maybe some worm castings ...can you get by just fine with what you have i believe yes because the soil has nutes and even the soil outside of your soil (the ground) has nutes..but outside it never hurts to add imo because your roots will grow past your nutes and the nutes will leach around it just seems to me its hard to over do it outside and quite easy to under do it!...the seaweed is going to add to the mocrobiology and organisms in the soil and increase nute uptake by the plant and the nitrogen is a must but you may at some point be lacking p and k if the plant is a heavy feeder and purges the soil thats why i suggest adding the other ammendments....ill be honest i never tried to nail a science when growing outside unlike indoor, id just nail the plants and see how they responded and they would always respond good never was able to overfeed outside!