Humus is decaying organic matter such as dead plants, leaf litter and the remains of dead animals in an advanced state of decomposition. Humus is usually dark in colour and rich in nitrogen. Life processes are those factors that separate living organisms from non-living things. Nitrogen-fixing bacteria in the soil and on the nodes on the roots of plants such as clover and beans are able to form these nitrates. The nitrates are absorbed in water by the roots and used by the plants to build plant tissue.
Animals receive nitrogen by eating plants.
When plants or animals die, bacteria and fungi return the nitrogen compounds back into the soil. Other bacteria break down these compounds and release free nitrogen back into the atmosphere. An organism is a living creature such as a plant or animal. See also life processes. In physical weathering, the rocks are broken down into smaller and smaller pieces as a result of wind, water, ice or temperature changes.