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Is Soft Water Killing Your Plants? The Helpful Facts

A Child Watering The Garden With A Hose, Hopefully It Isn't Soft Water That's Bad For Plants.

Most gardeners have heard that watering plants with softened water isn’t a good idea.  But is soft water killing your plants?  This article will uncover the facts that will answer this question. And we’ll also discover something interesting about watering plants with potassium-softened water.

Many sources say softened water is bad for plants, but they never back it up with data. That set us on the path to discovering the truth about softened water and supporting it with facts.

If your tap water supply comes from a water softener, read on to learn the interesting facts about using softened water on plants.

Here’s a brief run-down of what we’re going to find

  • We’ll discover that plants could suffer immediate harm if the softened water contains 265 mg/gal (70 mg/l) or more sodium.
  • This sodium level exists in softened water, starting at 8.83 gpg of hardness.
  • Preventing the harmful effects of sodium is vital.  We’ll find it’s better to water plants with the water feeding the softener.  The higher mineral content also ensures that the plants get the beneficial calcium and magnesium from that water.
  • For the rest of this article, we’ll call hard water “untreated water.” That’s what the water is before it enters the softener.
  • Watering plants with soft water eventually causes a build-up of excess sodium or potassium in the soil.  This harms indoor potted plants the most when they aren’t exposed to rainfall.
  • Watering your garden with softened water should only be a temporary solution.  Dilute it further by mixing it with distilled water or rainwater.
  • All things considered, avoiding softened water in the garden is best.

Let’s dive into the story of softened water…

Water, water everywhere, but what’s this “soft” stuff?

Almost all water in nature moves across rocks and soil.  The result is that flowing water picks up many things that could be good or bad.  One important group is the minerals that dissolve in the water.

Some of these minerals are compounds of calcium and magnesium.  When they’re present in water, the water becomes “hard.”  These minerals are vital nutrients for plant and animal life.  But untreated water causes problems with some essential things that people rely on.

Calcium and magnesium don’t cause health problems for people; we need them, too.  We can drink untreated water without issues (except for the “funny” taste).  But here are two significant problems people have with it:

  1. Calcium and magnesium scale can build up on expensive pipes and plumbing fixtures.  Over time, the scale causes damage and clogs pipes.
  2. The minerals in untreated water react with soaps and detergents and form a filmy scum.  This makes cleaning agents ineffective.  I remember a short time in my childhood when my family had to deal with this problem.

Those two critical points are the main reasons why water softeners exist.

We must remove the calcium and magnesium ions to solve these problems.  This happens in water softeners through a process called “ion exchange.”  Untreated water enters the water softener, and softened water comes out.

Water softeners need to use sodium chloride or potassium chloride salt pellets.  The salts provide the ions required for the ion exchange.  To learn more about this process, see the section How an Ion-Exchange Water Softener Works.”

A well-known fact is that salts in water or soil will kill plants.  That raises the question of whether water softened by salt is safe for plants.

A Note About Soft vs. Softened Water

The term “soft water” includes more than water sent through a softener.  Rainwater is soft since it doesn’t have dissolved calcium or magnesium ions.

It also has other characteristics that make it the best water for growing plants.  Nature has far more plants growing with rainwater than we have growing with some type of processed water.

The term “softened water” indicates water that became soft through an artificial process.  Ion exchange is the most common process, and it relies on salt.

How much salt do you need to soften water?

This is the section where we discover some facts about softened water and its effect on plants.  Sorry about the math I had to use below, but sometimes it’s necessary.

The tables below show the hardness of water based on the total dissolved solids (TDS).  That’s the term used in the water treatment industry.  It also shows the amount of salt (sodium chloride or potassium chloride) needed to soften the water.

As far as I know, this is the only place where this information is available in concise tables.

