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Humus is a dark soil material that is one of the three components resulting from the degradation of organic material in soil. The other two are heat and simple end products (like nitrogen, carbon etc.)

Humus is formed when organic material (such as leaves, dead animals etc.) is degraded by a combination of fungi, bacteria , microbes and other animals (earthworms for example) that reside in the soil. At first when the organic material is added to the soil, there is a great abundance of microbial activity as the number of these organisms increase with the fresh influx of "food". Gradually, however, the organisms numbers taper off to a final population that remains in the soil, essentially waiting for the next addition of "food". What is left behind is humus- a dark rich soil mixture composed of compounds that resist degradation.

Humus has the following characteristics:

•  It is composed mostly of carbon, hydrogen and oxygen in the form of microbial resistant material (such as lignin a complex compound that gives plants rigidity)

•  Has a very high water holding capacity

•  It does not stick together well, so when added to a soil, it tends to open the soil up to allow easier movement of gasses, water and roots in the soil.

The rate at which humus is formed varies considerably. Some of the factors that control this rate are:

•  Temperature. Even though the degradation of organic material releases heat as a by product, temperature plays a very important role. Too cool and the degradation slows down or stops and too hot will kill off the microbes that are doing the degrading.

•  Moisture. This is required for the degradation process. As with temperature, too wet or too dry will slow or stop the break down of materials.

•  Organic material type and particle size. Of course the type of "food" that is offered to the microbes will determine the speed at which it degrades. If the material has lots of resistant material (like lignin) in it, the breakdown process will be slower than if the material has very few resistant materials. Likewise, the size of the particles is important. Smaller particles offer a larger surface area for microbes to work on compared to large pieces.

The formation of humus in a soil takes a long time. In some climates it can take decades for material to completely degrade. For this reason, a class experiment to actually create humus in a soil may not be feasible.