pH is one of the most important things to measure in your nutrient solution. It's a measure of how acidic or basic something is. pH is about ion activity - specifically the balance of Hydrogen (H+) and Hydroxide (OH-) activity. More H+ means the solution is acidic and has a pH below 7. More OH- means the solution is basic, and has a pH above 7.
We'll explore the practical side of pH: How measuring and managing it can help your plants grow faster, bigger and healthier. If you're interested in details of pH in general, check out our other articles. As a general rule, you want your hydroponic water to have pH between 5.5 and 6.5.
How available nutrients are to plants depends on the pH of the nutrient solution. For example, at pHs above 6.5, some micronutrients, like iron, precipitate out of, and stick to walls and other surfaces. In other cases, plants may not be able to absorb nutrients efficiently if the pH is too high or low. If pH is very low, your plants may absorb too much of a micronutrient.
pH that changes suddenly, or drifts in one direction or the other can mean something's gone wrong with your system. For example, a dropping pH may mean there's high bacteria activity, or dying organic matter: Decomposition usually lowers pH. If your pH keeps dropping very low, consider emptying your reservoir, treating with hydrogen peroxide, and refilling. It might also mean you need to remove dead or rotting roots. Or, it could be thought of as a gentle reminder that it's time to change your reservoir water.
Your goal is to manage pH in your nutrient solution, so that it stays at the right point for your plants. This can vary depending on your plant, but it's usually slightly acidic. Here's a table showing pH for common types of plants:
(todo: ideal pH table)
There are a few ways to measure pH:
Paper strips are available that change color when inserted in your solution: They color they turn shows corresponds to pH. You read them by comparing the paper color to an included chart. They're cheap and easy, but aren't accurate or precise, and have to be replaced. pH-sensitive dyes are also available in liquid form: Add a few drops to a sample, and check the color.
Digital meters are the easiest way to measure pH. They come in a variety of styles and prices, with more accurate ones tending to be more expensive or rugged. Some styles:
Some digital sensors can be left submerged in your setup indefinitely, and continuously show you readings. This can be convenient. Look for sensors marked for continuous immersion, or double-junction.
pH measurement varies with temperature, which is why higher-end meters tend to have temperature sensors included. For indoor hydroponics, you can ignore this, since the temperature won't change enough to make much difference. Outdoor setups may need to take this into account.
You don't need to spend a lot for a pH meter and sensor: hydroponics doesn't require the precision required for laboratory and industrial uses. Any of the above method works fine, and which you choose depends on how much you want to spend, and easy it will be to use it in your setup.
pH meters need to be calibrated - check out our article on the topic.
The easiest way to change pH is to add buffers designed to do just that: They're commonly called pH up and pH down. They're designed to change pH in a controlled way, while being safe for your plants.
Household acids and bases like lemon juice, vinegar, and baking soda will change your pH, but they're not buffered, and, and may act as food for bacteria that are harmful for your plants.
How much modifier to add depends on a few things:
A good starting rule of thumb is to add 1 ml of modifier per 4 liters of solution. The key to success: Add a small amount, and wait several minutes to half an hour, then measure pH, and readjust if needed. Be careful about overshooting! After a few times, you'll get a feel for how your system responds.
Buffering is how well your system resists changes to pH. Buffer solutions can be added: These are a mixture of a weak acid and its conjugate base or vice-versa. Most pH up and pH down solutions are buffered, so adding them not only changes pH, but slows future changes.
Buffering can work against you too: For example, carbonates introduced to your system from source water or media can increase its buffering capacity, and make it tougher to low or a high pH. Usually pH is greater than 7 when this happens.
pH sensors, like all glass junction electrodes, must be stored wet. This might mean in your nutrient solution, or in a special storage solution. Nearly all of these sensors come with their measuring end capped with a small bottle containing this solution. It's usually a 4.0M KCl water solution. Water may be an acceptable substitute, but could lead to more sensor drift over time. If you leave the sensor dry for too long, it can become damaged.
How often to check depends on your system, what you're growing, and how precise you'd like to be. Start by checking once every few days. If you notice pH is stable, you can lower the interval. You should also check each time you change our your reservoir, after adding nutrients. Sensors that stay submerged and have a display make checking fast and easy.
It's possible for a computer or microcontroller to manage pH for you! The concept is that a device continuously measures pH, and compares it to a desired pH you set. It's connected to a pair of pumps (usually peristaltic pumps): One connected to a pH up, the other a pH down. When measured pH differs from desired, it activates one of the pumps, waits, measures again, then repeats as needed. Sound familiar? This is an automated version of the way we adjust by hand!
These systems are convenient, but currently expensive. We hope this changes soon! If you're interested in a DIY project, check out our article on the topic.
If you're measuring soil pH instead of a water solution, the techniques are different. Some techniques let you measure pH or soil directly, and others require sampling, and mixing with water.