Organic matter is an all-purpose soil enhancer. It helps sandy soil hold moisture and nutrients. It loosens clay soil and helps it drain and, when decayed into the sticky substance called humus, it binds the tiny clay minerals into sand-sized particles. It provides food for the microorganisms that are essential to the health of the soil. Unless you intend to grow desert plants that need to dry out quickly, organic matter in the soil will improve the health of everything you grow.
Dig over the patch of soil you're improving with the shovel or, if it has been tilled before, the garden fork. Break up the soil to a depth of at least 12 inches. Pick out any weeds and break up any large clods of dirt. If you are growing root vegetables such as carrots, remove all stones larger than a fingernail; a rocky soil can cause them to fork or deform. You may wish to remove larger rocks to make digging easier, but this isn't essential. Plant roots will simply grow around them without ill effects.
Spread a 2-to-4-inch layer of compost, bagged steer manure, peat moss, rotted straw or other organic matter across the soil. If you have very sticky clay soil or extremely sandy soil put as much as six inches of organic matter on top.
Mix the organic matter with the top 6 to 8 inches of soil using the garden fork or shovel. If you are adding 5 or 6 inches of material, it may be helpful to make a ditch at one end, removing the soil to a wheelbarrow. You can then fill the ditch as you mix the soil beside it, creating another ditch that you fill in with the soil beside that. Keep mixing and digging all the way across your garden patch. Mix the soil in the wheelbarrow with some extra organic matter and fill in the last ditch.
Rake the soil smooth and remove or break up any clods of dirt or clumps of compost.