its common nitrogen deficiency....
..."nitrogen is a very mobile nutrient. that means it can move throughout the plant as when needed. all the new leaves get plenty of nitrogen to make them green and help with photosynthesis because the leaves that get the most light are the newest. so the plant "wants" to give those leaves priority for getting light. makes sense? if the new leaves aren't getting enough nitrogen, the plant will start to take nitrogen from the older - lower leaves. this is what causes the yellowing and wilting. it's relatively normal for your plant's leaves to start turning yellow.
but, if your plant is losing lower leaves fast due to yellowing (if yellowing and dying leaves is "climbing" up the plant from the bottom), especially in the vegetative stage before plant is making buds, you have a problem that you will need to fix as soon as possible. it is not good if your plant is showing signs of an advanced nitrogen deificiency while still in the vegetative stage. it's normal to lose a few yellow leaves off the bottom of your plant here and there, especially with very big plants. But if you are losing a significant amount of yellow leaves, and the yellowing seems to be moving up the plant quickly feed it right away dont wait"...
i'm paraphrasing a cite growweedeasy... and yes it's nitrogen...