Nitrogen Deficiency in a cannabis plant is very easy to spot as it always starts with the lower leaves of the plant. It causes them to turn yellow and then eventually they will wilt and become crispy. If the problem worsens, it will move up the plant causing more leaves to die. Another sign of this nutrient deficiency, is the yellowing leaves will begin falling off of the plant. If you try pulling one of the affected leaves, it will come off very easily.
How to Cure:
If you are already watering your plants at the correct pH, then all you need to do is add a nutrient with a higher nitrogen content. On the front of all nutrient bottles are the N-P-K ratios, designated by 3 numbers like 6-4-4 for example. You want a nutrient that has a higher ratio of N, which is nitrogen, and this usually is found in veg/grow nutrients. For example Grow Big by Fox Farms, or Bio Thrive Grow by General Organics.