soil plant problems

So here's the deal. I have one plant about 8"-10" high in my closet. I have one 150watt HPS light on a 20/4 light cycle. There are four sets of branches and the top leaves are an emerald green. But the lower sets (total of four leaves total) are turning yellow. The ones closest to the surface of the soil are a green/yellow and the ones above it are a little yellow. Is this a major problem? if so how can i solve it? will this one light be enough to support the plant for its life cycle. I'm only planning on vegging until it reaches about 1-1.5 feet high. Its just my first grow and anything that deviates from the norm i think is a major problem any comments help. thanks - Ganja

2.http://www.icmag.com/ic/showthread.php?t=11688

http://images.google.com/imgres?img...tomato+leaf+chlorosis+picture&um=1&hl=en&sa=X

sounds like its time to fertilize,use half of recomended on bottle,then work your way up to full streangth next fert time