If you notice your lower cannabis 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.

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. In fact almost all plant nutrients of any kind will include nitrogen. If you haven’t been providing any nutrient to your plants, try supplementing your regular nutrients with a bit more nitrogen and see if the plant starts recovering.

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