Soil Quality – Soil Biology 

Technical Note No. 4 

Soil Quality – Soil Biology

Technical Note No. 4 

January 2004 

NRCS file code: 190-22-15 

Other titles in this series: 

#1 Soil Biology Primer 

#2 Introduction to 

Microbiotic Crusts 

#3 Soil Biology Slide Set 

 

NRCS Soil Quality

http://soils.usda.gov/sqi 

Soil Biology and 

Land Management 

Table of contents

The goal of soil biology management.......................................... 2

Why should land managers understand soil biology?.................. 2

The underground community....................................................... 4

What controls soil biology? ......................................................... 5

General management strategies................................................... 7

Considerations for specific land uses......................................... 10

Cropland.............................................................................. 10

Forestland............................................................................ 13

Rangeland............................................................................ 15

Assessment and monitoring....................................................... 17

Summary.................................................................................... 19

References and further resources............................................... 19

Introduction 

The purpose of this technical note is to provide information about the 

effects of land management decisions on the belowground component of 

the food web. It points out changes in soil biological function that land 

managers should look for when changing management practices. This 

information is not a set of prescriptive guidelines, but is designed to 

increase awareness and prompt field trials. 
The goal of soil biology management 

The goal of managing the soil biological 

community is to improve biological functions, 

including forming and stabilizing soil structure, 

cycling nutrients, controlling pests and disease, 

and degrading or detoxifying contaminants. 

Research shows that management practices and 

disturbances impact soil biological functions 

because they can 1) enhance or degrade the 

microbial habitat, 2) add to or remove food 

resources, or 3) directly add or kill soil 

organisms. Although management practices are 

known to impact soil biology, there is limited 

knowledge to support the development of 

detailed management strategies. A particular 

practice may have the desired result in one 

situation but have little effect in another because 

biological communities respond to the 

interaction of multiple factors including food 

sources, physical habitat, moisture, and impacts 

of historical land use. Therefore, before a new 

product or practice is applied to a large parcel of 

land, it should be tested on a limited area and 

results should be monitored in comparison to an 

untreated plot. 

Why should land managers understand soil biology? 

Energy and the food web 

Through agriculture, the sun’s energy is 

converted into food, feed, and fiber. However, 

most of the solar energy captured by plants is 

not directly harvested when crops are gathered; 

instead, it feeds the belowground food web. 

Feeding the “underground livestock” is essential 

to productive forests, rangeland, and farmland. 

Figure 1 shows how energy is recycled 

repeatedly through belowground soil organisms. 

The soil food web is part of energy, nutrient, and 

water cycles. The energy cycle begins when the 

sun’s energy is captured by the plant-based 

(aboveground) food web. Nutrient availability is 

governed by the detritus-based (belowground) 

food web. The water cycle is also influenced by

the interaction of plants, soils, and soil 

organisms. 

Functions of the soil food web 

Nutrient cycling 

In a healthy soil ecosystem, soil biota regulates 

the flow and storage of nutrients in many ways. 

For example, they decompose plant and animal 

residue, fix atmospheric nitrogen, transform

nitrogen and other nutrients among various 

organic and inorganic forms, release plant 

available forms of nutrients, mobilize 

phosphorus, and form mycorrhizal (fungus-root) 

associations for nutrient exchange. Even applied 

fertilizers may pass through soil organisms 

before being utilized by crops. 

Soil stability and erosion 

Soil organisms play an important role in forming 

and stabilizing soil structure. In a healthy soil 

ecosystem, fungal filaments and exudates from

microbes and earthworms help bind soil 

particles together into stable aggregates that 

improve water infiltration, and protect soil from

erosion, crusting, and compaction. Macropores 

formed by earthworms and other burrowing 

creatures facilitate the movement of water into 

and through soil. Good soil structure enhances 

root development, which further improves the 

soil. 

Water quality and quantity 

By improving or stabilizing soil structure, soil 

organism dynamics help reduce runoff and 

improve the infiltration and filtering capacity of 

soil. In a healthy soil ecosystem, soil organisms 

reduce the impacts of pollution by buffering, 

detoxifying, and decomposing potential 

pollutants. Bacteria and other microbes are 

increasingly used for remediation of 

contaminated water and soil. 

