Organic matter in the soil consists of anything that has once lived. It provides carbon and energy for microbes. Microbial activity creates the nutrients that feed plants and makes them more nutritious. Microbes physically stabilize the soil by binding particles together. Soil microbes become more active in warm weather, digesting organic materials and converting them to available plant nutrients. Microbes are added to the soil from several sources.
Bacteria and Fungi
The living microbes that break down first in the decomposition process are bacteria and fungi. Many plants decay quickly, thus adding valuable microbes and fungi to the soil. Organic matter in soil loosens the structure and increases its pore space. The soil becomes less compacted and forms granules that are the size of coffee grounds. The soil's water-retention abilities are increased. Bacteria and fungi are added to the soil by growing cover crops and green manure crops, or adding compost. It is more beneficial to continually add compost and green manure to soil than to till it in. Disturbing the soil reduces organic matter content. The "no-till method" is recommended for organic farming and home gardens.
Partially Decayed Plants
Materials that are slower to decay, such as plants and animal manure, also create microbes for healthy organic matter in the soil. As plant and animal materials break down in the soil they release nutrients that include nitrogen, phosphorus and sulphur. Nutrients in organic soil are slowly released to growing plants, rather than the sudden influx of nutrients from chemical fertilizers. A cycle of slow-release nutrients helps plants resist disease and harmful insect infestation. High organic matter content in soil is the beginning of healthy plant growth.
Humus is dark brown well-decomposed organic matter. It is the stable material that is formed from decayed plant and animal life. Humus is the slowest decaying material and provides soil stability. Humus is what you find on forest floors or undisturbed soil. It is matted and dark brown. It is also the end product of composting. When composted material becomes dark brown or black and granulated it is called humus. Composted humus is ready to use in the garden as fertilizer, soil amendment and mulch.