Plant Food??

I have 4 plants in pots and theyve been in the same soil for 4 weeks now and they are growing reeeally slow. What kind of nutrients can i buy and how do i use them. DO i have to transplant into new soil or can i buy a liquid. One that i can pour in with the watering?

Are they in at least 2-3 gallon pots 5 gallon are best? Did you use a good soil like Fox Farm or Oceans? Maybe Miracle grow moisture control anything less will slow you down? How much light are you useing? I like Fox Farm 3 part system grow big, tiger bloom, big bloom.You use the feeding schedule from each different manufacturer