while yellow leaves could be caused by alot of things if the problem starts with the older leaves and works its way up the plant it is usually nitrogen deficiency wouldnt worry to much about the very bottom leaves but if it continues to spread would probably feed them usually if you have decent hid lighting and grow room the plants will be able to handle full strength nutes by the time they are 3-4 weeks old while you should start them out at reduced strength just to see how they react.