The basic information for these tables is from the following sources:

Tables: Sodium and Potassium Levels Needed to Soften Water

Sodium Needed to Soften Water

Hardness LevelHardness as
grains/gallon
Hardness as
mg/l = ppm
mg of sodium
added/liter
to soften water
(mg/l)
mg of sodium
added/gallon
to soften water
(mg/gal)
Soft0.0 – 3.510 – 60softening not neededsoftening not needed
Moderately Hard3.57 – 7.0261 – 12028.3 – 55.7107 – 211
Hard7.08 – 10.53120 – 18056.0 – 83.5212 – 316
Very Hard>10.53>180>83.5>316

Potassium Needed to Soften Water

Hardness LevelHardness as
grains/gallon
Hardness as
mg/l = ppm
mg of potassium
added/liter
to soften water
(mg/l)
mg of potassium
added/gallon
to soften water
(mg/gal)
Soft0.0 – 3.510 – 60softening not neededsoftening not needed
Moderately Hard3.57 – 7.0261 – 12048.1 – 94.7182 – 358
Hard7.08 – 10.53120 – 18095.2 – 142360 – 537
Very Hard>10.53>180>142>537
Download These Tables Here

Notes to help understand these tables

  • The most common unit for water hardness is grains per gallon (gpg).  The unit parts per million (ppm) is in second place.
  • To get an idea of the size of a grain, a standard aspirin tablet is five grains.  One grain also equals 64.799 mg.  Using ounces with this information is cumbersome because a grain = 0.00229 ounces.  That would lead to some ugly numbers.
  • To convert hardness from parts per million (ppm) to grains per gallon, divide ppm by 17.1.
  • For sodium chloride softeners, every gpg of hardness needs 30 mg of sodium per gallon (7.9 mg per liter).
  • For potassium chloride softeners, every gpg of hardness needs 51 mg of potassium per gallon (13.2 mg per liter).

How much sodium in softened water will harm plants?

Now, we have it all broken down about water hardness and how much sodium to use to soften it.  The next step is determining what sodium level in softened water starts causing harm to plants.

According to the Salinity Management Guide, harm to most plants starts when sodium exceeds:

  • 70 mg/l in water,
  • 230 mg/l in soil,
  • or 5% of plant tissue.

70 mg/l of sodium in water puts us on the “Hard” row of the table above.  We must calculate that number since we can’t see it on the table.  We need the exact gpg of water hardness that needs 70 mg/l of sodium for softening.  Here are the steps:

  • First, we need to convert 70 mg/l into mg/gallon.  Since there are 3.785 liters in a gallon, 70 mg/l X 3.785 l/gal = 264.95 mg/gal.
  • We’ll round that off to 265 mg/gal.  That’s the amount of sodium in mg/gal needed to soften that water hardness.
  • Now that we know the sodium needed in mg/gal, we can figure out the hardness in grains per gallon.  The note above says every gpg of hardness needs 30 mg of sodium to soften it.  Since we have sodium equal to 265 mg/gal, then gpg = (265 mg/gal) / (30 mg/g) = 8.83 gpg.

So be wary if your untreated water tests at 8.83 gpg or higher before softening.  That’s the starting point for higher sodium levels that could quickly harm plants.

Let’s get a better idea of how much sodium that is

It helps to know what 265 mg/gal of sodium in water represents.  We all know how big an eight-ounce glass is, so it can help us.   Let’s calculate how much of that sodium would be in an eight-ounce glass of softened water:

(265 mg/gal) / (128 oz/gal) = 2.07 mg/oz of water.  That’s per ounce of water, so eight ounces of water = 8 oz X 2.07 mg/oz = 16.56 mg.

For anyone who doesn’t speak metric, there are 0.00003527 ounces in a milligram.  So, let’s convert 16.56 mg to ounces.  That would be 16.56 mg X 0.00003527 oz/mg = 0.000584 ounces of sodium in an eight-ounce glass of water!

No matter how you look at it, that doesn’t seem like a lot of sodium.  But it’s a high level for plants.  It could also be a high level for people on a low-sodium diet.

So, what do we know so far?

