MadSci Network: Environment & Ecology

Re: How is humus formed?

Date: Wed Nov 10 12:34:34 1999
Posted By: R. Ted Jeo, Bio Sci Tech, St. Paul, MN
Area of science: Environment & Ecology
ID: 941574839.En


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.

    Instead, to understand and observe the degradation process, you might want to try a compost experiment. With this type of experiment you can observe how fast material decomposes and you can control the factors that effect decomposition.

    A simple compost experiment might start with only one type of organic material (like leaves) or a mixture (vegetable scraps, leaves and grass clippings). Place the material in a pile outside. Ideal composting requires some moisture (usually the material composting has enough in it), chopping up the material into smaller pieces (to offer more surface area) and aerating the material (allowing air to move freely about the material). Aeration is an important factor, allowing aerobic organisms to thrive. If anaerobic (no air) conditions exist, the compost material may stagnate and start to really stink! To really speed up the decomposition of material, you may add some sort of innoculum, that is a mixture of microbes, bacteria or fungi that artificially starts the composting process. You may be able to buy pellets that are used in home compost piles to speed up degradation.

    The easiest way to check and see if you have active decomposition happening is to record the temperature of your mixture. You should see a rapid increase in temperature as the microbes start to work on the material, and then, over time, a decrease in temperature. After some time, you will be left with a rich dark humus like material.

    You might want to go to the web site of the International Humic Substances Society (IHSS) to learn more about humus and humic substances. As you are in Germany, you might be able to contact Dr. Fritz Frimmel at the Engler-Bunte-Institue of the University of Kalsruhe who is the vice president of the IHSS.

    Hope this is of some help.


    Personal communication with Dr. C.E. Clapp, USDA-ARS, St. Paul, MN
    Brady, N.C., "The Nature and Properties of Soils", 9th ed

    (Any commercial reference given, whether by name, in text or by subsequent web links is NOT an endorsement of any kind by myself, my employer or the Mad Scientist Network.)

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