Humus is the stable, amorphous, heterogeneous fraction of the soil organic matter remaining after the major portion of added plant and animal residues have decomposed. It is dark brown to black. It is bound to the surfaces of clays and hydrous oxides, which renders it resistant to microbial attack. The original source of the organic materials is not easily determined.
Humus has a low bulk density (0.05 to 0.25 grams per cubic centimeter), is very finely divided (colloidal), and has a high porosity and water holding capacity. It has a very high cation exchange capacity (greater than 200 milliequivalents per 100 grams of pure humus or 200 centimoles(+) per kilogram of pure humus) which allows it to retain cations such as Ca2+, H+, K+, Mg2+, Na+, and NH4+ ions). Humus has a variable composition but approximates 50% carbon, 35% oxygen, 8 % hydrogen, 5% nitrogen, and 2% sulfur plus traces of all other essential elements for microbial life.
The humus content of arable soils averages between 2-3% and has a range of <1 to over 12%. Humus significantly influences physical and chemical properties of soils. It commonly is responsible for 20 to 30 % or more of the cation exchange capacity of soils. It buffers the soil against drastic chemical changes in pH in the root environment. Humus provides stability to soil peds by acting as a binding agent promoting soil aggregation.
Two factors restrict the ability to increase the humus content of a soil. First, the amount of organic material that must be applied and incorporated into the soil will be very large. The amount of organic material necessary to maintain the normal humus level in soil ranges from 1000 to 5000 kg/ha per year (1000 to 5000 pounds per acre-furrow-slice per year). Second, the addition of organic material to the soil will not permanently increase the soil’s humus content. The natural physical and biological changes in the soil will hasten the decomposition of the organic residues resulting after a few years in only a small quantity of the added material remaining as humus. Cultivated soils lose humus by biological decomposition at a rate of 1 to 2% per year. Crop rotations including grasses and legumes in conjunction with conservation tillage and crop residue management can be used to sustain the humus level and productivity of agricultural soils.