Compost is what you should mix with your existing soil to enrich it. If you use ample amounts, it should contain nearly all the nutrients plants will need. For annuals and veggies, which need a steady supply of nitrogen, you will still want to fertilize periodically for best performance. Dont' be stingy with the compost -- amend your soil by thoroughly mixing in 6 inches or more of compost into the top 6-8 inches of existing soil. Then, your plants will thrive in it.