Without a doubt, organic matter is the most important soil component. It provides food and shelter for the flora and fauna in the soil. As it is broken down by microorganisms, it releases the nutrients that are essential to plants.
Humus binds the soil particles together, allowing them to form stable aggregates and to improve the soil structure.
A soil test will tell you exactly how much organic matter soil contains. Healthy soil contains at least 5% organic matter.
Compost and composted manure are the main sources of organic matter. How much you need to apply will depend on your soil test results and the needs of the plants you intend to grow.
Microorganisms in the soil break down organic matter, quickly releasing nutrients (mineralization) and forming humus (humification).
Mineralization: The process by which organic matter is decomposed into nutrients that can be directly assimilated by plants.
Humification: The process by which organic matter decomposes to form earthy-smelling dark brown to black material, called humus. Humus is mineralized in turn, but very slowly. This makes small amounts of nutrients available to plants over a very long time.