Rain water versus city water for health of plants

miasternAugust 14, 2008

Does anyone know about the difference between using city water - with its chlorine and flouride and everything else - and rain water for watering plants? Does it really make a difference? My gran says it does but I don't know.


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Water from a municipal system may be treated with chlorine and possibly floride (some are not), and many people think those things are harmful to plants, but rainwater contains things such as Sulfur Dioxide and many other potentially hazardous substances, acid rain, although the same people that say do not use "city water" will deny there is a problem with rainwater even though this is well documented.
I know people with gardens where most of the water comes from a municipal system (chlorine and floride) that grow just as well as the gardens of people that use well water. Not too many people have installed catchment systems, yet, but I alos know the gardens grow better after a good rain, probably because of the disolved Nitrogen in rainwater. So, is rainwater better? All plants need water from whatever source so they can uptake the needed nutrients from the soil they are growing in and so the leaves can manufacture more nutrients to feed the roots, water is essential to life. Use what you have.

    Bookmark   August 14, 2008 at 7:01AM
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joepyeweed(5b IL)

Chloramine is chlorine and ammonia mixed together - which helps the chlorine maintain its residual longer. Its not necessarily "harder" on the microbes, but it takes more effort to "dechlorinate"...chlorine alone will dissipate very quickly when exposed to the atmosphere. I have to filter my the chloramine out of my tap water, in order to make bread.

The biggest difference between rain water and tap water, besides the chlorine is the hardness. Rain water is soft water. Well water is hard water. Some tap water is hard, some is soft depending upon where you live,t he source of the water and type of treatment.

Rain water is naturally soft and has no chlorine so yes its good for the plants.

My plants prefer rain water to tap water, but they also prefer tap water to NO water. :-)

    Bookmark   August 14, 2008 at 10:23AM
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sudzy(5b IL)

Here's a surprise, at least to me. Wikipedia stats that according to the Unites States Geological Survey, 89.3% of homes in the US have hard water. I associate hard water with wells and small towns. hum

    Bookmark   August 15, 2008 at 2:23AM
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joepyeweed(5b IL)

Doesn't surprise me... most public water supplies rely on ground water for its source. And about 15% of USians have their own private supply.

    Bookmark   August 15, 2008 at 9:51AM
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So, let me see if I understand all of this right. The general consensus is that any water is better then no water. But rain water is better then city water if you live in an area with low air pollution. Like in a remote rural area far from a large metropolitan city or industrial factory's. Is this correct?

    Bookmark   October 24, 2011 at 5:16PM
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Maybe. Acid rain has been found in very remote areas, polluted rain water is found in very remote areas. It is true that any water is better then none because plants cannot uptake and utilize necessary nutrients without water.
Keep in mind that all of the water we have today is all of the water there will ever be and all of the water that ever was. What we have today has been recycled many times over and will be recycled many times into the future so it does make sense that we should take care of it and not add pollutants or poisons to that water.

    Bookmark   October 25, 2011 at 7:06AM
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IME, rainwater is much better than treated municipal water.

regarding the sulphuric acid in the rain: in most parts of north america, farmers have been having to increase their use of various forms of sulphur compared to the 60's and 70's when the soils had high levels of sulphur because of the highly acid rain. There is far less sulphur in the air now, and most high-rainfall regions have soils too low in sulphur, especially the leachable soils. Sulphur is critical for plants to be able to resist insects.

    Bookmark   October 25, 2011 at 8:24AM
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Keep in mind that all of the water we have today is all of the water there will ever be and all of the water that ever was.

Acutally water is being made and destroyed all the time.
Water is split apart into elemental hydrogen and oxygen both through hydrolysis in bodies of water and in the soil.*Poof* water is no longer water but elemental hydrogen and oxygen.
Also, when plants make sugar via photosynthesis, they split water into two hydrogens and an oxygen. Most of the hydrogen goes into sugar molecules while oxygen, to a large degree, goes into making the CO2 that plants "exhale". *Poof* Water was just destroyed and became things that aren't water but contain hydrogen and oxygen. There are an awful lot of plants doing that all day every day. (including massive amounts of algae)
On the flip side, every single time a base and an acid react with one another, the products are a salt and water. *Poof* Water was just created out of two things that weren't previously water but contained hydrogen and oxygen.

Those are just a couple of examples. So yes, a lot of water is recycled. I'm not denying that. But the statement quoted at the top of this post is not true by a long shot.

