Once your plants are in the ground, fertilizing for vegetative growth is the next step in growing a successful cannabis crop. Cannabis is what is known as a heavy feeder among plants, meaning it benefits greatly from nitrogen fertilization during the first part of its growth cycle. The plants respond with lush, vegetative growth and greater height. The tricky part at this stage is giving the plants all the nitrogen they need without overdoing it, and then tapering off of nitrogen when the flowering stage starts. Also, during the vegetative stage, cannabis plants still need other nutrients in moderate amounts, and the best vegetative growth fertilizers are organic ones with a balance of nutrients along with high nitrogen levels.
By starting plants in rich, fertile soil and giving them plenty of room to receive sunlight, the need for extra fertilization is reduced considerably. If you have transplanted your plants into high-quality soil, you can wait for a couple of weeks before you begin any supplemental fertilization. Plants primarily feed through natural means from the soil and environment are also more likely to remain pest and disease-free and to have a better quality crop at harvest. Over-fertilization can produce a finished product with lower quality flavor, smell, and potency. Time-release fertilizers are not recommended for cannabis, because they can damage the soil quality and release fertilizers at the wrong time. Liquid, organic fertilizers are best. They can be applied to the soil or through foliar feeding.
Fertilizers with a nitrogen content of nine to fifteen percent work well during the vegetative growth phase. This happens to also be an optimum nitrogen level for tomatoes, so balanced fertilizers for tomatoes can be used with cannabis during this period. Cannabis responds well to foliar application of fertilizers. Foliar feeding is done by mixing the concentrated fertilizer with water according to directions and applying it with a hand-pumped spray bottle or by using a hose-type fertilizer spray attachment. The fertilizer mix is then applied directly to the leaves where it is absorbed through the stomata. Liquid fertilizers, like fish emulsion and compost tea, can also be poured onto the soil where they are absorbed more readily than dry fertilizers. Be sure to dilute concentrated fertilizers according to instructions before applying them, and, if anything, use less rather than more.
Using fertilizers at half the recommended strength is a good starting point. Too much nitrogen can burn the plants and damage the soil. It is better to start with low levels and then add more, if it is needed. The primary symptom of nitrogen deficiency in cannabis is yellowing of lower leaves which then progresses up the plant. The yellowing also begins at the tips of the leaves and moves inward, as the plant struggles to shift available nitrogen to newer growth. After fertilizing, it takes about one week for the results to become visible. Resist the urge to fertilize again before giving the plants time to respond
As summer moves into fall, the hours of sunlight decrease. This is the trigger for most cannabis cultivars to shift to the flowering stage. When flowers begin to appear, gradually shift fertilizers toward lower nitrogen and higher phosphorus and potassium over a three-to four-week period.