Page 2 of 20 Soil Biology and Land Management 
Plant health 

A relatively small number of soil organisms 

cause plant disease. A healthy soil ecosystem

has a diverse soil food web that keeps pest 

organisms in check through competition and 

predation. Some soil organisms release 

compounds that enhance plant growth or reduce 

disease susceptibility. Plants may exude specific 

substances that attract beneficial organisms or 

repel harmful ones, especially when they are 

under stress, such as grazing. 

For more information about what lives in the 

soil and how they function, see the “Soil 

Biology Primer” (Tugel and Lewandowski, 

1999), and the soil biology glossary on the 

NRCS Soil Quality web site. 

Complexity and function 

Many soil biological functions emerge from the 

complex interactions of soil organisms and are 

not predictable by adding up the activity of 

individual soil organisms. How well the soil 

community performs each of these functions 

depends in part on the complexity of the 

biological community. Complexity is a factor of 

both the number of species and the different 

kinds or functions of species. Greater 

complexity may imply more diversity of 

functions and more redundancy of functions, and 

therefore more stability. For example, when 

multiple populations of microbes convert 

ammonium to nitrate, even if one population 

dies out, the function (nitrification) will continue 

to be performed. Functional redundancy is the 

underlying idea behind the "insurance 

hypothesis," which states that biodiversity

insures ecosystems against declines in function. 

 

Figure 1. The plant-based (aboveground) and detritus-based (belowground) food webs. 

Arrows represent energy flow (commonly measured in carbon units). Of the aboveground organic 

matter entering the pool of soil organic matter, 60%-80% of the carbon is eventually lost as CO2. 

(Based on Chapin et al., 2002, Fig. 11.12.) 

Plants

Soil organic matter 

Herbivores

 Carnivores 

 

Decomposers (bacteria, fungi) 

Predators

(mites, some beetles,

centipedes, etc.)

Microbivores 

(protozoa,

nematodes)

Stabilized organic matter (Heterogeneous pool of slowly decomposable,

physically protected, compounds)

Plant-based

food web

Detritus-based

food web

CO2

Soil Biology and Land Management Page 3 of 20 
The underground community

Soil organisms can be grouped by size as shown 

in figure 2, or by functions as described below 

(Wardle, 2002; Coleman & Crossley, 1996). 

Decomposers 

Bacteria, actinomycetes (filamentous bacteria), 

and saprophytic fungi degrade plant and animal 

residue, organic compounds, and some

pesticides. Bacteria generally, but not 

exclusively, degrade the more readily

decomposed (lower C:N ratio) materials, 

compared to fungi, which can use more 

chemically complex materials. (See boxes on 

pages 7 and 8.) Bacteria often degrade what they

can of a particular material; then fungi 

decompose the remainder. 

Grazers and predators 

Protozoa, mites, nematodes, and other organisms 

“graze” on bacteria or fungi; prey on other 

species of protozoa and nematodes; or both 

graze and prey. Grazers and predators release 

plant-available nutrients as they consume 

microbes. Often organisms specialize in one 

type of prey, such as either bacteria or fungi. 

Certain collembolans (springtails) even 

specialize on specific species of fungi. Other 

organisms are generalists and will feed on any

microbial species they encounter. 

Litter transformers 

Figure 2. Body width of soil organisms (Swift et al., 1979). 

1 um 10um 100um 1 mm 10 mm 100 mm

Isopods

Bacteria

Fungi

Nematodes

Protozoa

Mites

Enchytraeid

Symphylan

Termites

Centipedes

Millipedes

Spiders

Earthworms

Beetles

Collembola

Clay Silt Sand

Arthropods are invertebrates 

with jointed legs, including 

insects, spiders, mites, 

springtails, centipedes, and 

millipedes. Many soil arthropods 

shred and consume plant litter 

and other organic matter, 

increasing the surface area 

accessible to decomposers. The 

organic matter in their fecal 

pellets is frequently more 

physically and chemically

accessible to microbes than was 

the original litter. Some litter 

transformers, especially ants, 

termites, scarab beetles, and earthworms, are 

“ecosystem engineers” that physically change 

the soil habitat for other organisms by chewing 

and burrowing through the soil. Microbes 

(decomposers) living within their guts break 

down the plant residue, dung, and fecal pellets 

consumed along with the soil. 

