Nitrogen is one of the most important nutrients for all plants, and this is especially true of marijuana plants. When the plants are in the vegetative state (this means they are still growing but have not yet begun to flower) they require a lot of nitrogen. If your marijuana plants are not getting enough of this vital nutrient, the older leaves will begin to turn yellow and will eventually die and fall off of the plant.
Treating a nitrogen deficiency is not difficult once you have pinpointed the problem. Any local garden center will have available different soil treatments for plants lacking in nitrogen. They often come pre-mixed and only require minimal effort to be used. If you are reluctant to use a chemical product to treat this deficiency, an organic fertilizer will also do the trick. The main component in organic fertilizer is cow manure, which is naturally very high in nitrogen.
It is important to remember, though, that yellowing of the leaves just before your marijuana plants begin to flower and form buds is normal. At this point in the plant life cycle, not as much nitrogen is required.
It might seem strange that you can also provide too much nitrogen for your young marijuana plants, but that is also a very common problem. You will notice that the rate the marijuana plant grows will decrease considerably, and the leaves will turn a dark green color. If the problem persists, the leaves will ultimately turn a dark brown and will fall off of the plant. The treatment for this is simple: stop putting so much nitrogen on the plants! Once you adjust the level accordingly, the plants will start to grow more rapidly and the leaves will regain their bright green color.
Nitrogen is not the only nutrient that can be over-used. All of the nutrients that your plants might lack can also be ruined by having too much of them. This phenomena is called, simply, nutrient burn. The effects on your plants can be devastating if left untreated. If you notice the tips of your marijuana leaves turning black, you are probably giving them too many nutrients.
As with most deficiencies, the first step in treating nutrient-burned plants is to flush the system with water that has the correct pH level. Then, do not add any additional supplements to the water until you see a noticeable change in your plants. Marijuana plants are by nature hardy, so you should notice signs of recovery within a week.
Once your plants begin to lose the black tips and start to look healthy and vibrant again, you can begin adding nutrients back to the water. In order to avoid another care if nutrient burn. It is best to start with about a quarter of the recommended amount. Most supplements instruct you to use far more than the plants actually need. If they are looking better but are still not growing at their best, add more nutrients in very small increments.
Although the balance of nutrients might seem a hard line to walk, the best way to avoid problems is to keep an eagle eye on the pH level of your water and adjust it accordingly. Prevention will save you a lot of time and money in the long run.