding fertilizer, and also alternate between tap and rain water for normal watering.

I suggested drop-by-drop modification of the pH because I do not know the alkalinity of ChipMolly's water. A relatively few drops of acid into a low alkalinity water will produce a big pH change, the same amount of acid in a moderately high alkalinity water will produce a smaller pH change. The drop-by-drop method allows you to figure out the rough quantity of acid that needs to be added (final adjustment of pH to the target level can be made by adding a little more acid, or some tap water).

Ray's suggestion to find out the alkalinity (and other chemistry) from the local water department is a good one. Keep in mind that city water supplies can vary in chemistry on a seasonal basis - it is good to check the level of adjustment needed from time to time. I would do this maybe four times a year, especially if rainfall is highly seasonal in your area.

The summary at this link provides some good information on water quality for orchid growers, courtesy of Sue Bottom and the St. Augustine Orchid Society: http://staugorchidsociety.org/PDF/Wa...lizersRev2.pdf

[Optimist]

Thank you. I am just learning. I only want to be helpful. I have alkaline water myself. I am not worried about my own water, because it comes from a very mineralized aquifer.

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[Orchid Whisperer]

Originally Posted by Optimist

Thank you. I am just learning. I only want to be helpful. I have alkaline water myself. I am not worried about my own water, because it comes from a very mineralized aquifer.

No worries. In New Mexico, your water is probably both basic (pH above 7) and also alkaline (lots of buffering capacity due to all those minerals).

[ChipMolly]

Buffering capacity is to do with weak acids, which do not completely dissociate to produce equimolar amounts of H+ ions. Weak acids (which are almost always organic acids) have a weak dissociation constant, such that an equiilibrium exists. This is how buffering capacity works.

And pH >7 is alkaline. The measure of pH is the logarithmic concentration of [H+], not [OH-].