    Bookmark   October 25, 2011 at 10:33AM
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I must admit that with some trouble-prone hard-to-grow plants in containers the fluoride in the city water has crossed my mind.

The chlorine and chloramine I know are no big problem.

(BTW: chloramine is often, perhaps typically, used to reduce producing toxic disinfection byproducts that can be made by reaction of chlorine and traces of dissolved organic matter.)

    Bookmark   October 25, 2011 at 2:14PM
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feijoas(New Zealand)

My reticulated water has no added chemicals; I'm very, very lucky to be living over a 'fossil water' aquifer.
I'm pretty sure it hasn't been mentioned, but as far as I'm aware, rainwater contains quite a bit of dissolved atmospheric nitrogen, which I've always been told explains why plants seem to prefer a bit of rain over a heavy watering.
So even though my tap water's as good as it gets, I use rainwater if I can.

    Bookmark   October 25, 2011 at 9:03PM
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Belgianpup(Wa/Zone 7b)

I can't see how anything could be better for plants than rainwater. Or for washing your hair!

Municipal water has stuff like chlorine added to it to deal with problems, not because it's good for drinking or plants. Fluoride (IMO) is added because someone with political connections is making money on it.

Wherever you live, rainwater is cleaner than the groundwater, usually at least 20 times cleaner.

We've polluted the water, the soil, the air, so don't think any water is going to be perfectly clean, from land, sea or air, unless it is mechanically filtered or distilled. American babies are now BORN with more than 200 foreign chemicals in their bodies. YUCK!

If you live on the West Coast, where the prevailing wind comes from, the moisture has been carried across the Pacific Ocean and air-borne impurities have probably been diluted by gaining moisture. When it falls and enters the soil, it is contaminated by all the crap that's in the soil, farm chemicals, industrial pollution, spilled gasoline, etc.

If you live in a large industrial area, you probably have dirtier rainwater, but you are guaranteed to have far dirtier groundwater.

The hardness or softness of water is determined by the amount of chemicals dissolved in the water as it moves through the soil (mostly calcium and magnesium, the stuff that builds up around your faucets). Most of the water that is 'soft' is rainwater or distilled water.

Here in WA, we have pretty clean rainwater, but hard groundwater because the heavy rainfall leaches the calcium and magnesium out of the soil.

Rainwater has been found to contain trace amounts of nutrients: nitrogen, phosphorus, potassium, calcium, magnesium, sodium, ammonium, etc. It also has some Vitamin B12, which is helpful for plant growth.

Rainwater is normally slightly acidic, due to absorbing carbon dioxide from the atmosphere, thus making a weak carbonic acid solution. But it can become more acidic due to volcanic gas and dust, decaying vegetation, emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from power plants and other fossil fuel combustion. These gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds such as sulfuric and nitric acids.

For people, acid rain is nothing harmful. Years of acid rain can make the soil more acidic. Occasional testing of your soil will tell you if the pH is gradually sliding downward. If it is, you'll have to add some lime to counteract it, not really a big deal. When you get your soil test results, if it doesn't say what type of lime to use, call and ask. Some forms of limestone will raise the pH, and others won't.

    Bookmark   October 26, 2011 at 1:13AM
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Some forms of limestone will raise the pH, and others won't.
I'm sorry to harp but this one is driving me nuts. Please tell me which type of lime does not raise soil pH.

    Bookmark   October 26, 2011 at 8:19AM
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"Please tell me which type of lime does not raise soil pH."

A board like this is a good source of information from personal experience but often a very poor source for information on chemistry.

    Bookmark   October 26, 2011 at 4:05PM
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True. I see this statement made over and over by the same person.
This one bit of misinformation in particular bothers me because it could quickly and easily have difficult to correct real word consequences if somebody decides to lime their already alkaline soil because they've read that "some forms of limestone will raise the pH, and others won't."

I've explained to this particular poster how lime affects pH yet the same problematic misinformation keeps showing up in their posts. I would like that person to name a particular "lime" so that I can explain in detail how it will effect the pH or why the material that they are thinking of is not lime. Then maybe we can put this one to bed.

    Bookmark   October 26, 2011 at 4:25PM
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Belgianpup(Wa/Zone 7b)


Gypsum isn't really lime, it doesn't usually act like lime, but everyone seems to call it lime.

