Water Hardness and Growing – What You Need to Know!

How does Water Hardness Affect My Grow?

Water hardness is caused by the presence of dissolved minerals such as calcium, magnesium and carbonates. The hardness of the tap water in the UK (and indeed around the world) varies a lot by region and the supplier. Some people have soft water, others have normal hardness water, while yet others have very hard or "bad" water.

Some people say they can tell the softness or hardness of water by taste alone, particularly in their tea or coffee where they say the minerals "mask" the true flavours and add a somewhat metallic taste. Another way to spot the hardness of water is by how difficult it is to achieve a good lather with soap. Soft water makes it relatively easy to produce a lather, while the dissolved minerals in hard water make it more difficult. Another way to spot hard water is to check the inside of your kettle after a few weeks of use. If you see a deposit of a light grey crust on the bottom and around the inside then chances are you have hard water, at least to some extent.

When we feed our plants we want to deliver a balanced nutrient profile. We want to include the right amount of all the nutrients that the plant needs in the right amounts.

The importance of giving plants the right amount of Calcium and Magnesium

When we feed our plants, we give them water (H2O) and a mixture of elements that they  need to grow. There are a total of 3 macronutrients (nitrogen, phosphorous and potassium) which are needed in relatively large amounts. There are also 3 secondary nutrients (calcium, magnesium and sulphur), and  7 micronutrients (iron, zinc, copper, manganese, molybdenum, copper and boron). As stated before, in an ideal plant feed, those elements should be present in the correct quantities and proportions. However, when there is already an amount of calcium and magnesium in the starting water, we need to take account of that before adding in any more.

Not only do we want to supply our plants with exactly what they need but there should be very low or zero amounts of elements that are not needed by plants. Any elements present that are not required by the plant can have a negative effect on plants' ability to absorb and uptake the ones that they do need. Take sodium (Na) for example. Plants don't need sodium. If there is sodium present in feed water then that will make it difficult to uptake some of the nutrients that it does need. On top of that, if the essential elements are present in the wrong proportions then that likewise can interfere with the uptake of each other (which will likely lead to deficiencies).

So, calcium and magnesium are both essential secondary nutrients. If a plant becomes deficient in calcium then (among other effects) its immune and defence systems will become compromised and dark spots will appear on the leaves. A lack of magnesium will cause (among other effects) a reduction in chlorophyll production.

Too much of anything can be as bad as too little!

On the other side of the coin, too much of either of these secondary nutrients will reduce the availability of other nutrients, particularly potassium (as well as several micro nutrients).

However, they are also the most usual cause of hardness in tap water. Given that the amount of calcium and magnesium in tap water also varies a lot around the country, how can a grower ensure that his final feed water contains the right amount (not too much and not too little) of both of these elements? In this article we are going to explore how to do just that, and ensure that your feed water is well balanced in all the different elements.

In order to do just that, we first need to get a good idea of the levels of what water hardness means and of what is already in your starting water. Then we can take account of these "already present" minerals before creating our feed water. In order to do this, it is useful to be familiar with types of hardness and how it is measured:

Temporary Hardness and Permanent Water

For the end-user, hardness is often described as one of two types. One is called "temporary hardness", the other is called "permanent hardness". Temporary hardness is caused by dissolved calcium hydrogen carbonate, Ca(HCO3)2, and it is called temporary because it can be removed from the water by boiling (hence the deposit on the inside of the kettle above!). Permanent hardness is caused by calcium sulphate, CaSO4, and it cannot be removed by boiling.

General Hardness and Carbonate Hardness

There are simple water test kits available at aquarium suppliers which allow for simple measuring of water hardness. A small sample of tap water is taken and reagents (chemicals) are added drop by drop until a change in colour is noticed. There is usually one test that will tell you something called the general hardness of the water (GH), and another test that will tell you just the "carbonate" hardness (the amount of hardness due to just the amount of carbonate present and is abbreviated to KH). The carbonate (KH) hardness test gives us a measure of temporary hardness.

The general hardness test measures the both the carbonate hardness (temporary hardness) and the hardness caused by sulphates (the permanent hardness) as well.

