The Soil Food Web

Real soil is a living system. There is an incredible diversity of organisms that make up this living system. This system is known as the soil food web. These organisms 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. 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 .

There are many ways that the soil food web is an integral part of the gardening processes. Soil organisms decompose organic compounds, including manure, plant residue, and pesticides, preventing them from entering the 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.

Not only do soil organisms do all this, but they also prey on crop pests and are food for above-ground animals. This is great news for the home gardener. This community of organisms live all or part of their lives in the soil. The food web diagram below illustrates the conversion of energy (indicated by arrows) and nutrients as one organism eats the other.

What Do Soil Organisms Do?

Individual plants and soil organisms depend upon interactions with each other. By-products from growing roots and plant residue, also known as exudates, feed soil organisms. In turn, soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance soil structure, and control the populations of soil organisms which also includes crop pests!

What fuels the Food Web?

Simply put it is organic matter that feeds the food web. It is a storehouse of energy and nutrients which are used by plants and organisms. It is the bacteria and the fungi, as well as other organisms in the soil that help release those nutrients from the 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.

What are the Functions of Soil Organisms?

There are 10 categories that soil organisms are divided into. These categories have major functions.

Photosyntesizers capture energy. They use solar enger to fix CO2 and add organic matter to the soil (biomass as dead cells, plant litter and secondary metabolites). Type of soil organisms in this group include:

Plants
Algae
Bacteria

Decomposers break down residue and immobilze nutrients in their biomass. This group of organisms create new organic compounds (cell constituents, waste products) that are sources of energy and nutrients for other organisms. Decomposers such as bacteria help bind soil into aggregates. Fungal hyphe then binds the soil aggregates. Nitrifying and denitrifying bacteria convert forms of nitrogen. Both bacteria and fungi compete with and inhibit disease-causing organisms. Type of soil organisms in this group include:

Bacteria
Fungi

Mutualists enhance plant growth by protecting plant roots from disease causing organisms. Someof these even fix N2. Soil organisms in this group include:

Bacteria
Fungi

Pathogens promote disease. They consume roots and other plants parts causing disease. Type of soil organism in this group include:

Bacteria
Fungi

Parasites are organisms that can include disease-causing organims. Some parasitize nematoes can be benefical when they attack harmful organisms. Types of soil organisms in this group include:

Nematodes
Microarthropods

Rootfeeders consume plant roots. This can potentially cause significant crop yeild losses. These are not the good guys! Type of soil organisms in this group include:

Nematodes
Macroarthropods (cutworm, weevil larve and symphylans)

Bacterial-feeders graze on bracteria. This releases plant available nitrogen (NH2) and other nutrients when they feed on bacteria. They control many root-feeding or disease-causing pests. Another one of their functions to to stimulate and control the activity of bacterial populations. Type of soil organisms in this group include:

Protozoa
Nematodes

Fungal-feeders also graze. As the name suggests, they graze on fungi. They also release plant available nitrogen NH2 and other nutrients when feeding on fungi. And they control many root feeding or disease causing pests. Fungal-feeders stimulate and control the activity of bacterial populations. Types of soil organisms in this group include:

Nematodes
Microarthropods

Shredders breakdown residue and enhance soil structure. They shred plant litter as they feed on bacteria and fungi. Shredders provide habitat for bacteria in their guts and fectal pellets. They also enhance soil structure as they produce fecal pellets and burrow through the soil. Types of soil organisms in this group include:

Earthworms
Microarthropods

Higher-level predators control populations of lower trophic-level predators. Larger organisms improve soil structure and by passing soil through their guts. They will also carry smaller organisms long distances. Types of soil organisms in this group include:

Nematode-feeding nematodes
Larger arthropods (mices, voles, shrews, birds, other above ground animals.

How is the Soil Food Web Structured?

Looking at microorganisms in a soil sample under a microscope

The "structure" of a food web is the composition and relative numbers of organisms in each group within the soil system . Each type of ecosystem has a characteristic food web structure

(see table) . Some features of food web structures include:

The ratio of fungi to bacteria is characteristic to the type of system. Grasslands and agricultural soils usually have bacterial -dominated food webs that is, most biomass is in the form of bacteria. Highly productive agricultural soils tend to have ratios of fungal to bacterial biomass near 1 : 1 or somewhat less. Forests tend to have fungal-dominated food webs. The ratio of fungal to bacterial·biomass may be 5 : 1 to 10 : 1 in a deciduous forest and 100 : 1 to 1000 : 1 in a coniferous forest.
Organisms reflect their food source. For example, protozoa are abundant where bacteria are plentiful. Where bacteria dominate over fungi, nematodes that eat bacteria are more numerous than nematodes that eat fungi.
Management practices change food webs. For example, in reduced- tillage agricultural systems, the ratio of fungi to bacteria increases over time, and earthworms and arthropods become more plentiful.

What are the benefits to having a healthy Soil Food Web?

There are numerous benefits to your vegetable garden when you have a balanced and healthy soil food web. These benefits include:

Nutrient cycling

When organisms consume food , they create more of their own biomass and they release wastes . The most important waste for crop growth is ammonium (NH4 +) . Ammonium and other readily utilized nutrients are quickly taken up by other organisms, including plant roots. When a large variety of organisms are present, nutrients may cycle more rapidly and frequently among forms that plants can and cannot use.

Nutrient retention

In addition to mineralizing or releasing nitrogen to plants, the soil food web can immobilize or retain nitrogen when plants are not rapidly growing. Nitrogen in the form of soil organic matter and organism biomass is less mobile and less likely to be lost from the rooting zone than inorganic nitrate (NO3 ) and ammonium ( NH4 + ).

Improved structure, infiltration, and water-holding capacity

Many soil organisms are involved in the formation and stability of soil aggregates. Bacterial activity, organic matter, and the chemical properties of clay particles are responsible for creating microaggregates from individual soil particles. Earthworms and arthropods consume small aggregates of mineral particles and organic matter, and generate larger fecal pellets coated with compounds from the gut. These fecal pellets become part of the soil structure. Fungal hyphae and root hairs bind together and help stabilize larger aggregates. Improved aggregate stability, along with the burrows of earthworms

and arthropods, increases porosity, water infiltration, and water holding capacity.

Disease suppression

A complex soil food web contains numerous organisms that can compete with disease- causing organisms . These competitors may prevent soil pathogens from establishing on plant surfaces, prevent pathogens from getting food, feed on pathogens, or generate metabolites that are toxic to or inhibit pathogens.

Degradation of pollutants

An important role of soil is to purify water. A complex food web includes organisms that consume (degrade) a wide range of pollutants under a wide range ofenvironmental conditions.

Biodiversity

Greater food web complexity means greater biodiversity. Biodiversity is measured by the total number of species, as well as the relative abundance of these species , and the number of functional groups of organisms .

Learn More: The Soil Biology Primer

This is an excellent resource to learn more about soil health and each individual player in the soil food web. This publication gives a more in depth understanding of how soil structure works and how its principles ultimately benefits not only your vegetable garden, all of the ecosystems around you. A digital version of the Soil Biology Primer can be found online for you to download. This publication was produced by the USDA and the Soil Quality Institute of the Natural Resources Conservation Service.

Download Soil Biology Primer

References: Ingram, Elaine R; Oregon State University, "Soil Biology Primer" USDA, Soil Quality Institute of the Natural Resources Conservation Service