Hey there,
what sort of light cycle do you have your plants on? If they're already into flowering, its quite normal for the bottom leaves to turn yellow as the plant draws nitrogen up from the lower leaves during flowering ( as you reduce the nitrogen level in your nutrient feed as you go through the flowering stage). But if your still in veg state i.e still growing for size, then it could be simply to add more nitrogen in your feed. Are the plants in general good condition apart from this??? If not, may just need more frequent feeds.
Don't hold me to any of this as Im far away from being an expert, just what ive picked up on from growing my first lot?
Hope this helps...any other takers on this one???

MNSN