Looks like a magnesium deficiency, or maybe even nitrogen. Magnesium is essential for the green part of chlorophyll, as the very color in it requires an ion of magnesium to be green. Some cheap epsom salts might correct this. I am also gauging the other leaves which seems a little yellow then they should be, even for its age. Also although others may disagree with me, I am not a fan of using bark or other types of wood chips, branches or similar in a soil mix. Reason being that they can drain the soil of nitrogen, because they take a long long time to break down and compost. Some pieces can take years to breakdown. This composting action requires nitrogen and can rob the plant. One good rule of thumb is to avoid any thick pieces of wood or branches because of this reason. I have had lots of failures growing with too much un-composted woody material. I avoid anything that takes a significant period of time to breakdown, and that generally speaking is pieces of wood. If its been composted already and breaks easily in your hand into fine particles, its okay. If it remains hard, throw it out.