You CAN add GYPSUM to high-pH soils to add CALCIUM because it won't raise the pH levels unless you have ACIDIC, HIGH-SODIUM SOILS (over 15% CEC) WITH SOLUBLE ALUMINUM.

If you have high-sodium soil (common in the U.S. SW): "If free lime is present in the soil, it can be dissolved by applying sulfur or sulfuric acid. Sulfur products reduce the pH which dissolves the lime, thus freeing up the calcium. If free lime or gypsum is not present in adequate amounts as determined by a soil test, then add calcium. The most common form of calcium used for this purpose is gypsum. Although calcium chloride, which reacts more quickly, can also be used it is usually more expensive. After broadcasting the calcium source on the soil surface, mix it, and make sure adequate moisture is present to
dissolve it. Recovering a foot depth of sodic soil on one acre requires approximately 1.7 tons of pure gypsum (CaSO4-2h2O) for each milliequivalent of exchangeable sodium present per 100 grams of soil." from U. of MN (http://www.extension.umn.edu/cropenews/2002/02MNCN07.htm)


Gypsum is NOT lime. In order to adjust soil pH, liming products must contain carbonate (CO3-) which reacts with hydrogen ions to neutralize soil acidity. Gypsum is calcium sulfate (CaSO4). While the calcium will displace hydrogen ions, these ions will remain in solution and will not adjust soil pH... Gypsum can be used as a source of calcium and sulfur, however, remember [that] Gypsum is more soluble than lime and can add calcium more rapidly to the soil. This may result in decreasing potassium or magnesium levels in the soil. Monitor this by soil testing. Gypsum can be used as a sulfur source, however, it tends to be less soluble than other sources such as ammonium sulfate." From MI St. U (http://www.turf.msu.edu/gypsum-use-it-or-lose-it/)

"Gypsum does not change pH nor improve drainage in non-sodic situations. Gypsum is used to add calcium to soils." From UofCA Davis (http://cesonoma.ucdavis.edu/files/27200.pdf)

"Although gypsum, like lime, contains calcium, the calcium is not responsible for an increase in pH where soils are limed." From U of MN (http://www.extension.umn.edu/cropenews/2002/02MNCN07.htm)

"Gypsum (CaSO4) is considered both a soil ammendment and a source of plant nutrients...Gypsum can be added to the soil to supply the needed calcium with out altering the soil pH value." From Clemson U, SC (http://hubcap.clemson.edu/~blpprt/bobweb/BOBWEB12.HTM)

"What is gypsum? Gypsum, is calcium sulfate. It doesn't change the soil pH... Use lime until your pH is at the desired level. Then use gypsum to add more calcium. From NC State U Coop. Extension

"Calcium is nearly always only marginally sufficient and often deficient in developing fruits. Good fruit quality requires an adequate amount of calcium. Calcium moves very slowly, if at all, from one plant part to another and fruits at the end of the transport system get too little. Calcium must be constantly available to the roots. In very high pH soils, calcium is not available enough; therefore, gypsum helps." From USA Gypsum (http://www.usagypsum.com/agricultural-gypsum.aspx)

"High quality gypsum will not change the soil pH, although contaminants in some by-product gypsum may have some effect (raising or lowering pH) on soils." From Spectrum Analytic Lab (http://www.spectrumanalytic.com/support/library/rf/Gypsum.htm)

"Lime vs Gypsum:
• Both used to change the pH, and add Calcium.
• Lime to raise the pH, Gypsum to lower it.
From Cabrillo College, CA (http://www.cabrillo.edu/~dobrien/pH%20lime%20gypsum.ppt.pdf)

"Muddy Water in Ponds: Agricultural gypsum is another material for removing suspended clay and does not cause the concern of a fish kill associated with adding hay. Gypsum is also chemically neutral and therefore does not cause possible pH problems." From OH St. U (http://ohioline.osu.edu/a-fact/0006.html)

"The most economical way to get sufficient calcium to plants in a high pH environment is to use SuperCal SO4 [dihydrite gypsum that is 95 percent pure]... In the list of over 30 favorable reasons for using gypsum on agricultural land, one item suggested that gypsum could result in decreased pH of the photosphere near the active roots. The mechanism for this effect is that many plants take up calcium quite readily, but take up sulfate much more slowly. Since the differential uptake of calcium and sulfate results in a buildup of hydrogen ions in photosphere equal to the differential uptake, the pH adjacent to the roots will decrease. The advantage for the effect is that in high-pH soils, uptake of the micro-nutrients, iron, zinc, maganese, and copper can be increased....The use of Super Cal SO4 alone will probably not reduce your pH, but it will create a more friendly environment for your plants... There is a growing body of literature indicating that gypsum combined with lime is more effective than lime alone." From Calcium Products Inc. (http://www.calciumproducts.com/farming_gypsum.cfm)



    Bookmark   October 27, 2011 at 6:53PM
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Aaaaaaand there it is.

