y yellow leaves??

[sc0usa]

can ne1 help me? my plants are about 4 weeks old now and all of the bottom leaves are turning yellow quickly,my ph is 5.5,im using a hydroponics kit with a 400w growell lamp,rockwool cubes and i have a fan on so they are not burning , but im using "power-gro" nutrients, its says it has nitogen in it so can ne1 tell me what im doing wrong?

[godsbackyard]

Its normal for the bottom leaves to die off. The plant is fine.