Too Soon To Feed Again? [PICTURE]

So my plants are 40 days old and the bottom leaves started to yellow about 5 days ago so I fed them a small amount of nutes for the first time... the leaf color has not gotten better... how long should I wait to feed it again...

The first TWO pictures are from the 35th day
The last two pictures are from today ( day 40 )

bump......

looks like a nitrogen def. to me.

replant them they look like theyre in small pots when you do feed them a little not too much