Ph is one of the most overlooked metrics when it comes to growing plants both indoors and outdoors, many of the yellowing and deformed growth in plants are often tied to ph that's beyond acceptable ranges for your plants.
Invest in a good pH meter, both for your nutrient solutions and your soil mixes, that'll save you a lot of headaches.
The one with the long probe is the soil ph, the other one is for liquids. Stay away from low quality brands which tend to be innacurate.
by ShogunPeppers