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Hydroponics: the importance of pH

Updated: Jan 28, 2021

This week’s blog is going to cover a very important topic in hydroponic growing: pH. You’ll learn what pH is, how it affects your plants and how to measure it. There’s also a step by step set of instructions on how to measure and tweak your pH so your plants are thriving and not just surviving!

What is pH?

pH is a measurement scale of how acidic or alkaline a water-based solution is. It is an abbreviation for ‘power of hydrogen’ or ‘potential hydrogen’ and this is because it measures how many hydrogen ions there are in a solution. The pH scale is inversely related to the amount of hydrogen ions - so a lower pH indicates a higher concentration of hydrogen ions.

The scale goes from 0-14; 7 on the pH scale indicates a neutral solution, lower than 7 is considered acidic and higher than 7 is alkaline. A few examples: Lemon juice has a pH of 2, coffee has a pH of 5, baking soda has a pH of 9 and bleach has a pH of 13.

Why does pH matter in hydroponics?

Hydroponic growing revolves around using a water-based nutrient solution, so you can immediately see how important pH will be in this discipline. To complicate things, different types of plants have a fixed pH range in which they will grow optimally. If our nutrient solution deviates from this range, the plants won’t be able to uptake certain nutrients, leading to deficiencies. You will produce lower quality crops and may even experience crop loss!

The graph below shows how nutrients are taken up differently by plants depending on pH. Straying to the extremes will lock these nutrients up, unable to be used by the plant. So even if you have the most expensive nutrients on the market, you could end up with a poor harvest!

What is the right pH to grow at?

The pH level that you need use will depend specifically on what you’re growing. The range that most common crops need will be around 5.8-6.5. You’ll notice in the graph above that this is a sweet spot, going more acidic (<5) reduces uptake of the three important macronutrients, Nitrogen, Phosphorus and Potassium, as well as Magnesium and Calcium. Going more alkaline (>7) will reduce uptake of minerals like Iron, Manganese and Boron.

Below is a table of the ranges you can keep certain crops within to ensure perfect growing conditions.

From the table above we can see some more pH sensitive crops (Lettuce, Mint and Rosemary) as well as some resilient plants which can withstand a wider range. If you want plants which aren’t bothered by pH fluctuations, grow Kale, Bok Choy, Chives, Fennel and Oregano.

How do I measure pH?

There are two main ways that we can test pH. The first is using a colour indication and the second is by using a pH probe, the latter being far more accurate. Determining pH by colour indication relies on paper strips or by adding some drops of indicator solution into a sample of water. If it turns a light green colour this indicates it is slightly acidic around 5.5-6.5. If it is more acidic than this it will start to go yellow, but these are all estimations

pH probes are far more accurate because they give you a digital reading. Most will give a reading +/- 0.1 and this is close enough for our purposes. Place the probe into the nutrient solution and in no time at all you will have a pH reading. Avoid cheaper probes because they will give results that jump around a lot, making it hard to get a solid reading.

How do I alter my pH levels?

It’s unlikely that your local tap water will be the perfect pH for what you intend to grow, which means you need to have a strategy for tweaking it to the right level.

Here in London the average tap water pH is between 8.0-8.5. I find in most cases to alter your pH you’ll be using a product called pH Down. It's simply an industry name for Phosphoric Acid which will bring your pH down. I recommend buying the litre bottle because you’ll be using it quite a bit. I recommend also buying a small bottle of pH Up (Potassium Hydroxide), so if you make the mistake of making your nutrient solution too acidic, you can bring it back up with pH Up.

Here’s what you’ll need:

  • pH strips or pH probe

  • 1L pH Down (Phosphoric Acid)

  • 0.25L pH Up (Potassium Hydroxide)

  • Pipettes

  • Stirring Rod (bamboo cane)

  • Goggles and Gloves

1. First you need to fill your reservoir with water and add the appropriate amount of nutrients, mixing it thoroughly with your stirring rod. Always remember to add nutrients before measuring pH because they can alter the results.

2. Measure the pH, ideally with a probe. In this example we will be growing Lettuce at a pH of 6.5, but our readings give a pH of 7.2 in our reservoir.

3. Wearing eye protection and gloves, fill a pipette full of pH Down solution and begin to add it drop by drop. If there is a large difference in pH it is sometimes okay to add the solution more liberally.

4. Mix the whole solution thoroughly with your stirring rod and then take another pH reading.

5.Repeat this until you get your desired pH of 6.5. If you overshoot (<6.5) you can add pH Up by the drop until you're back into range.

6. Check your pH every couple of days and every time you fill up your reservoir.

That's it!

I hope you found this helpful and informative. Don't hesitate to get in touch if you've been experimenting with hydroponics or if you're planning to, we're here to help and happy to answer your questions!

Square Mile Farms bring vertical, urban farming to city dwellers in their homes and in the workplace. We aim to bring people closer to food production and help to create a culture of healthy, sustainable living. Find out more about our offering for homes and offices and get in touch here with any queries. Sign up to our newsletter for tips on a healthy lifestyle and a round-up of relevant news. You can also follow us on social media to stay up to date with our journey, find us on Instagram, Twitter and Facebook. Why not join our online Urban Growing Community for advice on growing at home and much more!

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