Looks like you need more Nitrogen. Nitro deficiency starts showing in the lower leaves as nitrogen is a mobile nutrient. When there isn't enough in the soil the plants takes it from its lower leaves. You may want to check your feed amounts and adjust them. Happy growing.
If they get more yellow, then add more nitrogen, but your plant looks very healthy. Go easy on the nitrogen, it's easy to burn her which is much worse than a slight deficiency.