Mutualists 

Mycorrhizal fungi, nitrogen-fixing bacteria, and 

some free-living microbes have co-evolved 

together with plants to form mutually beneficial 

associations with plants. Mycorrhizae are 

associations between fungi and plant roots in 

which the fungus supplies nutrients and perhaps 

water to the plant, and the plant supplies food to 

the fungi. These fungi can exist inside 

(endomycorrhizae) or outside (ectomycorrhizae) 

the plant root cell wall. The common arbuscular 

mycorrhizae (AM or VAM fungi) are 

endomycorrhizae. 

Pathogens, parasites, and root feeders 

Organisms that cause disease make up a tiny

fraction of the organisms in the soil, but have 

been most studied by researchers. Diseasecausing organisms include certain species of 

bacteria, fungi, protozoa, nematodes, insects, 

and mites. 

Page 4 of 20 Soil Biology and Land Management 
What controls soil biology? 

People can adapt management strategies to 

affect the factors that control soil biological 

communities. Soil biological activity is 

determined by factors at three different levels. 1) 

At the scale of individual organisms, biological 

activity is determined by conditions such as 

temperature and moisture in the microbial 

habitats. 2) At the scale of populations, 

biological activity is determined by the amount 

of habitat diversity, the types of habitat 

disturbances, and the diversity and interactions 

among various soil populations. 

3) At the scale of biological processes, functions 

such as nutrient cycling or pest control are 

affected by the interaction of biological 

populations with physical and chemical soil 

properties. 

For example, consider the effect of tillage on 

earthworms at each of these scales. At the scale 

of individual organisms, a single tillage event 

may kill as many as 25 percent of individual 

earthworms. At the scale of populations, a single 

tillage pass may have little effect after a few 

months as the earthworms reproduce and rebuild 

their population. At the scale of soil processes, 

tillage will weaken soil structure over time and 

reduce the amount of surface residue available to 

fungi and earthworms. As fungal and earthworm

activity declines, soil stability declines and alters 

the microhabitats for other organisms. 

Microscale factors 

The following environmental factors affect the 

types and activity levels of soil organisms. 

These factors may vary over short distances in 

the soil. Consider how each factor is impacted 

by climate, soil texture, time of day or season, 

and management practices including tillage, 

crop rotation, and irrigation. 

Food (nutrients and energy) 

All organisms require a source of food that 

supplies nutrients and energy. “Primary

producers” are organisms that use 

photosynthesis to make their own food from

sunlight and CO2. “Consumers” are organisms 

that use organic compounds from other 

organisms as their source of both carbon and 

energy. A small group of bacteria get their 

energy from inorganic nitrogen, sulfur, or iron 

compounds, rather than from sunlight or organic 

compounds. These bacteria are important in 

cycling some nutrients required by plants. Soil 

organisms also require varying amounts of 

macronutrients (N, S, and P) and micronutrients 

(e.g. Fe, Cu, Zn). The amount of all these 

nutrients and the quality of nutrient sources will 

favor some organisms over others, depending on 

each species’ requirements and preferences. 

Oxygen 

Animals and most soil organisms are obligate 

aerobes, meaning they require oxygen. Some

bacteria are obligate anaerobes, meaning they

require oxygen-free conditions to function. 

Many organisms are facultative anaerobes, 

meaning they can switch metabolic pathways

and function as either aerobes or anaerobes 

depending on environmental conditions. 

Anaerobes use nitrate, sulfate, or iron instead of 

oxygen as an electron acceptor. Aerobic 

respiration is the most common form of 

metabolism and typically produces ten times 

more energy per unit of organic matter than that 

generated through anaerobic metabolism.

Anaerobic conditions and anaerobic microbes 

dominate in marshes and other saturated soils. 

