Compost and dead leaves are excellent. Do not use sawdust, wood chips, or similar materials. These require a long time to break down and rob the soil of nitrogen. Avoid uncomposted manure. It contains large numbers of weed seeds.
Organic matter holds more water and nutrients than any other soil constituent.
It breaks up heavy soils, improving water intake and air exchange to plant roots. Organic matter firms light soils, making them richer and less drought prone. In each case, adding organic matter modifies a soil so that it behaves more like a loam. The benefits of adding organic matter include increased seedling survival, better root development, and faster plant growth. These crops improve the soil by bringing up nutrients from the lower soil and converting them into plant organic matter.