A healthy soil depends on the interaction of many organisms that make up the soil food web. These
organisms live all or part of their life cycle in the soil and are responsible for converting energy as
one organism consumes another.
The soil food web begins with the energy from the sun, which triggers photosynthesis in plants.
Photosynthesis results in plants using the sun’s energy to fix carbon dioxide from the atmosphere. This
process creates the carbon and organic compounds contained in plant material. This is the first trophic
level.
Then begins building of soil organic matter, which contains both long-lasting humus, and active organic
matter. Active organic matter contains readily available energy, which can be used by simple soil
organisms in the second trophic level of the soil food web.
The second trophic level contains simple soil organisms, which decompose plant material. Organisms such
as pathogens, parasites, and root feeders reproduce in this environment. Bacteria can use more simple
organic compounds such as soluble sugars and fresh plant residue, whereas fungi feed on more fibrous
plant residue.
Soil tillage stimulates bacteria, which rapidly consume active organic matter and deplete this source of
energy while releasing excess carbon dioxide.
The third trophic level of the soil food web contains larger soil organisms, which are shredders,
predators, and grazers such as protozoa, nematodes, and arthropods. These feed on second level
organisms. The fourth and fifth trophic level contains higher-level predators, which feed on smaller
soil organisms.
Thus, the sun’s energy is converted to higher-level mammals, which can be a source of food for humans.

Agriculture can enhance the soil food web to create more soil life by better utilizing the sun’s energy.
Growing cover crops which photosynthesize at times of the year when grain crops are not growing or
active will extend the time period of sunlight capture. Also, reducing or eliminating soil tillage will
prevent needless waste of active organic matter consumed by soil bacteria.
To order the bulletin (Soil Biology Primer) – www.swcs.org