How Soil Biology Affects Your Garden
An incredible diversity of organisms make up the soil food web. They range in size from the tiniest
one-celled bacteria, algae, fungi, and protozoa, to the more complex nematodes and micro-arthropods,
to the visible earthworms, insects, small vertebrates, and plants.
There are many ways that the soil food web is an integral part of landscape processes. Soil organisms
decompose organic compounds, including manure, plant residue, and pesticides, preventing them from
entering water and becoming pollutants. They sequester nitrogen and other nutrients that might
otherwise enter groundwater, and they fix nitrogen from the atmosphere, making it available to plants.
Many organisms enhance soil aggregation and porosity, thus increasing infiltration and reducing runoff.
Soil organisms prey on crop pests and are food for above-ground animals.
Organisms live in the microscale environments within and between soil particles. Differences over short
distances in pH, moisture, pore size, and the types of food available create a broad range of habitats.
As these organisms eat, grow, and move through the soil, they make it possible to have clean water,
clean air, healthy plants, and moderated water flow.
The soil food web is the community of organisms living all or part of their lives in the soil. A food web
diagram shows a series of conversions of energy and nutrients as one organism eats another.
All food webs are fueled by the primary producers: the plants, lichens, moss, photosynthetic bacteria,
and algae that use the sun's energy to fix carbon dioxide from the atmosphere. Most other soil
organisms get energy and carbon by consuming the organic compounds found in plants, other
organisms, and waste by-products. A few bacteria, called chemoautotrophs, get energy from
nitrogen, sulfur, or iron compounds rather than carbon compounds or the sun.
As organisms decompose complex materials, or consume other organisms, nutrients are converted from
one form to another, and are made available to plants and to other soil organisms. All plants (grass,
trees, shrubs, agricultural crops) depend on the food web for their nutrition.
Growing and reproducing are the primary activities of all living organisms. As individual plants and
soil organisms work to survive, they depend on interactions with each other. By-products from
growing roots and plant residue feed soil organisms.
Soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance
soil structure, and control the populations of soil organisms including crop pests.
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 humus and active organic matter.
Active organic matter is the portion available to soil organisms. Bacteria tend to use simpler organic
compounds, such as root exudates or fresh plant residue. Fungi tend to use more complex compounds,
such as fibrous plant residues, wood and soil humus.
Intensive tillage triggers spurts of activity among bacteria and other organisms that consume
organic matter (convert it to CO2), depleting the active fraction first. 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.
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. These microshredders, immature oribatid mites, skeletonize plant leaves.
This starts the nutrient cycling of carbon, nitrogen, and other elements.
The organisms of the food web are not uniformly distributed through the soil. Each species and group exists
where they can find appropriate space, nutrients, and moisture. They occur wherever organic matter occurs
which is 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 cycling
and disease suppression needed by plants occurs immediately adjacent to roots.
On humus. Fungi are common here. Much organic matter in the soil has already been decomposed
many times by bacteria and fungi, and/or passed through the guts of earthworms or arthropods. The resulting
humic compounds are complex and have little available nitrogen. 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. Within large aggregates,
processes that do not require oxygen, such as denitrification, can occur. Many aggregates are actually
the fecal pellets of earthworms and other invertebrates.
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 desiccation,
such as protozoa and many nematodes, live in water-filled pores.
The living component of soil, the food web, is complex and has different compositions in different ecosystems.
Management of croplands, rangelands, forestlands, and gardens benefits from and affects the food web.
The next unit of the Soil Biology Primer, The Food Web & Soil Health, introduces the relationship of soil
biology to agricultural productivity, biodiversity, carbon sequestration and to air and water quality.
Healthy Soil - Healthy People
What Happens To The Soil In Winter?
How To Apply and Use Compost
How To Apply and Use Bone Meal