Yellow leaves

The bottom leaves going yellow is pretty normal at this age and not something to be overly worried about. They just don't get enough light to keep going, besides, their main purpose of getting the seedling established has been done. Some of the yellowing could also be attributed to the leaf in the picture rubbing on the edge of the pot. Either way, not much to be concerned over. The fact they are going yellow slowly, indicates the plant is translocating the elements contained in the leaf, things like carbs, sugars, amino acids etc to be used elsewhere in the plant and this is why it is vital to not remove any leaves until they have gone yellow, for by the time they are yellow, they have served their purpose. Hope this helps,......... Organoman.