  • If water hardness exceeds 8.83 gpg before softening, the softened water’s sodium can immediately harm plants.  Please don’t water plants with this softened water.
  • If your water hardness is 8.83 gpg or higher, water the plants with the untreated water instead of the softened water.
  • Water with hardness up to 26.3 gpg is safe for watering plants.  Please see our article about using untreated water for plants for more information.
  • The ultimate solution is a home water system with a water softener and a reverse osmosis filter.  After softening the water, the RO filter removes excess sodium or potassium.  Reverse osmosis water is free of most contaminants and minerals.
  • Rainwater will dilute or wash away any excess sodium or potassium in the soil outside.Collecting rainwater is a great way to get a backup supply to water your garden.
  • Indoor plants in pots can suffer the most from excess salt or minerals.  Since they don’t get rainfall, each watering with hard or softened water builds up minerals or salts.  Give house plants some rain and extra sunlight by moving them outside occasionally.
  • Dilute hard or softened water with distilled water or rainwater.  That will decrease the rate of build-up in the soil.
  • At best, using softened water below 265 mg/gal of sodium on plants should be temporary.  Using the untreated water that feeds the softener is always a better option.  This is the system we used back home when I was growing up.  The outside faucet had a dedicated pipe that went straight to the untreated water line feeding the softener.

Watering Plants With Potassium-Softened Water

Well, that was quite a lot of information about water softened with sodium chloride! The situation changes, though, when softening water with potassium chloride.

Potassium is an essential nutrient for plants, and they tolerate much higher levels than sodium. Potash is a significant component of many plant fertilizers, and its main component is, you guessed it, potassium chloride!

Canada is the world’s largest potash producer, with 31% of the total. 95% of potash used worldwide goes into fertilizers. That’s a lot of potassium chloride.

I’ve had some significant experience with potash mining. Before I retired, I had an enjoyable career as an electrical engineer. I did many projects in Canada, and some of them were at their potash mines.

The point of all that is to show that watering plants with potassium-softened water is harmless under normal circumstances.

But always remember the old saying, “It’s not the poison, it’s the dose.” If plants or people try to consume any good thing in massive quantities, it will affect our health.

If you’re watering your plants with potassium-softened water and you see whitish deposits forming on the soil, it’s time to use distilled water to leach out the excessive salt.

The Effects of Softened Water on Plants

We’re keeping with the term “softened” water since it differs slightly from “soft” water.  Rainwater is soft water, and except for floods, it isn’t harmful to plants.  The harm from softened water depends on the amount of salt used to soften it, as we showed earlier.

1. Sodium or Potassium Content

Sodium is an essential micro-nutrient for people, but it’s far more “micro” for plants. Plants have some particular problems with excess sodium.  First, let’s look at the reasons plants need these minerals:

Benefits of Sodium for PlantsBenefits of Potassium for Plants
Build osmotic potential for cells (the ability of water molecules to move across a membrane).Build and maintain the xylem and phloem, the tissues that move water, sugars, and nutrients through the plant structure.
Water absorptionNecessary for the operation of the stomata.  These are organs on leaves used for plant respiration.
Maintain turgor (cell rigidity so plants can stay vertical).Aids photosynthesis
Improves drought tolerance, root growth, and resistance to pests and diseases.
Aids in starch and protein production (makes plants healthier to eat).

Living cells, including plants, have more need for potassium than sodium.  The problem is plant cells can absorb sodium as easily as potassium.

Exposing plants to excess sodium in water or soil causes the plants to absorb sodium more than potassium.  The sodium continues to accumulate and causes sodium toxicity. This causes yellowing leaves, stunted growth, and, in severe cases, plant death.

2. Sodium interferes with water balance

As sodium builds up in plant tissues, they’re less able to absorb water.  That leads to dehydration and death.  If excess sodium is in the soil or water, dilute it or wash it away.

3. Lack of Essential Minerals

Calcium and magnesium are essential plant nutrients, and softened water lacks these minerals. They’re crucial for cell wall structures, enzymes, and the creation of chlorophyll.

I remember a biology class I took way back in my college years.  The professor said that the magnesium in chlorophyll is somewhat analogous to the iron in our hemoglobin.  Every molecule in life serves a vital role.

4. pH Levels

Softened water tends to be more acidic than untreated water.  Most plants prefer slightly acidic conditions, while a few others may suffer.  Knowing the pH preferences of plants is an essential rule for successful gardening.