However, even well-drained soils can have 

anaerobic and aerobic microsites within 

millimeters of each other. See box (next page) 

for a list of processes performed by aerobic and 

anaerobic organisms. 

Soil Biology and Land Management Page 5 of 20 
Other physical factors 

Moisture, temperature, light, pH, and electrical 

conductivity (salinity) are other critical factors 

determining the level of biological activity

within an ecosystem. Each species has different 

optimal conditions, but overall bacterial activity

is highest at temperatures between 20°C and 

40°C, at pH levels between 6 and 8, and when 

pore spaces are about 60% water-filled. Soil 

texture and porosity determine the amount of 

space available for soil organisms and for the 

movement of air and water through the soil. 

Thus, porosity, aeration, and moisture levels are 

linked. Relatively larger organisms, such as 

nematodes and small mites (figure 2), require 

large pore spaces to move. Many organisms, 

including protozoa and nematodes, are 

essentially aquatic and require water films. 

Community-scale factors

The microscale factors listed above directly

affect soil organisms. However, to understand 

soil biological function we also have to consider 

large-scale factors such as heterogeneity of 

habitat, disturbances, and biological interactions. 

Heterogeneity of resources Anaerobic soil biological processes 

Fermentation – Conversion of sugar to 

alcohol. 

Denitrification – Reduction of nitrate to 

gaseous nitrogen. 

Methane production – Reduction of CO2 to 

methane (CH4) in marshes and rumens.

Sulfur reduction – Reduction of sulfate to 

hydrogen sulfide or sulfur. 

Iron reduction 

Aerobic soil biological processes 

Respiration – The conversion of oxygen 

eventually to carbon dioxide and water. 

Ammonification – The creation of ammonia 

from organic compounds. May also 

occur anaerobically slowly.

Nitrification – The oxidation of ammonium to 

nitrite and then nitrate. 

Heterogeneity can refer to variation in food 

sources or any of the other microscale conditions 

listed above. Heterogeneity of soil habitats 

creates diversity and complexity in the structure 

of the soil food web. Plant diversity is an 

important means for creating heterogeneity

because plants affect the food sources, the 

physical habitat (e.g. root structures and soil 

structure), and the chemical attractants and 

deterrents for soil organisms. 

Disturbance 

All human land uses, especially agriculture, are 

subject to natural and human disturbances 

including fire, harvest, tillage, compaction, 

overgrazing, disease, or pesticide applications. 

The frequency, severity, and timing of 

disturbances determine their effect on soil 

biological activity. According to the 

intermediate disturbance hypothesis, the greatest 

level of biological diversity and stability occurs 

with a “Goldilocks” amount of disturbance – not 

so much that processes are continually disrupted, 

and not so little that just a few species gain 

dominance. Conventional cropping systems are 

highly disturbed systems. Low-input, 

conservation tillage systems with crop rotations 

may be an example of an intermediately

disturbed ecosystem. 

Interactions with other organisms 

Soil populations are affected by interactions 

with other soil organisms. One type of biotic 

interaction is competition for limited food and 

habitat. A second type is predation by larger 

organisms, such as nematodes and mites. A third 

type is mutualism – interactions beneficial to 

both parties, such as those involving 

mycorrhizae, symbiotic nitrogen fixers, many

rhizosphere microbes, and microbes living in 

earthworm guts. When land management 

practices disproportionately affect one group of 

organisms over another, they impact the 

interactions among soil organisms. 

Page 6 of 20 Soil Biology and Land Management 
General management strategies 

Four broad management strategies are presented 

below. The diversity and functioning of a soil 

biological community are likely to improve 

when these strategies are used. Management 

plans should consider both the timing of 

management practices and disturbances, and the 

duration and degree of their effects on soil 

biology. The effects of management and 

disturbances vary by season, and the capacity of 

the soil community to recover from a particular 

practice or disturbance ranges widely. 

1) Manage organic matter. 

