Tap Water, RO Water, Softened Water, Rain Water – What Are The Differences?

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Robert Pavlis

There are many different sources of water that can be used for plants including tap water, reverse osmosis water, softened water, and rain water. What are the differences between these and which are the best ones for plants?

When someone says, ” I just use tap water for my plants” you really have no idea what type of water they are using.  It could be water straight from a municipal service or from a well. It could also be water that has gone through a softening process or a reverse osmosis system. It could be naturally hard or soft or it might contain a lot of sodium chloride. All of these can come out of a water tap.

In this post I will discuss the various types of water and how they affect plants. In a future post I will give some practical advice for solving problems you might have.

Tap Water, RO Water, Softened Water, Rain Water - What Are The Differences?
Tap Water, RO Water, Softened Water, Rain Water – What Are The Differences?

What is in Natural Water?

Tap water starts from various sources. Some homes may be on a well, but even municipalities can get their water from wells. My town has large wells dug into local aquifers to get water. Other nearby towns get water from rivers and lakes. A friend of mine used to use rain water collected into a large cistern for all his water.

Compost Science for Gardeners by Robert Pavlis

Water that comes from the ground has collected all kinds of minerals as it passed by various types of rocks. The chemical properties of the water depend very much on the ground through which it passed. Rain water has very little dissolved minerals but is acidic, so it readily dissolves minerals as it passes through soil.

These minerals are critical to your understanding of the suitability of the water for your plants. There are three important characteristics.

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Water Hardness

Hardness is a measure of how much calcium and magnesium you have in the water. If you have very little, it is considered soft water. Rain water is soft. If you have a lot, it is hard water. One way to measure hardness in GPG (grains per gallon, where 1 grain/gallon = 17.1 mg/L = 17.1 ppm) . A value under 3 is considered soft. Anything over 7 is considered hard.

Stores selling water softeners will usually measure your hardness for free. You can also contact your municipality and ask them. For a rough guide you can use this map in the US and similar maps exist for some other countries.

Water Hardness Map for the US, source: h2odistributors
Water Hardness Map for the US, source: h2odistributors

If you get a lot of white deposits in your kettle, you probably have hard water. The calcium and magnesium also produce white deposits on plant leaves.

Calcium and magnesium can occur in quite high concentrations but these ions are not very toxic to plants. Values of 8.8 grains or 200 ppm will not harm plants.

Water pH

Most plants like to grow with a pH between 6.5 and 7, but they can do quite well outside this range provided it is not far outside. Hard water generally has a pH above 7, and soft water is usually below 7 and can be as low as 5.

The calcium and magnesium do not increase pH, as commonly thought. However, these two minerals usually occur along with carbonate ions. Limestone is mostly calcium carbonate. The carbonate makes the pH higher.

Hardness and pH are different properties and should be treated as such. Adding acids to water can lower the pH but usually does not lower the hardness. Citric acid and acetic acid (vinegar) can be used for this, but if you plan to do this make sure you measure the pH after making any changes.

Acidic fertilizers will also lower pH a certain amount, depending on their contents.

Water Alkalinity

Alkalinity is a measure of the total carbonates (CO3), bicarbonates (HCO3) and hydroxyl ions (OH). The problem with high alkalinity is that these ions can combine with nutrient ions, and form solid salts. These show up as white deposits on top of the soil and as white crusts on the outside of clay pots.

High alkalinity is not directly a problem for plants, but it does tend to raise the pH of the media and it is this higher pH that causes problems for plants.

Note that the term alkalinity and alkaline mean different things; one talks about pH and the other about carbonates.

Salt in Water

The salt we are talking about here is sodium chloride, not the general chemists use of the word salt, which includes all metal ions.

Some areas have a high salt level in soil. Water flowing through this soil will wash out the salt which then ends up in wells and aquifers.

Sodium and to a lessor degree, chloride, are very toxic to plants.

Fluoride in Water

Fluoride is also a concern for municipal water, but this is better left to a separate post.

Softened Water

Hard water is a big problem for many regions and it causes problems in the home, including stains and clogged pipes. It also makes soap less effective for washing. For these reasons some homes use a water softener.

Lots of health promotions make it sounds as if hard water is not healthy to drink. That is a myth. There is nothing wrong with drinking hard water and in fact, it may be better for you.

A water softener works by replacing each calcium or magnesium ion (molecule) with two sodium ions ( or potassium in some cases). This works well for the home problems mentioned above because sodium is very soluble and washes away without leaving much of a deposit. It also doesn’t affect soap. So the homeowner is happy – until they grow plants.

Sodium is very toxic to plants, so they don’t do well with softened water.

