What is soil biodiversity?

Soil biodiversity reflects the mix of living organisms in the

soil. These organisms interact with one another and with

plants and small animals forming a web of biological

activity.

Soil is by far the most biologically diverse part of Earth.

The soil food web includes beetles, springtails, mites,

worms, spiders, ants, nematodes, fungi, bacteria, and

other organisms. These organisms improve the entry and

storage of water, resistance to erosion, plant nutrition,

and break down of organic matter. A wide variety of

organisms provides checks and balances to the soil food

web through population control, mobility, and survival

from season to season.

What are the benefits of soil

organisms?

Residue decomposition

Soil organisms decompose plant residue. Each organism

in the soil plays an important role. The larger organisms in

the soil shred dead leaves and stems. This stimulates

cycling of nutrients. The larger soil fauna include earthworms, termites, pseudoscorpions, microspiders, centipedes, ants, beetles, mites, and springtails.

When mixing the soil, the large organisms bring material

to smaller organisms. The large organisms also carry

smaller organisms within their systems or as “hitchhikers”

on their bodies.

Small organisms feed on the by-products of the larger

organisms. Still smaller organisms feed on the products

of these organisms. The cycle repeats itself several times

with some of the larger organisms feeding on smaller

organisms.

Some larger organisms have a life span of two or more

years. Smaller organisms generally die more quickly, but

they also multiply rapidly when conditions are favorable.

The food web is therefore quick to respond when food

sources are available and moisture and temperature

conditions are good.

Infiltration and storage of water

Channels and aggregates formed by soil organisms

improve the entry and storage of water. Organisms mix

the porous and fluffy organic material with mineral matter

as they move through the soil. This mixing action provides

organic matter to non-burrowing fauna and creates

pockets and pores for the movement and storage of water.

Fungal hyphae bind soil particles together and slime from

bacteria help hold clay particles together. The waterstable aggregates formed by these processes are more

resistant to erosion than individual soil particles. The

aggregates increase the amount of large pore space which

increases the rate of water infiltration. This reduces

runoff and water erosion and increases soil moisture for

plant growth.

Nutrient cycling

Soil organisms play a key role in nutrient cycling. Fungi,

often the most extensive living organisms in the soil,

produce fungal hyphae. Hyphae frequently appear like

fine white entangled thread in the soil. Some fungal

hyphae (mycorrhizal fungi) help plants extract nutrients

from the soil. They supply nutrients to the plant while

obtaining carbon in exchange and thus extend the root

system. Root exudates also provide food for fungi,

bacteria, and nematodes.

Soil Quality Information Sheet

Soil Quality Resource Concerns: Soil Biodiversity

USDA Natural Resources Conservation Service January 1998
When fungi and bacteria are eaten by various mites,

nematodes, amoebas, flagellates, or ciliates, nitrogen is

released to the soil as ammonium. Decomposition by soil

organisms converts nitrogen from organic forms in

decaying plant residues and organisms to inorganic forms

which plants can use.

Management considerations

Cultivation

The effects of cultivation depend on the depth and

frequency of the cultivation. Tilling to greater depths and

more frequent cultivations have an increased negative

impact on all soil organisms. No-till, ridge tillage, and strip

tillage are the most compatible tillage systems that

physically maintain soil organism habitat and biological

diversity in crop production.

Compaction

Soil compaction reduces the larger pores and pathways,

thus reducing the amount of suitable habitat for soil

organisms. It also can move the soil toward anaerobic

conditions, which change the types and distribution of soil

organisms in the food web. Gaps in the food web induce

nutrient deficiencies to plants and reduce root growth.

Pest control

Pesticides that kill insects also kill the organisms carried

by them. If important organisms die, consider replacing

them. Plant-damaging organisms usually increase when

beneficial soil organisms decrease. Beneficial predator

organisms serve to check and balance various pest

species.

Herbicides and foliar insecticides applied at recommended rates have a small impact on soil organisms.

Fungicides and fumigants have a much greater impact on

soil organisms.

Fertility

Fertility and nutrient balances in the soil promote biological diversity. Typically, carbon is the limiting resource to

biological activity. Plant residue, compost, and manure

provide carbon. Compost also provides a mix of organisms, so the compost should be matched to the cropping

system.

Cover crops and crop rotations

The type of crops that are used as cover or in crop

rotations can affect the mix of organisms that are in the

soil. They can assist in the control of plant pests or serve

as hosts to increase the number of pests. Different species

and cultivars of crops may have different effects on pests.

However, the organisms and their relation to the crop are

presently not clearly understood.

High-level

Carnivores Carnivores Aboveground

Crops Herbivores

Weeds

Full Web

Roots

Pests & Pathogens

* Root-feeding

Nematodes

& Insects

* Fungi

* Bacteria

Omnivorous

Nematodes

Belowground

Earthworms

Millipedes Fungal-feeders

* Centipedes

* Nematodes

* Mites

Bacterial-Feeders

* Nematodes

* Protozoa

• Ciliates

• Amoebae

• Flagellates

* Enchytraeids

Mutualists

* Mycorrhizal

Fungi

* N-fixing Bacteria

Decomposers

* Fungi

* Bacteria

* Shredders

Organic Matter

Algae

Lichens

Bacteria

Aboveground

Birds

Mice

Beetles

Spiders

Predatory Insects

and higher ... Tertiary

Consumers

Secondary

Consumers

Primary

Consumers

Photosynthesizers

Predatory

Nematodes

Predatory Mites

Crop residue management

Mixing crop residue into the soil generally destroys fungal

hyphae and favors the growth of bacteria. Since bacteria

hold less carbon than fungi, mixing often releases a large

amount of carbon as carbon dioxide (CO2). The net result

is loss of organic matter from the soil.

When crop residue is left on the soil surface, primary

decomposition is by arthropod shredding and fungal

decomposition. The hyphae of fungi can extend from

below the soil surface to the surface litter and connect the

nitrogen in the soil to the carbon at the surface. Fungi

maintain a high C:N ratio and hold carbon in the soil. The

net result is toward building the carbon and organic

matter level of the soil. In cropping systems that return

residue, macro-organisms are extremely important.

Manage the soil to increase their diversity and numbers.

(Prepared by the National Soil Survey Center, NRCS, USDA in cooperatio

n with the Soil Quality Institute, and the National Soil Tilth Laboratory,

Agricultural Research Service, USDA).

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