Regular inputs of organic matter are essential for 

supplying the energy that drives the soil food 

web. Each source of organic matter favors a 

different mix of organisms. (See boxes, this page 

and next.) Thus, a variety of sources may

support a variety of organisms. The location of 

the organic matter—whether at the surface, 

mixed into the soil, or as roots—also affects the 

type of organisms that dominate in the food web. 

Under any land use, organic matter inputs to the 

soil can be increased by improving plant 

productivity and increasing annual biomass 

production. In particular, good root growth is 

important for building soil organic matter. High 

biomass production should be combined with 

other organic matter management practices 

including minimizing residue removal and 

tillage, growing cover crops, and adding manure, 

mulch, or other amendments. 

2) Manage for diversity. 

The diversity of plant assemblages across the 

landscape and over time promotes a variety of 

microbial habitats and soil organisms. Up to a 

point, soil biological function generally

improves when the complexity or diversity of 

the soil biological community increases. 

Many types of diversity should be considered, 

such as diversity of land uses (buffers, forest, 

row crops, grazing land), plant types (perennial, 

annual, woody, grassy, broadleaf, legume, etc.), 

root structures (tap, fibrous, etc.), and soil pore 

sizes. Diversity is desirable over time as well as 

across the landscape. Land managers can 

increase diversity with appropriate grazing 

management, patchy or selective tree harvest (in 

contrast to broad clear-cutting), vegetated 

fencerows, buffer strips, strip cropping, and 

small fields. These landscape features provide 

refuges for beneficial arthropods. Diversity over 

time can be achieved with crop rotations. 

Rotated crops put a different food source into 

the soil each year, encouraging a wide variety of 

organisms and preventing the build-up of a 

single pest species. 

Components of organic matter

Organic matter is composed of a 

heterogeneous mix of compounds with 

various chemical bonding and branching 

characteristics. Each organism has the 

necessary enzymes for decomposing some 

compounds but not others. For example, 

lignin is a recalcitrant organic compound that 

can only be broken down by white and 

brown rot fungi.

The composition of organic matter from 

plants varies considerably, but generally 

comprises 60-70% carbohydrates 

(polysaccharides), 

15-20% lignin, and 15% other compounds 

including proteins, nucleic acids, lipids, 

waxes, and pigments. 

3) Keep the ground covered. 

Ground cover at or near the surface moderates 

soil temperature and moisture; provides food 

and habitat for fungi, bacteria, and arthropods; 

and prevents the destruction of microbial habitat 

by erosion. Minimize the length of time each 

year that soil is bare by maintaining a cover of 

living plants, biological crusts, or plant residue 

at the surface. 

Living plants are especially important as cover 

because they create the rhizosphere—that area 

within one or two millimeters of living roots 

where soil biological activity is concentrated. 

Soil Biology and Land Management Page 7 of 20 
Microbes around roots take advantage of plant 

exudates and sloughed-off root cells. 

Maintaining a rhizosphere environment is one of 

the important benefits of using cover crops. In 

addition to preserving microbial habitat, cover 

crops help build and maintain populations and 

diversity of arthropods by preserving their 

habitat for an extended portion of the growing 

season. 

4) Manage disturbances. 

Some soil perturbations are a normal part of soil 

processes, or are a necessary part of agriculture 

and other land uses. However, some

disturbances significantly impact soil biology

and can be minimized to reduce their negative 

effects. These disturbances include compaction, 

erosion, soil displacement, tillage, catastrophic 

fires, certain pesticide applications, and 

excessive pesticide usage. 

Compaction 

Ideally, soils are approximately 50 to 60% pore 

space comprising a variety of pore sizes and 

lengths. The size and continuity of pores 

controls whether larger microbes, such as 

protozoa, can prey upon bacteria and fungi. 

Compaction reduces the diversity of pore sizes 

and the amount of space and pathways available 

for larger organisms (figure 2) to move through 

the soil. This favors bacteria and small predators 

over fungi and the larger predators. Arthropods 

are severely affected by compaction. Among 

nematodes, the predatory species are most 

sensitive to compaction, followed by fungalfeeders, then bacterial-feeders. Root-feeding 

nematodes are least sensitive to compaction—

perhaps because they do not need to move 

through soil in search of food. Compaction 

changes the movement of air and water through 

soil, can cause a shift from aerobic to more 

anaerobic organisms, and may increase losses of 

nitrogen to the atmosphere (denitrification). 

