The Impact of Hard and Soft Water in the Growroom

By Rich Hamilton
Published: November 25, 2019 | Last updated: April 30, 2021 12:38:55
Key Takeaways

When growing indoors we often hear about hard water and soft water, but do you really know what the difference between the two is? It’s worthwhile to look at exactly what hard water and soft water are, what their differing components are, and what positive or negative effects they have upon your plants and equipment and why.

Hard Water: High Calcium and Magnesium Levels

Hard water contains a higher-than-normal concentration of calcium (Ca) and magnesium (Mg). These minerals have had a longer or greater exposure to mineral-rich rocks and soil as it flows naturally through rivers and waterways. It also increases the chemical reactive levels within the water in terms of pH, making it much more alkaline. Generally, whether you have hard water or soft water is determined by your geographical location.


What impact do these higher concentrations of minerals have on your plants? Generally, the presence of more calcium and magnesium isn’t the problem in itself — it’s the balance between all the nutrient elements that can and does cause problems. The more calcium and magnesium in the water will directly influence how much of the other nutrients, such as potassium and phosphorus, get locked out and not taken up by the plant, causing deficiencies which in turn lead to undernourishment and growth problems. Further to this, the positive ions in the calcium and magnesium will serve to increase the pH of the feed solution and then any excess of CO2 carbonates in the water will exacerbate this situation further, causing the pH to become more and more alkaline in not only the solution but in the medium itself. The harder the water is, the more acid is needed to bring the pH back down to a plant-friendly level.

Maintaining Proper pH with Hard Water

So how do you combat this effect? Well before you begin growing you should find out whether the water source you intend to use is hard or soft. You can do this by looking online as there are maps available which will show which areas have hard or soft water. You can also call your city services and ask. If you suffer with lime scale build up in pipes, kettles, or other household appliances and systems, then chances are you have hard water. Measurement-wise, anything between 17.1-60 ppm is classed as slightly hard and shouldn’t cause you any noticeable problems. You can buy strip tests online which, when dipped into a water sample, will change color and the corresponding color will tell you how hard the water is (like a pH test). In fact, pH is good indicator as to how hard your water is.


Read also: Top 4 Reasons Why Your Garden's pH is Out of Whack

Ways to Deal with Hard Water

Hard water feeds are available from many of the big nutrient companies. They work by lowering the final pH of your feed but should only be used if you have very hard water, so a pH of 7.8 and above. If you use a hard water feed and your pH is still too high, you can use ‘pH down’ to reduce it further to the plant’s sweet spot of 5.5-6.5. Just remember to not try and adjust your solution until all your other nutrients have been added.

Another solution is using reverse osmosis on your water. This is a filtering system where the water is pushed through a set of membranes which have pores that decrease in size. This process removes calcium, magnesium, and other hard water culprits by blocking molecules over a certain size from passing through the membranes and so softens the water. Be warned, however, that this can be a costly process to run and maintain. If you are considering reverse osmosis for all of your water, it can also strip other nutrient elements from the water which could leave you with another set of problems if you use it too much.

Hard water may also impact equipment, where you will see a white chalky buildup of magnesium or calcium carbonate. This can accumulate quickly because the presence of heat within the growing environment creates an endothermic reaction in the solution, where the warmer it gets the more carbonate is created and deposited, creating blockages, restrictions, and, ultimately, underperforming or failing equipment. This is why hard water is not recommended for use in recirculation systems and such.


Ins and Outs of Soft Water

By contrast, soft water has had minimal exposure to rocks and earth that are rich in minerals, especially calcium and magnesium. It is water originating from surface pathways like rivers and streams where the basins are formed from hard, impervious rocks. Water can also be made soft via treatments like reverse osmosis and through using a water softener. Water softening is where hard water ions of calcium and magnesium are flooded with and replaced by sodium ions, which lowers the hardness of the water. This is a popular and useful treatment for clothes washing and bathing as it reduces the amount of chalky build up in pipes and machinery. Soft water is not the best for human consumption or for feeding plants. In measurement terms anything below 17.1 ppm or with a pH of 7.8 or below is classed as soft water.

Read also: What Every Grower Needs to Know About Water Quality


Soft Water Feeds

Just as there are many feeds created especially for hard water, there are also feeds for soft water, however, just keep it in mind soft water is not a problem in anywhere near the same respect as hard water. Soft, slightly acidic water provides perfect conditions for nutrient uptake in your plant both by roots and by foliar feeding, and it causes no known problems to equipment, so unless your water is extremely soft and your pH is very low, a universal feed will do the job without any trouble. Universal feed works well in soft to mid-range hard water. If you do choose to use a soft water feed, then be aware your pH will already be low and soft-water feeds do not really affect the pH of the nutrient solution, so you will definitely need to adjust it yourself using ‘pH up’.

Hopefully this has given you more of an understanding of your water source, why it is important to know what’s in it, and how it can affect not only your plants but also your equipment and other nutrient elements. If you can do your research and testing to help get and keep the conditions as near to perfect as possible before you start growing, then you will have a sound foundation to build upon and you won’t go too far wrong.


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Written by Rich Hamilton | Writer, Consultant, Author of The Growers Guide

Profile Picture of Rich Hamilton

Rich Hamilton has been in the hydroponics industry for more than 20 years, working originally as a general manager in a hydroponics retail outlet before becoming an account manager at Century Growsystems. He enjoys working on a daily basis with shop owners, manufacturers, distributors, and end users to develop premium products.

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