It holds apart the tiny particles of clay so they can drain excess water more readily. It provides clay soil with pore space, which lets in the air essential to good plant growth, and makes it easier for plant roots to penetrate the soil. It fills in excess pore space on sandy soil, slowing down drainage and increasing the water-holding capacity. It regulates soil temperature, releases nitrogen and other nutrients for plant use through the process of decay, and promotes growth of microorganisms which help condition soil. It may be necessary to supplement the organic matter with some fertilizer.
Most organic sources are low in phosphate and only produce limited available nitrogen.
Fertilizers should be added at planting time and as side dressings during the season. Once you have chosen a spot and have taken steps to improve the soil, it's time to decide what your are going to grown. Your aim should be to get as much high quality food from the area as your can. Try to grow family favorites and develop a good variety of crops to make meals more interesting. A small, well-kept garden will be more rewarding than a large one that becomes a bore.