I don't know why we are discussing this; what Jim said is completely
accurate. Bonsai represent a special way of raising plants without
natural soil. In such a situation, micronutrients are necessary all
year long because the trees have no way to get them unless they are
added exogenously. Since bonsai soils don't store much nutrient, the
fertilizer needs to be added all the time.

NPK fertilizers are meant to add the top 3 limiting nutrients to soils
that otherwise DO NOT LACK for fertility. I always use a full
fertilizer complement with my bonsai because I keep them in turface,
bark and grani-grit, a mixture that lacks micronutrients.

Nina, who is putting her trees in winter storage, except that crummy
abutilon and her pathetic collection of indoor bonsai.