I need some help with my plant

My plant is 1.5 months old, and older leaves are getting yellow and dying, this happens in a pattern, when 3 floors of healthy plants have developed the bottom one starts dying, and so on, as you can see in the picture.
I´m not using any ferts, i water (every 4 or 5 days) directly into the soil without watering the leaves. I´m using a 60 watt flourescent light(swirly tubes) 5 inches away from the plant, the plant is well ventilated. It has already lost some 10 or more leaves in total.
So, how is she doing? is she gonna make it?
Why are the leaves becoming yellow?
Thanks four your time!

 

yeah I had this problem too bud, I asked about it on here and I was told that I needed to start using fertiliser, that solved the problem within a week for me. Nitrogen (one of the things in fertiliser) is important for the growth of the plant. When the plant isn't able to extract enough nitrogen from the soil to give to the new leaves that are growing, it will take the nitrogen from the older leaves at the bottom to give to the fresh leaves at the top. Thats why the oldest bottom leaves are turning yellow and dying. So just buy some fertiliser and start mixing it in when you water, that should solve your problem really quickly.