Fertilizers are food for plants. They come in either a solid pelleted form or in a nutrients solution form.  They both serve the purpose of replacing/reintroducing vital nutrients that the plants have absorbed during their growing stage. Without the addition of nutrients/fertilizers, crop yields and production would be significantly reduced and impacted. 

In order to have a healthy & growing crop, the key is to maintain a level of nutrients in the substrates (soils, peats or water) that will support the growth of  our crops without applying excess.