Removed answered grow question 4 years ago How much is needed? 0.5 EC /250 PPM and if it's a seedling half that amount (assuming that your water is RO water or close to it). You need it for the whole grow, although after the stretch, when the plant is forming buds, you could switch to epsom salts, which is magnesium plus sulfur (actually switching to epsom salts is the ideal)