Humus soil is soil made up of decayed organic material. Popularly, it refers to soil that is light in texture, dark brown or black in color, and sweet-smelling. It is considered the richest soil and is usually described as the ideal soil type. Most soils are a combination of the soil types, and tend have either more clay or more sand, with varying amounts of humus.
In terms of chemistry, humus is made up of completely decomposed plant and animal material. The material is broken down to its elemental form, and it is largely composed of carbon, hydrogen, oxygen, and a small amount of nitrogen. Humus does not contain active bacteria or microbiotic life, which makes it a stable substance because it will not decompose any further.
With regular cultivation, including the addition of soil amendments such as minerals to balance pH and fertilizers made of organic materials, the humus content of soil gradually increases over time. Although gardening stores sell bags of material labeled as “humus,” it is not really possible to create or harvest the pure substance. In nature, it is generally a combination of materials at various stages of decomposition. Pure humus soil contributes to the soil's overall quality. It is an integral part of healthy soil structure, which also includes minerals from sand or clay and organic matter that is still decomposing.
Colloquially, humus can refer to any organic soil matter. It is often confused with compost, since the two can look alike. Finished compost should bear little to no resemblance to the original material that went into it and, like humus, compost has a light texture, dark color, and sweet-scent. The difference between compost and humus is that compost is still undergoing decomposition, and it contains live bacteria and other nutrients that directly contribute to plant growth.
Almost no soil is completely sand, clay, or humus. Sandy soil is generally poor in nutrients, and cannot hold water well, so it may not be able to hold up shallow-rooted plants. Clay soil is generally richer in nutrients, but water cannot drain from it and plants' roots often cannot penetrate it. Humus has the ability to hold moisture, which is why it improves sandy soil. At the same time, its light texture allows moisture to drain and oxygen to circulate, which is how it improves clay soil.