Bluelab pH PenPH Pen pH measurement Plants get their nutrients from minerals, but they can only get all the right minerals they need if certain conditions are right. Think of nutrients like a lock. pH is the key. A key factor that affects food uptake is whether your solution is acid or alkaline. We measure this in pH. The Bluelab pH Pen also measures the temperature of the solution, as root health is vital to the success of a crop. Temperature affects the growth