The golden rule of gardening says, "If you treat your soil well, it will treat your plants well." Healthy, fertile soil is a mixture of water, air, minerals, and organic matter. In soil, organic matter consists of plant and animal material that is in the process of decomposing. When it has fully decomposed it is called humus. This humus is important for soil structure because it holds individual mineral particles together in clusters. Ideal soil has a granular, crumbly structure that allows water to drain through it, and allows oxygen and carbon dioxide to move freely between spaces within the soil and the air above.
Application of organic matter to the soil adds carbon, which promotes the growth of beneficial bacteria, which increases the likelihood of hearty plants. Another benefit is when crops grow and demand more nutrients, added organic matter can be used as plant food. Remember that every time you disturb soil by turning or tilling, oxygen also is added to the soil. This increases microbial activity, which feeds on organic matter. Therefore, soil disturbance can decrease the soil's organic matter reserves and should be kept to a minimum.