I have no idea why you felt the need to bend over backwards to make the case that gypsum does not typically affect soil pH. As you stated yourself, it isn't lime.

So, now that you've finally come to the realization (hopefully) that gypsum isn't lime, can you please stop misinforming people by telling them that lime doesn't always raise the pH? Because, ya know what? It does.

    Bookmark   October 27, 2011 at 7:39PM
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I for one appreciated the information contained in belgianpup's post.
Getting back to the water. It would seem to me that if one was using chlorinated city water that using a sprinkler system over a drip system for irrigation would be preferred so the chlorine would aerate out. Also for those using city water and watering with a bucket I have herd that adding hydrogen peroxide helps dissipate the chlorine?

    Bookmark   October 29, 2011 at 7:33AM
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Soil pH is a measure of the "potential Hydrogen" in the soil. In soils with low pH there are more free hydrogen ions while in soils with a high pH there are few. If you need to decrease the number of hydrogen ions Calcium Carbonate (CaCo3, lime) has been found to be the best way to do that while adding sulfur will increase the number of hydrogen ion.
Rain water typically is acidic and has, over time, lowered the pH of some soils and some bodies of water. Most all municipal water systems add, in addition to Chloramine and Floride, something to make that water close to neutral since water very acidic or alkaline simply does not taste good.
Adding Gypsum to soils that are not sodic is usually a waste of time.

    Bookmark   November 1, 2011 at 6:28AM
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Just a quick note. "pH" does not stand for "potential hydrodgen". The "H" part is hydrogen, so that's correct. However, the "p" doesn't stand for a word. It's a mathematical expression that means "find the inverse log of".

It's much the same as the "X" in the expression 5X5 doesn't stand for a word. It means "multiply by".

So, pH means "find the inverse log of Hydrogen (expressed as molarity)."

It's like saying the "S" in U.S.A. stands for the word "spaces". People will get the gist of what you're saying but you sound pretty silly saying it.

    Bookmark   November 1, 2011 at 10:35AM
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If one reads this they would find that a definition of pH is Potential Hydrogen.

Here is a link that might be useful: a definition of pH

    Bookmark   November 2, 2011 at 10:11AM
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Or one erroneous definition among many correct ones.

And it is certainly not the capital "P" Potential of a formal name.

That said, I have heard informal use of "potential" in describing a pH value. That was probably "potential" in its physics sense of local strength or conceivable originally derived from the electrical potential (voltage) developed in a pH electrode.

But pH is not Potential Hydrogen in any formal sense that I have ever heard.

    Bookmark   November 2, 2011 at 10:32AM
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If one reads this they would find that a definition of pH is Potential Hydrogen.

If one learned their chemistry from somewhere other than "thefreedictionary.com" one might realize that "freedictionary.com" has both accurate and inaccurate information side by side and, therefore, cannot be considered a reliable source for such things.

    Bookmark   November 2, 2011 at 10:51AM
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greenleaf_organic(8, TX)

Funny here how the OP asks a question and sets off a flurry of sometimes heated postings. Mia, if you are still out there, what did you decide to do? For what it's worth I have lived in 4 states as well as various municipalities in my adult life and without exception have found rain water to be better for plants. That is why I installed a system of barrels for rain water cachment. Hopefully in my own little way it conserves ground water here in hot and dry S TX.

    Bookmark   November 21, 2011 at 2:55PM
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OK! I have found rain water works best, and NO, I have NO science to back that statment up lol. It could be that our tap water is just too cold and slows down plant growth.


    Bookmark   November 30, 2011 at 4:18PM
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My comment won't rival the above scientist-like comments above. But, all I know....is that when it rains, my veggies get a boost of steroids I tell ya'. I rent so I can't make a proper water tank....but whenever I can somehow catch a bit and use it, I do. It is magic from the sky on my tomatoes especially.

    Bookmark   December 12, 2011 at 8:47PM
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