It is now time to clarify two terms; softened water and soft water. They are not the same. Soft water contains very few minerals of any type, including things like calcium, magnesium and sodium. Softened water contains less calcium and magnesium, but more sodium.

Sodium is also not great in our drinking water, so when homes are built properly the cold water tap in the kitchen and all outdoor taps use untreated water. The other taps, including all hot water taps, use water from the water softener.

Provided you use the right tap, and the home was built correctly – they are not all done this way – your plants will be exposed to hard water but not high sodium levels.

Some people use potassium in the softener instead of sodium. Potassium is a plant macronutrient and is good for plants in reasonable amounts and its much less toxic than sodium. But at high levels even potassium is a problem.

Using a water softener results in less hardness, higher sodium or potassium, but the pH and alkalinity does not change.

Reverse Osmosis Water

Reverse Osmosis (RO) is a different way of purifying water. In its simplest terms RO is a filtering system that pushes water molecules through a membrane that does not allow other larger molecules or ions to go through. Water comes out, and sodium, calcium, magnesium and larger organic molecules are all left behind.

This sounds great for plants, but ….

Water that is too pure is also a problem for plants. When roots are placed in pure water, nutrients leach out of the roots. To solve this problem you can add some minerals back by mixing the untreated water with the RO water. You can also add some fertilizer to the RO water.

Personally, I’m not a big fan of RO water because it wastes a lot of water to make a bit of pure water. You waste anywhere from 4 to 20 gallons to produce 1 gallon of RO water. Some systems use the waste water elsewhere in the home, but many just pour it down the drain.

RO water has a pH of 7.0, but as it sits exposed to air it will pick up CO2 and become more acidic and can get down of a pH of 5.5. Its hardness, sodium levels and alkalinity are also very low – essentially zero.

Rain Water

As water vaporizes it turns to a gas. Any solids that were dissolved in the water stay behind so water vapor can be thought of as pure water. When this vapor condenses – turns back into water – it is still pure water not unlike RO water.

In nature this happens high in the sky and shows itself in the form of clouds which are really just droplets of water. This water picks up all kinds of chemicals from the air. One thing that is absorbed is CO2 which makes the rain acidic. Once it falls to earth it has a pH of about 5.5.

It also picks up other chemicals from your roof, or other collection devices.

The resulting water has low pH, sodium, alkalinity and harness, making it ideal for watering plants, especially if a bit of fertilizer is added to increase the mineral content and raise the pH a bit.

Watering Plants

You are probably sitting there thinking to yourself, I now know all about various water options, but what should I use for watering plants? If I don’t have perfect water, what can be done to improve it? That is a bigger topic and will be dealt with in the next post: Solving Water Problems for Indoor Plants

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

7 thoughts on “Tap Water, RO Water, Softened Water, Rain Water – What Are The Differences?”

  1. Great posts. I grow peppers each year and if I use tap water straight from the mains they don’t do very well. I’m guessing it has something to do with the carbonate or bicarbonate content which raises the soil pH. The TDS of our mains water in summer normally sits around 200ppm. I normally mix 1/4 tap water with 3/4 reverse osmosis water and this is works fine. We have a 50 gallon barrel in the garden for collecting rain water and in the summer it often runs dry and we have to fill it using the hose pipe. The odd thing is, when the water is left overnight I can use it on the pepper plants and they do just fine. So to clarify, they hate tap water straight from the mains, but tap water left sitting in the barrel overnight is okay. Do you know why this is? I’m no expert on these things, but my initial thought was that perhaps the tap water contains too much chlorine or chloramine and this was why the plants didn’t like it. And when filling the barrel with tap water the chlorine reacted with residual organic material at the bottom and this got rid of the chlorine.

    Reply
  2. I have collected snow and melted it for use in watering my plants and when I start my vegetable seedlings. Is this good for the plants?

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  3. That’s good to know. Thank you. I hope the pure well water without the neutralizer improves the seedlings growth. Did I understand you to say that it won’t matter either way? Perhaps my grow lights are not close enough and it’s not the water at all…

    Reply
  4. Thank you. I have hard water with a neutralizer system. My seedlings that I start indoors never do well. I just had them set up a separate hose to use for my next seedlings so that I can access the water before it goes through the neutralizer. I hope that helps. In years past, my water measured very acid because of the run off from street salting. I live in Maryland. Still, I find everything in the garden does great after a good rainfall. I look forward to your next article. I just have not solved this water issue yet. THANKS.
    Diane

    Reply
    • It is unlikely the acidic water is from street salting. Neither Sodium chloride nor calcium chloride would acidify water.

      Reply

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