Seeds will use most of the nutrients in the soil when germinating. They will then continue to get their nutrition from the first leaves that are emerging from the sprout. Once the plant becomes big enough, it will need to get more nutrients from an outer source to help it grow stronger.We recommend using liquid fertilizers, or water soluble plant food. It is very common and easy to get with a simple online search. You should always read the instructions for the fertilizer you decided to use, as each kind will have  different dilution quantities and different feeding frequencies, according to different stages of the growth.