Trees need large amounts of nutrients, especially fruit trees. Growing in a pot brings with it more limited access to nutrients and, therefore, they must be replenished.
During the transplants we can add a new substrate, if it is of good quality it will incorporate the necessary nutrients for a few months.
You can choose to use liquid fertilizer or fertilizer on a regular basis following the instructions on the package and the needs of your tree. Usually they are applied monthly and it is necessary to avoid applying fertilizers or fertilizers during the winter period in deciduous trees.
The top 2-4 cm of the pot substrate can be replaced with new compost. This technique is not always viable, it depends on whether our tree has very superficial roots that we could damage.
The slow release fertilizer distributed around our tree (away from the trunk) in the amount indicated by the manufacturer will allow us to spend a lot of time without having to worry about its nutrient needs and allowing us to worry almost exclusively about watering.
In the event that we see that the leaves are wrinkled, have a different tone, spots or similar, we must check the health of our tree. In the case of there being no pests or diseases that cause this problem, surely we are facing a lack of some type of nutrient. We must compensate him to help him recover.
For example, citrus fruits are prone to iron deficiency, showing yellow leaves except for the veins. It is easily correctable with a supplement.