Are those yellow leaves in the new grow? Please check this:
"Generally speaking, mobile elements are those that can be moved from older to newer tissue in the plant, while immobile ones cannot be. For example, calcium is incorporated into the cell wall, so it is immobile; it cannot be relocated later.
Plants with a deficiency of mobile elements (e.g. nitrogen) will exhibit damage to older tissues as the nutrients are moved to new growth. Conversely, plants lacking immobile elements (e.g. calcium) will exhibit stunted new growth while the older tissues stay healthy. Some elements (e.g. sulphur) fall somewhere between these two extremes"
Nutrient Profiles and Deficiencies by Mr. HIGHway
Nutrient deficiencies and excesses in Cannabis
Perhaps Nitrogen deficiency?
That plant doesn’t look too bad, you should see mine’s 
. Northern Lights is a very interesting strain, I live in the north coast of Spain and with my climate and humidity it might be appropriate, she’s on my wishlist … 