yellowing, starting at the bottom and moving up, indicates a deficiency of an element that is mobile within the plant. The plant is stealing nutrients stored in those lower leaves and sending them up to the top growth where they are needed.
Luckily, there are only 3 mobile nutrients and one semi-mobile... so diagnosis is pretty easy. It has to be showing you a lack of either or a combination of Nitrogen, Potassium or Phosphorus. Since these are also some of our macro nutrients, or the ones that the plant uses the most of, it is easy to assume by seeing this, and if everything else is correct, that the problem is lack of feed.
Remember too that nutrient dosage levels on a lot of these cannabis fertilizers are assuming that you are under a 1000w HID light... but you are outside. Expect your plants to have at least a 50% greater need for nutes.