pH stands for “Potential Hydrogen” ions and uses a scale of 0-14 to tell us how acidic/basic something is. In the case of gardening it’s the pH of your grow medium: whether that be soil, water, coco or another soilless medium. Unlike most scales, this one starts in the middle and works outwards. 7 is Neutral, 7-14 is basic, 0-7 is acidic. 0 and 14 being the two extremes.
Plant growth can be heavily affected by the pH of the medium it is grown in because the pH can cause the nutrients to be more or less available to the plants. Most plants like a slightly acidic root zone, somewhere around 6-6.2. However some people obsess over the exact pH of their plants and are convinced it must remain at 6.2 at all times but there is certainly an acceptable range.
It’s important to know the pH of the medium you decide to work with as it can mean the difference between smooth sailing and a headache. This is the reason most peat mix comes with Lime in it. The Lime acts as a pH buffer: it helps to pull up the pH of peat based mediums to keep it constantly near neutral and not as acidic as peat tends to be without a buffer. If you decide to grow with an acidic medium and buffer be sure to keep a close eye on your pH as buffers tend to get washed out over a series of waterings. For this reason a recirculating system can allow for better control of your pH as the lack of physical medium and buffers will keep those acids and bases out of your nutrient solution. It is important to note that pH UP and pH DOWN products are adjusters and not buffers. These are much more temporary but can play a big part in getting you to your optimal pH when measuring down to the decimal.
Check out this helpful video from The Garden Sage for more information!