Courtesy of Rastadub What N.P.K does for your plants and when: N-Short for nitrogen, is associated with fast lush growth. Nitrogen is needed in steady supply throughout a plants life. Cannabis requirements for nitrogen are the highest during the vegetative growth stage, and tapers off during flowering. P-Short for phosphorus, another nutrient needed by plants for photosynthesis and respiration. Cannabis' takes up large amounts of phosphorus during germination and seedling stages and then again later in flowering and seed set. K-Short for potassium, Is associated with developing sturdy stems and resisting deseases. Cannabis uptake of potassium is highest during its earliest vegetative growth stages.