With iron deficiency it would be the youngest top leaves that turn yellow first and the leaf veins would, at least to start with, remain green, so it's not that. Because it's the lower leaves I would look first at nitrogen deficiency, although if it is only yellow between the veins and the veins are still green it could be magnesium deficiency. That's assuming it is a nutritional issue. It may be nothing more than the plant is shedding its older leaves. Did you get them from a garden center in pots?
