You are hereNitrogen Deficiency
Problem: A nitrogen deficiency will cause the older, lower leaves on your plant to turn yellow, wilt away and eventually die.
It's relatively normal for your plant's leaves to start turning yellow towards the end of your flowering cycle as the plant becomes nitrogen deficient while it's creating its buds.
However, if your cannabis plant is losing lower leaves fast due to yellowing, especially in the vegetative stage (before plant is making buds), you have a problem that you will need to fix.
If you notice your leaves turning yellow in the vegetative stage or in the beginning part of the flowering stage, your plant may be experiencing a nitrogen deficiency which will need to be treated.
The first picture is a plant in the vegetative stage which is experiencing the beginnings of a nitrogen deficiency. The second plant is showing signs of nitrogen deficiency late in flowering and is completely normal. The last picture is an infographic about nitrogen and your marijuana plant.
Solution: You can find many pre-mixed nutrients from the store which contain nitrogen or you could use nitrate of soda or organic fertilizer which are both good sources of nitrogen. Try supplementing your regular nutrients with a bit more nitrogen and see if the plant starts recovering.
More About Nitrogen and Your Marijuana Plants
Sometimes you can get the signs of a cannabis nitrogen deficiency if the pH at the plant root zone is too low, even if the nitrogen is there. This is because when the pH at the roots is not right, your plant roots can't properly absorb nutrients. If you aren't sure about your root pH, learn more about pH & growing cannabis plants here.
Nitrogen is especially important during the vegetative stage of your cannabis plants. As your plants start flowering, they will need lower amounts of nitrogen.
When looking at plant nutrients, you'll almost always see 3 numbers listed, like 3-12-6 or 5-10-5. These numbers represent the percentage of Nitrogen (N), Phosphorous (P) and Potassium (K) contained in the bottle. Just about all plant life on Earth needs these 3 elements to grow.
The very first number, "3" in the case of the picture to the right, always displays the proportion of nitrogen in this nutrient bottle compared to the other 2 nutrients (Phosphorus and Potassium respectively).
The reason nitrogen is in all plant nutrient formulations is because it's vital to plant processes.
Note: During the last few weeks before harvest, marijuana plants starts pulling all the remaining nitrogen from her leaves as part of the bud-making process. This causes yellowing leaves starting towards the bottom of the plant. This is part of the natural flowering process and you don't need to fight it.You may notice that marijuana leaves are yellowing in almost all pictures of marijuana plants with big buds that are close to harvest. You tend to get smaller yields from nitrogen-toxic plants with dark green leaves at harvest.
Remember: It's Normal For Marijuana Leaves To Start Turning Yellow As Harvest Time Approaches
Occassionally a nitrogen toxicity is mistake for a deficiency. Could your plant actually be nitrogen toxic? (pictured below)
This picture shows a Nitrogen Toxicity