Solving Water Problems For Indoor Plants: Hardness, pH, Salts, Alkalinity & Chlorine

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Robert Pavlis

Indoor plants are not too fussy about the water they get, but certain conditions like pH, alkalinity, hardness, chlorine and sodium levels are a concern. If the water is not suitable, plants fail to grow properly and may die. Many gardeners blame themselves for having black thumbs when in fact their problem is “black” water.

In this post I will discuss things that you can do to try and correct any water issues you might have.

Hard Water Stains: Solving Problems With Your Water - For Indoor Plants
Hard Water Stains: Solving Problems With Your Water – For Indoor Plants, Source: Good to Grow

Types of Tap Water

I have discussed different types of water and possible issues with them in another post. For a background primer on these see:

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

Determining Your Water Conditions

Before you start making changes to your water it’s important that you know the amounts of various chemicals in your water so that you can determine if you have a problem. If you use municipal water, the municipality should be able to provide these values, and many now publish them online. If not, you can get the water analysed by a lab.

Soil Science for Gardeners book by Robert Pavlis

You should know the following values: pH, alkalinity, sodium and harness.

Unfortunately, various units are used to quantify these chemicals. Note that 1meq/l CaCO3 = 50 mg/l CaCO3 = 50 ppm CaCO3.

This link has a nice table of minerals and water properties along with recommended limits for each. It will help you determine if you have an issue with things other than the ones listed here.

Chlorine and Chloramine Levels

I have discussed this in a separate post. Contrary to a lot of what you read on social media, it is unlikely that either chlorine or chloramine levels, in tap water, will cause harm to your plants. This is contrary to what you might have read in social media; a lot of which is just plain wrong.

Water With High pH and High Alkalinity

High pH is anything over 7.5 and high alkalinity is more than 120 ppm CaCO3. Note that high pH is not necessarily a problem. A pH of 8 with low alkalinity (30-60 ppm) will not harm most plants.

You only have a problem when both values are high.

Water that has high alkalinity ( i.e. high carbonates) tends to also have high pH. The solution is to add some acid to bring the pH below 7.5. An ideal pH would be closer to 6.5.

The acid neutralizes the carbonates, which caused the pH to fall.

Phosphoric and nitric acid are normally used in the horticulture industry, but for a homeowner it is better to use citric acid or acetic acid (vinegar) which are safer and easier to buy.

How much do you need to add? That depends on the pH and alkalinity values. Don’t listen to recipes on the internet – nobody can tell you how much to add without knowing your data.

the pH can be measured at home with a simple garden pH tester. Note that ‘soil pH testers’ are usually not recommended for use in liquids. Add some acid and measure the pH again. If you are using citric acid make sure it is dissolved before measuring pH. Once the pH is around the target value, stop adding more.

You could also mix your water with either rain water or RO water and this will lower the alkalinity and in turn lower the pH.

Once you know how much acid you need to add, you can just use the same amount each time. You don’t need to measure pH each time you make up some water but it is a good idea to measure it a couple of times a year just to be sure the water has not changed.

Water With High pH

Example of an acidic fertilizer
Example of an acidic fertilizer

Water that has a high pH, but low alkalinity, is normally not an issue, except for some specialty plants that require very low pH. You can adjust the pH as described above, or just use an acidic fertilizer. Such fertilizer will provide the nitrogen in the ammonium form, which is acidic, and the packaging is usually labeled as being acidic, or for acid loving plants.

You will not need to measure pH. Just add the recommended amount of fertilizer and use the water.

Water High in Sodium

A high sodium level is anything over 50 ppm.

If you are using water softened with sodium chloride, stop using it.

If your water has naturally high sodium levels, the only thing you can do is to dilute it with water that has low sodium levels. This can be rain water, RO water, or purchased distilled water, all of which contains no sodium.

Assuming the added water has no sodium, a ratio of 1:1 (50/50) will cut the sodium level in half. If that is not good enough to get it below 50 ppm, use a 1:2 ratio to cut it into 1/3.

High Hardness in Water

High harness is anything over 150 ppm, but values up to 200 ppm usually don’t cause growth problems for plants because the calcium and magnesium causing the hardness are not very toxic to plants.

Many people worry about their hard water unnecessarily, mostly because social media incorrectly tells them hard water is not good for plants. In most cases the real problem is the alkalinity and pH, not the hardness.

If your water really is too hard, dilution as explained for high sodium, will work.

An alternative is to use a water softener, but use potassium chloride instead of sodium chloride for recharging the water. This adds excess potassium to your water, so you might want to look for a fertilizer that has low potassium levels.

Modify Your Watering Method

All of the issues discussed in the post will be less of a problem if you water correctly. When you water, allow lots of water to run through the pot. This washes out excess salts so they are less likely to accumulate in the pot.

My Water

I thought I would check my local city water. I contacted them and got the following values:

  • Sodium – 75 ppm
  • Hardness – 450 ppm
  • pH – 7.8
  • Alkalinity – 300 ppm
  • Chlorine – 1 ppm

All of these values, except chlorine, are over the suggested limits. Sodium, hardness and alkalinity could be reduced by mixing with distilled water, rain water or melted snow. The pH and alkalinity can be reduced by adding acid.

I don’t grow a lot of houseplants. My orchids, which are very sensitive to salts, are doing just fine without adjusting the water. I have had some issues with small seedlings and maybe the alkalinity and pH explains part of the problem.

Other houseplants seem to do OK. This goes to show you that many of these recommended limits have a level of safety margin built into them. Just because you have values that are higher than recommended, don’t panic. Your plants may be just fine.

In my case, I think I will try to dilute the water by half to get sodium levels down, and then adjust with vinegar to reduce the alkalinity and pH.

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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!

6 thoughts on “Solving Water Problems For Indoor Plants: Hardness, pH, Salts, Alkalinity & Chlorine”

  1. Paraparaumu Reticulation

    g/m3
    Chloride 20.40
    Sulphate 6.08
    Calcium 7.72
    Magnesium 1.60
    Sodium 11.7
    pH 7.8
    Total hardness (as CaCO3/m3) 26
    Total dissolved solids 77
    Estimated Bicarbonate 31.5
    Estimated Carbonate 0.1
    Can you please let me know what you would class this water as,. , Am using it for (everything haha it’s my tap water)hydroponics more ( recirculating DWC system I’ve built) is there enough cal mag in the water already to be considered hard? Learnt heaps in half hour on your blogs,, choice bro thnx, anyway I also HV veg garden outside with rainwater tank for that, should I use rainwater in hydro system or a mix? And I grew up with beautiful water here in nz, now that council are molesting it….. Urgh lol gta become a scientist just to b a bloody anything haha

    Reply
  2. Hi:

    I live in the Puget Sound area of WA state and we have extremely soft water with very low mineral content. This poses a problem when using most liquid fertilizers, even such as fish fertilizer. The water has almost no ability to buffer changes in PH so even mixed at the recommended level it drives the water PH into the 3.0 to 4.0 PH level which is massively acidic.
    I used to brew a lot of beer and used calcium hydroxide to raise PH and improve the buffering ability of the water. What would you recommend for adding buffering ability to water mixed with liquid fertilizer?

    Reply

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