Athosanswered grow question 5 years ago It is not always needed, like every PK booster you use it when the plant has enough nitrogen (dark geen leaves), but shows P or K deficiency, usually after stretch when all it's energy is going to bud production. This situation happens, when using prefertilized soil like heavy or all mix, the media usually has an excess of nitrogen, but will sometimes have a lack of P or K. Hope it helps