There are so many ways to add nutrients to your garden. But you must be careful to only include fertilizers that are 100% organic. The best way to do that is to make it yourself. Yes, it does take more time and effort. Ultimately you will be rewarded with healthy living foods that will nourish you and your family.

Most systems you will use to feed you garden will involve the initial outlay of time and some equipment, but the maintenance is quite minimal.
You could start out with a very simple liquid fertilizer, while getting a compost heap (or two) on the go.

There are also some very useful commercial organic fertilizers that are widely available. Blood and Bone is an excellent nitrogen and phosphate fertilizer.
Seaweed extracts are a valuable liquid fertilizer as they contain a broad spectrum of micro-nutrients, promoting increased bacterial and enzyme activity. Fish waste and fish emulsions are an excellent source of nitrogen and phosphorus.

You will get the best results by creating your own compost. 

Apart from the fact that you will put to good use all of your kitchen scraps, animal manure if you have it (only herbivores - never include dog, cat, human or pig manure), weeds, leaves and grass clippings, you will get exercise putting it all together and turning it, and be out in the fresh air getting a natural dose of vitamin D!