Rooting depth may be limited in highly

compacted soils. This restricts the depth of the 

rhizosphere environment that supports microbes. 

Carbon-to-nitrogen ratio 

The carbon-to-nitrogen ratio (C:N) of organic 

matter can vary from about 4:1 (low carbon, 

high nitrogen) for bacteria to more than 

200:1 to 600:1 for woody materials. Ratio

for wheat straw are about 80:1, and youn

legumes may be 15:1. The C:N ratio of soil 

organic matter in agricultural soils averages 

10:1. Fungi have a fairly constant C content 

of 45%, but N levels vary, resulting in C:N 

ratios of 15:1 to 4.5:1. 

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Low N materials have a low nutritional 

quality for microbes. When organic materia

are added to soil, the carbon triggers 

microbial growth. If the amount of N in th

added material is inadequate to suppo

increased growth, the microbes will absorb 

nitrogen from the soil and immobilize it in 

their tissues, thus depriving plants of 

nitrogen, at least temporarily. As a rule-ofthumb, materials with C:N ratios less than 25 

or 30:1 will not trigger this N deficiency in 

plants. Materials with lower C:N ratios tend 

to decompose quickly. Materials with highe

C:N ratios are slow to decompose and can 

lead to carbon storage or sequestration in 

the soil when accompanied by addition

nitrogen inputs in reduced tillage systems. 

C:N ratios of soil organic matter provide 

clues about the microbial community. For 

example, higher ratios tend to support more 

fungi compared to bacteria. A labile pool of 

soil organic matter with a low C:N ratio 

implies that the SOM consists of a high 

proportion of microbes. 

Erosion and sedimentation 

Most soil organisms – especially larger ones – 

live in the top few inches of soil. Erosion 

disrupts and removes that habitat. Sedimentation 

buries the surface habitat and deprives 

organisms of space and air. 

Soil displacement and tillage 

Displacement and mixing of the soil occur 

during many activites including tillage, land 

leveling, grading, intense grazing, and site 

preparation and harvesting on forestlands. Some

soil displacement can be useful such as tillage 

for seedbed preparation in cropland, limited 

disturbances in highly productive grassland 

Page 8 of 20 Soil Biology and Land Management 
systems, and soil scarification to ensure success 

of some types of reforestation. However, soil 

disturbances significantly change the biological 

habitat of the soil. If the extent of the 

disturbance is limited to small areas, the overall 

impact will also be limited. Broadly applied 

practices such as tillage, grazing, or clear-cutting 

can impact large areas. Even a single tillage or 

compaction event can significantly affect the 

location and quantity of the food supply and the 

physical habitat of soil organisms. If enough 

nitrogen is present, tillage and other practices 

that mix the soil usually lead to a flush of 

microbial activity and nutrient release, and loss 

of soil organic matter via CO2 respiration. 

Where there is a loss of soil organic matter, 

microbial activity will eventually drop to a rate 

that is lower than the initial rate. Over time, 

tillage shifts the food web from being dominated 

by fungi to being dominated by bacteria. 

Pesticides and herbicides 

All pesticides impact some non-target 

organisms. Heavy pesticide use tends to reduce 

soil biological complexity. Total microbial 

activity often increases temporarily as bacteria 

and fungi degrade a pesticide. However, effects 

vary with the type of pesticide and species of 

soil organism. Labels generally do not list the 

non-target organisms affected by a product. In 

fact, few pesticides have been studied for their 

effect on a wide range of soil organisms. 

Pesticides that kill aboveground insects can also 

kill beneficial soil insects. Foliar insecticides 

applied at recommended rates have a smaller 

impact on soil organisms than fumigants or 

fungicides. Herbicides probably affect few 

organisms directly, but they affect the food and 

habitat of soil organisms by killing vegetation. A 

pulse of dead vegetation may trigger a flush of 

biological activity and decomposition. Crop 

rotations are useful for breaking pest cycles, 

reducing pesticide application rates, and for 

varying the families of pesticides used. 

