can someone help please

my plants are about 1 month old and have been under 8 5000 k florescents, the temp is 75f and i watter 1 time a day ony gave mericle grow 1 time and not more than half adult dose.
any one know why the bottom leaves are turning yellow and falling off ? here are some pics

first glance looks like N def. wich could be cuased by something being off in your mix cuasing the lockout. whats your growing medium and ph of your water going in and ph of run off after water? need a little more info to properly diagnose. =)