Humus and Organic Matter
Organic Matter Fuels the Food Web
Soil organic matter is the storehouse for the energy and nutrients used by plants and other organisms. Bacteria, fungi, and other soil dwellers transform and release nutrients from organic matter.
Organic matter is many different kinds of compounds, some more useful to organisms than others. In general, soil organic matter is made of roughly equal parts of humus and active organic matter. Active organic matter is the portion available to soil organisms. Bacteria tend to use more simple organic compounds, such as root exudates or fresh plant residue. Fungi tend to use more complex compounds, such as fibrous plant residues, wood, and humus.
Intensive tillage triggers spurts of activity among bacteria and other organisms that consume organic matter (convert it to carbon dioxide), depleting the active fraction. Practices that build soil organic matter (reduced tillage and regular additions of organic material) will raise the proportion of active organic matter long before increases in total organic matter can be measured. As soil organic matter levels rise, soil organisms play a role in its conversion to humus, a relatively stable form of carbon sequestered in soils for decades or even centuries.
Where Do Soil Organisms Live?
The organisms of the food web are not uniformly distributed throughout the soil. Each species and group exists where they can find appropriate space, nutrients, and moisture. They occur wherever organic matter occurs, mostly in the top few inches of soil.
Soil organisms are concentrated:
Organic Matter Fuels the Food Web
Soil organic matter is the storehouse for the energy and nutrients used by plants and other organisms. Bacteria, fungi, and other soil dwellers transform and release nutrients from organic matter.
Organic matter is many different kinds of compounds, some more useful to organisms than others. In general, soil organic matter is made of roughly equal parts of humus and active organic matter. Active organic matter is the portion available to soil organisms. Bacteria tend to use more simple organic compounds, such as root exudates or fresh plant residue. Fungi tend to use more complex compounds, such as fibrous plant residues, wood, and humus.
Intensive tillage triggers spurts of activity among bacteria and other organisms that consume organic matter (convert it to carbon dioxide), depleting the active fraction. Practices that build soil organic matter (reduced tillage and regular additions of organic material) will raise the proportion of active organic matter long before increases in total organic matter can be measured. As soil organic matter levels rise, soil organisms play a role in its conversion to humus, a relatively stable form of carbon sequestered in soils for decades or even centuries.
Where Do Soil Organisms Live?
The organisms of the food web are not uniformly distributed throughout the soil. Each species and group exists where they can find appropriate space, nutrients, and moisture. They occur wherever organic matter occurs, mostly in the top few inches of soil.
Soil organisms are concentrated:
- Around Roots. The rhizosphere is the narrow region of soil directly around roots. It is teeming with bacteria that feed on sloughed-off plant cells and the proteins and sugars released by roots. The protozoa and nematodes that graze on bacteria are also concentrated near roots. Thus, much of the nutrient recycling and disease suppression needed by plants occurs adjacent to roots.
- In Litter. Fungi are common decomposers of plant litter because litter has large amounts of complex, hard-to-decompose carbonaceous material. Fungal hyphae (fine filaments) can "pipe" nitrogen from the underlying soil to the litter layer.
- On Humus. Only fungi make some of the enzymes needed to degrade the complex compounds in humus.
- On the Surface of Soil Aggregates. Biological activity, in particular that of aerobic bacteria and fungi is greater near the surfaces of soil aggregates than within aggregates. Many aggregates are actually the fecal pellets of earthworms.
- In Spaces Between Soil Aggregates. Those arthropods and nematodes that cannot burrow through soil move in the pores between soil aggregates. Organisms that are sensitive to dryness, such as protozoa and many nematodes, live in water-filled pores.