d, when they need it.
How do I check the pH?
There are two methods for checking the pH at the plant's roots. The most common method is to water the plant until there is a runoff and then check the pH of the runoff water using a hand-held pH meter. If the pH that comes out is higher or lower than what you put in, then you know that it is unbalanced.
The second method uses a soil pH meter. These unit have probes that allow them to test the soil near the roots after a watering.
How do I balance the pH?
To balance the pH, flush with water until the runoff is in an acceptable range. After the pH near the roots is at the right level, follow up with a hearty dose of nutrients and the problem should be solved.
To correct a deficiency, aim for a pH of around 7 in the soil, or 6.5 in coco, to get the most potassium uptake. If you notice the yellowing continuing, you likely have a root condition. If the pot is too small, transplant into a larger container.