Step 1: Creating a well-balanced feed for your plants: Find out what your water supplier says about the quality of your water

The very best way to find out the composition of your tap water (including the hardness) is by sending a sample of it off to a laboratory for chemical analysis. However, many people would feel that this is going a bit far. Fortunately there are a couple of things you can do to get a decent idea of what you are dealing with without having to send off a water sample.

Second to having a laboratory test your tap water is to consult your water supplier. Most water suppliers in the UK have a website where you can get a report on the quality of your water by simply entering your postcode. (Check out the links at the bottom of the blog)

The reports will usually tell you the degree of hardness of your tap water. This may be as a description such as "fairly hard", or in terms of one of the commonly-used measurements of water hardness such as degrees in Clark, French or German scales. Sometimes the report will contain all 4. The water here in Stoke-on-Trent is supplied by the Severn Trent Water Authority. The report for our postcode, ST4 2TE tells us that our tap water is "Hard". It also tells us that it is 17.66 on the Clark Hardness scale, 25.23 on the French Hardness Scale, and 14.13 on the German Hardness scale.

The German Hardness scale (sometimes referred to as dGH, or degrees of General Hardness) is probably the most useful scale for us growers because measuring the General Hardness of the water gives us a more accurate picture of what we are dealing with. Unless we are going to boil the temporary hardness out of your feed water and allow it to cool (which few of us have the time for!) then we need to take account of all the hardness in the water. How the German Hardness Scale relates to what you measure in the next step is explained below:

Step 2: Finding out the level of mineral content in your water with your nutrient strength meter

Pure water from a reverse osmosis unit has a CF / EC / PPM of effectively zero (i.e. it is pretty much just pure, clean water with nothing else in it). However, if you draw some water straight out of your tap and then test it using a nutrient strength meter (such as the Bluelab Truncheon) in almost all cases you will see that the water that you thought was fairly pure and clean actually has a CF / EC / PPM all its own. In most cases, nearly all of the reading is because of the calcium and magnesium present in the water. These elements get dissolved in our water as it passes through rock and countryside on its way to the water treatment plant and then to your tap.

This test is to find out how much total dissolved solids (TDS) are in your tap water picked up during its journey through the countryside and water treatment plants. This can be done by measuring your straight tap water with a nutrient strength meter:

Take a sample of your tap water and test it with your nutrient strength meter. In the picture above, we tested the tap water at our Stoke shop with our favourite - a Bluelab Truncheon. You can see that we got a reading of EC 0.4 (or a CF of 4). We know that on some days this reading can be EC 0.5 (or a CF of 5). Most water supplies vary a little from day to day, week to week, and even from month to month.

If you looked up the German Hardness of your water on your water supplier's website and divide it by 4 then this will give you approximately the reading you will get on your nutrient strength meter (in CF), or dividing it by 40 will give you the approximate reading in EC.

What the measurement means:

If the reading you get is:

  • between 0 and 0.3 EC then you have a low amount of mineral content. The water almost certainly contains less than ideal amounts of calcium and magnesium. Ideally, a Cal-mag supplement should be added to bring the E.C up to around 0.4 or 0.5.
  • between 0.4 and 0.7 EC then you have a fairly normal amount of mineral content. The water likely contains enough calcium and magnesium for good plant growth.
  • 0.8 EC or higher then you have a high amount of mineral content. The water probably has too much calcium, magnesium and other unneeded minerals with could cause lock-outs, deficiencies and other problems.

So, going back to our reading with the Bluelab Truncheon, the display on it of 0.4 EC is suggesting that here at our shop the mineral content of our tap water is fairly normal.

At this point it is worth pointing out again that while the nutrient strength meter test gives us an indication of hardness, actually it does not tell us the whole story. What we are reading here is not just the hardness, but the the total amount of dissolved minerals, not all of which are a cause of hardness. However, usually the minerals which cause hardness contribute towards the bulk of of the reading you see on your meter.

Step 3: Use the readings to help create an ideal feed

Water with a CF of 0 (for example, from a reverse osmosis unit) contains very little of anything, let alone calcium and magnesium. In this case it is important to use a nutrient which has been designed to be used with reverse osmosis water. Some nutrient manufacturers that highly recommend reverse osmosis water are Cyco and Advanced Nutrients.

