Safe-T-Sorb, Turface, Soil Master Select, and many other substrates do not come pre-loaded with fertilizer. You need to add the nutrients plants need.
Sort of like buying a set of dishes and hoping they came pre-loaded with dinner.
Substrate is the holding material, it is not the food.
High CEC substrate will hold onto the fertilizers and release them slowly as the plants need them. But you need to keep on top of adding more fertilizer. The plants are sequestering fertilizer in their leaves, roots and stems. When you prune the plants you are removing the fertilizers from the system. You need to put some fertilizers back.
This is true of ALL substrates. Some come with certain amounts of some nutrients pre-loaded, but these nutrients get used up and removed from the system. They need to be put back. The high CEC substrates provide a very good cushion, an excellent reserve of fertilizers so you can be a bit forgetful about dosing and the substrate provides the back up. Doing an 'overdose' sort of fertilizer regime like EI can really add lots of fertilizers to the substrate. Then, when you go on vacation you do not need to worry about dosing. You have built up a reserve that the plants can call on while you are gone.