3 weeks is a good age to manually fertilize if the soil doesn't already have fertilizers. generally soil that's been used for 3 months is used up and needs new nutrients. again, I'm speaking of commercial soils. if you're using your own concoction or if it's outdoors, you should introduce the fertilzer slowly, with every 3rd watering or som starting in small quantities and gradually increasing (avoids burnage). just follow the instructions on the nutes