Why Does pH Matter in Hydroponics?

The pH level of your hydroponic nutrient solution plays a large role in your plants' ability to grow and thrive. All throughout the internet however, you will find different pH recommendations for the same plant. It is difficult to know what applies to you without a solid understanding of the role of pH in hydroponic gardening. This post will explain what pH is, what environmental factors change your pH level, the impact of different pH levels of your hydroponic plants' growth, and how to manage your garden's pH.

What is pH

pH is a measure of the number of H+ (Hydrogen) ions in a volume of something (in our case, our nutrient solution). pH ranges from 0-14. 0-6.9 is acidic and 7.1-14 is alkaline or "basic". 7 is neutral. The human body has a pH of 7.4, whereas coffee, which is acidic, is somewhere around 5.

If you want to get really technical, a liquid with a pH of 6.2 contains 10^-6.2 moles/Liter of Hydrogen ions. The definition of neutral pH is that the number of H+ equals the number of OH- (Hydroxyl) ions.

I measure pH with a pH meter.

pH and Nutrient Availability

The pH level of your hydroponic solution affects the availability of nutrients to your plants. The recommended pH range for hydroponic plants' is usually between 5.5 and 6.5. This is because the macronutrients and micro nutrients that are vital for life are most available in this pH range when you plant with water instead of soil. The below chart depicts which nutrients are available at which pH ranges.

In the above picture (credit to hydro experts), you can see that some nutrients, like calcium and iron, are minimally available even in the 5.5-6.5 range, but nitrogen, phosphorus, and potassium are much more available. This is a tradeoff that makes sense for most plants. Leafy greens thrive very well in this range, for example.

pH and Plant Growth Potential

Plants actively maintain an pH level at their root zones that is ideal for them, whether that is neutral or otherwise. Changes in pH in your water solution therefore, affect mostly the solubility of nutrients in the water solution. If the pH is too high or too low, different nutrients will precipitate out of solution. At higher pHs, you will see iron precipitate out.

Incorrect pH is one factor that leads to nutrient deficiency. Extended periods of time spent without proper nutrients will lead to permanent morphological changes in your plants. Your plants' roots may fail to grow thick or long enough, which directly impacts your plants' size and quality. Another specific example is that if your plant goes too long without enough nitrogen, it will suffer from low chlorophyll production and small leaf size. This directly impacts your plants' ability to photosynthesize, which therefore stunts their growth.

Why Does My pH Change So Much?

Strangely enough, the pH of your hydroponic nutrient solution fluctuates over time even when you do nothing! In many hydro systems, you can't just set and forget the nutrient solution. This can be due to several common factors.

Breakdown of Nutrients

A common cause of pH change stems from the breakdown of nutrient salts in water when you set up your system as well as your plants' consumption of the nutrients. These causes of pH change are completely unavoidable! Plants tend to consume cations (ions that include H+), so this directly leads to an increase in pH. Less H+ means higher pH.

Planting Medium

Planting Mediums are not completely inert. Just like plain old soil, mediums like hydroton clay pebbles or sand provide sites which H+ ions will bind to. The action of binding H+ ions removes them from the water solution, which means the pH of the hydro nutrient solution increases.

Different mediums will bind more or less H+. Popularly used mediums like sand, hydroton clay pebbles, and rockwool may technically affect the pH. They are however, popular for a reason, which is that they don't affect it that much. I wouldn't shy away from those mediums on account of the pH affect.

Algae

Microorganisms like algae respirate. They use up oxygen and output carbon dioxide. CO2 when combined with water forms carbonic acid, which is acidic. So, presence of respirating organisms in nutrient solution, like algae, will lead to lower pH.The double whammy here is that the algae will consume oxygen that has been dissolved in the water, which your plants also need to survive. Make sure to avoid algae at all costs!

How to Manage pH for Maximum Growth!

Here are my top tips for pH management:

Keep out algae

Keep out algae by blocking all light from entering your hydro system. Use dark colored materials if possible and fill caps in net cups with hydroton clay balls or use large pieces if rockwool.

Changing water every 7-10 days and washing your system in order to kill anything that's growing will also help.

Adjust pH Manually Every Few Days

Adjust the pH using a hydroponic formulated pH Up and pH Down solution. Your plants will also regulate the pH of the solution that is right next to their roots. At different stages of life they may require more of some nutrient and less of another. Their self regulation accomplishes this, so it's also important to not micromanage the pH level (unless of course, the pH goes above 6.5).

Don't Let pH Go Above 6.5

When the pH goes above 6.5, we leave the golden zone for a lot of plants. Some nutrients like iron will fall out of solution and they may not become available again even after you adjust the pH down. If it goes above 6.5, it might be a good idea to change out your water. If you plants are still very young, you want to make sure that they get a good foundation for future growth.

Use a Well Formulated Hydroponic Nutrient Blend

Different hydroponic blends will make your nutrient solution have wider or smaller swings in pH. When I go on vacation, I'd rather use a hydroponic nutrient blend like this one from General Hydroponics, because the pH changes very little as nutrients break down. On the other hand, the General Hydroponics MaxiGrow has much larger pH swings. It is however, much cheaper and easier to use. Most of the time, I would just go with MaxiGrow and manually adjust pH more rigorously

Thanks For Reading!

I hope you learned something useful. Hopefully you now understand the optimal pH range for hydro gardens and how to maintain it. I hope you use this information to help your hydro garden reach its full potential!