As discussed in our article, “Cannabis Fertilizers, Supplements, and pH”, the pH of the nutrient solution affects the solubility of the nutrients in the solution. If the pH of the solution that you add is outside of the acceptable range, some nutrients will be unavailable to the plant.
Acceptable pH Range for Coco: 5.5-6.5
Coco does not buffer the pH of nutrient solution as effectively as soil and therefore it is critical to set the pH of the nutrient solution inflow within the range of 5.5 to 6.5. Because different nutrients are soluble at different pH, it is best to allow the pH to drift up and down within the range on successive fertigation events. During the majority of the grow your inflow pH should fluctuate around an average of 5.9. However, during the seedling and ripening phases it is best to target somewhat higher in the range (6.1-6.3).
There are two options for measuring pH: a color indicator test which uses liquid drops, or a pH meter. Other pH tests or meters, such as test strips, are ineffective. Many novice growers purchase two-prong “triple meters” that are supposed to measure “soil pH” along with water and light, these, in particular, are useless. You do not need to test the “soil” or the coco, you need to test the pH of the nutrient solution that you are going to add.
These are a cheap and easy solution to get started. Many pH adjustment kits come with a small liquid drop color test kit. These are an excellent option to start managing pH. The color test is not particularly accurate, but pH does not need to be a specific target and it is actually better to allow it to vary from fertigation to fertigation. The drawback to the drops is they become practically useless if you add a supplement that significantly darkens the color of the nutrient solution.
These range from cheap meters at about $15 to expensive models over $100. Unlike EC meters, the technology involved in measuring pH is sensitive and it is worth avoiding the cheapest meters. Cheap meters are unreliable and have short life spans. That said, you do not have to buy the most expensive meters either. We use and recommend this Apera Instruments pH meter. It comes with small jars of calibration fluid and is easy to use and store.
All pH meters require maintenance and calibration. Many meters come with powder to mix and use to calibrate the meter. These powders are useless unless you have laboratory grade instruments to measure the amount of water to dissolve them into. It is far better to purchase calibration fluid that is premixed at an actual laboratory. To maintain the meter between uses, it should be stored in either storage solution or calibration fluid. This can be done by putting a small amount of the solution in the cap of the meter or in the bottom of a jar and storing the meter in that jar immersed up to the “immersion line”.
Adjust Inflow pH:
Most of the nutrients and supplements that you add to the nutrient solution have an impact on pH. The base nutrients lower the pH and certain supplements like silicon and SM-90 raise it. Whether and how much you will have to adjust the pH of your nutrient solution depends on the pH of your starting water and your specific recipe for each stage of growth.
Give nutrient solutions several minutes to stabilize before measuring pH. To adjust pH it is best to use a product specifically designed for pH adjustment of horticultural nutrient solutions. We use and recommend General Hydroponics “pH up” and “pH down” products. They allow significant pH adjustment with very low doses and have minor impact on the EC of the nutrient solution. It should generally require less than half a milliliter of either product per gallon to successfully adjust the pH. After adjusting, be sure to re-measure to verify. We recommend recording inflow pH values in your grow journal.
Ignore Run-Off pH
Although it is critical to adjust the pH of the inflow, you should not be concerned with the pH of the run-off water. Your only concern with pH is always providing nutrient solution (inflow) that is in the appropriate range. Many growers make the bad decision to provide nutrient solution that is outside of the acceptable range in a misguided attempt to correct run-off pH values. This can destabilize the pH of the nutrient solution in the root zone and make nutrients unavailable. To avoid this common pitfall, simply ignore the pH of the run-off.
See our complete tutorial, “How To Mix Nutrient Solutions”