My initial reaction would be you need more light hitting those bottom leaves, if they do not produce in terms of photosynthesis the leaves will begin to wither, and eventually die and fall off.

But, if you are not using nutes yet, now would be the time to start adding more Nitrogen to the diet, could also be magnesium, but more commonly you see N deficiency. Start out using half the recommend dosage, I recommend Jack's Classic All Purpose, it is very cheap about $6.00 at your local nursery that carry's it, effective, and 100% organic. Eventually you will need Blossom Booster, but that is later in the game. Since you are most likely using a soil that does not have very many nutes, you may want to invest in Fox Farms Ocean Forest Organic mix, mixed with 40% perlite (miracle grow brand is good - ONLY FOR PERLITE!!)

You can find it by typing your zip code in here:

Buy Jack's Classic Plant Food -- J.R. Peters, Inc., Fertilizer Manufacturer

Good luck, post back as soon as you give it a try, let us know how you do!

On a side note:

It could be possible that it is normal for those leaves to die off, its natural, but I would still give the half strength nutes a shot.