i couldnt say weather it is really needed or not, but i make sure just to use plain water for the last 2 weeks and hope the plant uses everything up. (i dont flush) these are powdered organics though not liquid. right now with 2 weeks to go the leaves are turning purple from what i have read is phospherous deff. i wont add any more now, but next time i think i will add an extra tbs. while mixing the dirt up. i dont really think i want deff. during the last 2 weeks of flowering.