yellow baby

one of my 5 day plants has got a yellow color between the leafes, the leafes are very small and i`m worried, i tried to fert it with a very weak fertilizer, and i`m still wating for results, do u think it`s gonna make it or is it doomed?
I was thinking of putting 2 plants in the same pot, would they be succesfull ?
Does anyone know what is the averege yeld for this specie, if i leave it for 2 moths in veg and one in flowering?
Please help me out,
Thanx

What time of soil are you using?

garden soil, ph 6-7, with nutrients included. Not sterilized. But my other plant is doing ok in it ...