Mitigating the Effects of Sodium-Softened Water

Here are some ways to lessen the adverse effects of sodium-softened water on plants:

  • Bypass the Softener.  Most water-softening systems come with a bypass valve that will provide untreated water for your plants.  Don’t forget to check our article about using untreated water for plants for more information.
  • Rainwater HarvestingRainwater is always the best option for watering plants, but it isn’t always available.  Collecting rainwater could solve that problem.  Some misdirected local governments have regulations about rainwater collection, so check your area.
  • Dilution.  Mix sodium-softened water with rainwater, distilled water, or RO water.  That will lessen the effects of excess sodium or potassium.
  • Add Essential Minerals.  Calcium and magnesium supplements are available to replace these minerals lost by softening.

Additional Information

How an Ion-Exchange Water Softening System Works

Water Softener Salts

These water softeners use either sodium chloride or potassium chloride.  Here are some comparisons between these two salts:

  • Potassium chloride can be three times more expensive than sodium chloride.  It also takes about 30% more potassium chloride to equal the softening of sodium chloride.
  • Sodium chloride is more effective at softening water than potassium chloride.  So you use less sodium chloride, and it costs less.
  • Potassium chloride is essential in softeners for people on low-sodium diets.  It also has more nutritional value for plants and people.
  • Extra potassium could cause problems for people with chronic kidney disease.

The Ion-Exchange Process

One sentence can give a basic explanation of ion-exchange water softening:

“Water softeners exchange the calcium and magnesium ions in untreated water with sodium or potassium ions from salt.”

Here’s a step-by-step explanation of the process
  1. Brine Tank.  This could be a separate unit or combined with the water softener tank.  With a new system, the happy homeowner puts a few gallons of fresh water in the brine tank.
  2. Salt Pellets.  The brine forms after dumping a bag of water-softener salt pellets into the water.
  3. Softener Tank.  The tank is full of tiny resin beads made of polystyrene and divinyl benzene (DVB).  These beads, rather than the salt, actually soften the water.
  4. The Regeneration Stage.  This is when brine water pumps through the resin beads.  It causes the sodium or potassium ions in the brine to charge the beads.
  5. The Backwash Stage.  After regeneration, the brine shuts off. Then fresh water flows through the beads to flush and drain all the brine.
  6. Untreated Water Inflow.  After the backwash, the softener is ready to treat water.  As water flows into the softener tank, it comes into contact with the resin beads.
  7. The Ion Exchange Process.  The calcium and magnesium ions from the untreated water displace the sodium or potassium ions on the resin beads. This releases the sodium or potassium ions into the water.  So, calcium and magnesium ions are “exchanged” for sodium or potassium ions.
  8. Softened Water Outflow.  After the ion exchange, the water that flows out of the softener is now “softened.”  It contains more sodium or potassium ions and fewer calcium and magnesium ions.
  9. Regeneration Again.  Over time (about a week), the resin beads get full of calcium and magnesium ions.  That makes the beads unable to capture any more ions. To refresh the system, a new regeneration cycle starts.

Brine has a high concentration of sodium or potassium ions.  Pumping in brine causes those ions to displace the calcium and magnesium ions on the resin beads.  This “recharges” the beads so they’re ready for another softening cycle.

Another backflush cycle removes the wastewater containing the displaced calcium and magnesium ions.

While water softeners reduce water hardness, they don’t remove other contaminants.  These other contaminants include things like water treatment chemicals, sediments, and microbes.  A reverse osmosis (RO) filter can remove all those other contaminants.

The Facts on Salt-Free Water Softeners

“Salt-free water softener” is a marketing term used for water conditioners.  These units can improve the taste and smell of water.  They do this by removing chlorine, gases, lead, etc., but they don’t soften water.

Water conditioners take the calcium and magnesium compounds in untreated water and alter their structure, so they become crystals.  In this form, these minerals are less able to form scale on plumbing.  The minerals remain in the water.

The main purpose of salt-free water softeners/water conditioners is to improve the taste and smell of water.  Protecting the plumbing is a secondary feature.

Do you want to learn more about water conditioners?  Check this information from Culligan.

Water Testing Kits That Also Check Salinity

Bob Styer

As a child, I hated gardening. That was mainly because Dad expected us to work in the garden every so often even though we thought play was more important. Over the years, though, I've developed a real appreciation for growing things. Whether you're growing plants for food or to enjoy their beauty, gardening can make your life better. Seize the moment!

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