A nitrogen deficiency is a very common deficiency because plants use a lot of nitrogen. If there is a general issue with a lack of fertilizer, a nitrogen deficiency will be one of the first symptoms to appear.
During the vegetative phase, plants take up nitrogen in high quantities to support growth, and also to store for use later during the bloom phase.
Plants in vegetative growth that are deficient in nitrogen will experience small leaves, stunted growth, and sparse branching.
A nitrogen deficiency shows as lower leaves turning pale green and then yellow. The yellowing starts at leaf tips and moves inward.
Plants will take nitrogen from older growth to protect younger growth.
Yellowing will start on the old growth at the bottom of the plant first and then travel up to the the new growth as the nitrogen in the lower leaves gets sucked dry.
A deficiency will likely first show up during a phase of rapid vegetative growth when nitrogen is getting used up quickly. To prevent nitrogen deficiency, add extra nitrogen to the reservoir before periods of rapid vegetative growth.
If extra nitrogen is added to solution after a deficiency has already begun, it will take about a week for plants to recover. Leaves will turn from pale green and slightly yellow back to a healthy green. Very damaged, badly yellowed leaves won’t recover and will die.
An application of a foliar spray with fertilizer containing nitrogen will help speed up recovery and prevent further leaf damage. Only use a foliar spray during vegetative growth.
Cannabis plants that are grown in hotter than average environments use approximately 10%-20% less nitrogen. If the grow room is warmer than average, consider reducing nitrogen. If the grow room is colder than average, consider raising nitrogen.
If nitrogen is toxic, leaves will become dark green and droop, starting from the bottom of the plant.