If you want something water soluble and simple, some kind of organic liquid fish hydrolysate fertilizer, ideally with humates and kelp as well. I get my transplants all the way to transplant stage without any added fert by adding compost and a number of other amendments to the mix, including a pelletized organic fertilizer available in our area called MicroLife. I also use their liquid fert called ocean harvest for a root dip before transplanting in the field, if you look that up you'll prob be able to find something comparable in your area. #grownostr