Water with an EC of below 0.4 contains low amounts of calcium and magnesium. Either use a nutrient that has been specifically designed for Soft Water or use a calcium-magnesium additive product to bring the EC to 0.4 before continuing to make up your nutrient solution.

Most nutrient manufacturers have a CalMag additive in their line-up


Water with a EC of between 0.4 and 0.7 most often contain enough calcium and magnesium in it for feeding to plants without having to resort to using a calcium-magnesium additive alongside your nutrient. In our experience, water with a starting CF of 4 works ok with most nutrient lines. If you don't need to remove chlorine or chloramine from the water to protect your beneficial microbes then you will probably have successful grows. If you decide to use a nutrient which has 2 versions, (one designed for Hard Water and another designed for Soft Water) then use the one designed for Hard Water. Some manufacturers that make Hard and Soft Water versions of their nutrients are:  Canna, Dutch Pro, General HydroponicsMills Nutrients, Plant Magic, Shogun Fertilisers and Vitalink.

The excellent Dutch Pro and Shogun Fertilisers nutrients have different formulation for hard water and soft water.

Dutch Pro Nutrients

Shogun Fertilisers

To summarise:

  • For low mineral content water with a reading of between 0 and 0.3 EC all that needs to be done is to add a quality calcium-magnesium additive to bring the EC up to 0.4
  • For water with normal mineral content with an EC reading of between 0.4 and 0.7 nothing needs to be done.
  • For water with a high mineral content with an EC of 0.8 or over, it would be wise to seriously consider getting and using a reverse osmosis unit to bring the EC down to 0, and then add a quality calcium-magnesium additive to bring the EC back up to 0.4 before adding anything else. Alternatively, use a nutrient specifically designed for Reverse Osmosis water

Making the jump to using Reverse Osmosis water

Using Reverse Osmosis water gives us growers absolutely pure, unadulterated water to begin making our nutrient solution with. With Reverse Osmosis water, all the extraneous minerals (which are of no use to our plants, and possibly cause problems with the uptake of necessary nutrients) have been taken out. It is the ideal "Clean Sheet" to start with. A Reverse Osmosis unit is not just great for fixing water with high hardness/mineral content. Bringing the starting water down to an EC of 0 (and removing all traces of those extraneous minerals first) will almost always improve plant growth for any grower with tap water which has a starting water of 0.4 or above. If you use a humidifier or fogger then you really need to be using RO water in it:

When making nutrient solution with RO water always remember to use a nutrient designed for Reverse Osmosis water, or bring the EC up to around 0.4 with a quality Calcium-Magnesium additive before continuing to make up your feed water.

As touched upon in a previous article, Reverse Osmosis water also has a special place for those growers who utilise friendly microbes in their grow. A Reverse Osmosis unit removes the chlorine/chloramine content which inhibits (and can even kill off) the friendly microbes in your grow. Part of organic gardening is about building a thriving microherd in the soil which then go on to feed the plants indirectly as well as defending the roots against pathogens. For this reason chlorine and/or chloramine content is not very compatible with organic gardening where we want the soil microbes to thrive. Recently, a Product has come along which quickly neutralises all chlorine and chloramine in tap water. It is made by Ecothrive and it is appropriately called Neutralise

Some Resources that May Help:

Your local water supplier can usually provide details about the water quality in your area. For example, for the West Midlands (home of the Stoke Store of One Stop Grow Shop) the water provider is Severn Trent Water. Their website here: Severn Trent Water allows you to check your water hardness (as well as other quality measurements) by postcode.

Here is a list of the UK Water Authorities websites:

Anglian Water (East of England)

Castle Water (Thames Valley) No hardness or quality by postcode check.

Dwr Cymru / Welsh Water (Wales)

Northern Ireland Water (Northern Ireland)

Northumbrian Water (North East England) No hardness or quality by postcode check.

Scottish Water (Scotland)

Severn Trent Water (East and West Midlands)

Southern Water (South East England)

South Staffs Water (South Staffordshire)

South West Water (South West England)

Thames Water (Greater London and Thames Valley)

United Utilities (North West England)

Wessex Water (South West England)

Yorkshire Water (Yorkshire and Humber)

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