The NPK ratio is basically a measure of the concentration of 3 key nutrients—nitrogen, phosphorous, and potassium—that can be found in fertilizers. There really is no ideal fertilizer and every marijuana grower has his own theory, but, as a general rule of thumb, a 7-5-5 ratio works for vegetative growth.
When marijuana plants go into flowering stage, the nitrogen concentration can stay at a similar level, but, as flowering progresses, it should be reduced. Higher phosphorous promotes flower formation, but it is highly acidic. This is balanced out by a corresponding increase in potassium. For adequate flowering, you’ll want a formula of 3-10-10. Nutrients need to be diluted to 1,200 to 1,800 parts per million (ppm). A garden’s fertilizer requirements are individuated for a variety of reasons.
First, water quality—including its mineral content—varies, which alters the fertilizer needs. Second, different marijuana strains might use nutrients at different rates. Third, the plants’ nutritional needs shift with environmental changes in the garden. Light and temperature conditions can have huge effects on plant needs. Fourth, marijuana’s nutritional needs change based on its stage of life. Early and late vegetative stages and early, mid, and late flowering all have unique nutritional requirements.4