Soil Biology and Land Management Page 9 of 20 
Considerations for specific land uses 

The effects of management practices, operations, and natural disturbances often are specific to particular 

land uses, such as those discussed in this section. Each of the considerations below relates to the general 

management strategies described in the previous section. 

Cropland 

The highly disturbed soils of cropland may have 

as many bacteria and protozoa as other 

ecosystems, but tend to have far fewer fungi, 

nematodes, and arthropods. Reduced tillage and 

perennial cropping systems will support more of 

these larger soil organisms. 

Crop biomass additions 

Roots and surface residue from crops are 

convenient and valuable sources of soil organic 

matter and food for soil organisms. Corn 

harvested for grain will generate 3 to 4 tons of 

surface residue per acre and 1 to 2 tons of root 

biomass. Dense, sod-type crops produce 

generous amounts of root biomass. Recent 

research suggests that root contributions are 

more significant for building soil organic matter 

than are contributions from aboveground plant 

residue. 

Surface residue encourages the decomposers—

especially fungi—and generally increases food 

web complexity. Residue provides food and 

habitat for surface-feeding organisms, such as 

some earthworms, and for surface-dwellers, such 

as some arthropods. It also changes the moisture 

and temperature of the soil surface, and protects 

soil organic matter from erosion. The residue 

will increase some pathogens and reduce others. 

Soybeans, peanuts, and many vegetables leave 

little surface residue and should be rotated with 

high residue crops or cover crops. 

Animal manure 

Dung pats provide food and habitat for a variety

of larger soil organisms. Manure in any form is a 

significant source of nutrients. Manure 

application substantially changes the mix of 

organisms in the soil compared to plant sources 

of organic matter. The implications of these 

differences are not clear, but they probably

affect disease levels and nutrient cycling. Overapplication of N or P (whether from organic 

amendments or synthetic fertilizer) can suppress 

certain soil organisms, especially mycorrhizal 

fungi, as well as lead to degradation of air and 

water quality. 

Compost 

Compost can be used to inoculate the soil with a 

wide variety of organisms and to provide a high 

quality food source for them. Composts have 

also been credited with reducing the incidence of 

plant disease (Ceuster and Hoitink, 1999). Some

species thrive in both compost and soil, but 

many prefer one or the other. For example, the 

redworms (Eisenia fetida) commonly used to 

make vermicompost do not survive well in soil. 

The quality of compost varies substantially

depending on the material used, peak 

temperature during the composting process, and 

the level of aeration. Organic materials 

decomposed with little oxygen (e.g. liquid 

manure) will contain a very different set of 

organisms and compounds than well-aerated 

compost. (For information about how to make 

compost, see NRCS conservation practice 

standard #317, Composting Facilities.) 

Sewage sludge 

Like manure, sludge can be an excellent food 

source for organisms. However, high levels of 

metals or salts in some sludge kill or reduce the 

activity of some organisms. 

Cover crops 

Cover crops have several positive effects on soil 

communities. Most soil organisms live in the 

rhizosphere – the area directly around living 

roots. By planting cover crops (also called green 

manure) the rhizosphere environment is 

available to soil organisms for a longer portion 

of the year. Cover crops typically increase the 

amount and diversity of roots and aboveground 

Page 10 of 20 Soil Biology and Land Management 
growth that become part of soil. Because of each 

crop’s unique physiology, populations of 

specific soil organisms will increase or decline 

depending on the crop. For example, some cover 

crops exude compounds that inhibit diseasecausing organisms. 

Inorganic fertilizers 

Fertilizer provides some of the nutrients needed 

by soil organisms and will favor those species 

that can best utilize the forms of nutrients found 

in fertilizers. The effect of acidity, alkalinity, or 

salt of some fertilizers (e.g. ammonium nitrate, 

ammonium sulfate, and urea formaldehyde) 

tends to reduce populations of fungi, nematodes, 

and probably protozoa. It is not clear how 

persistent these population reductions are in 

various situations. 

Judicious fertilizer use can be positive for 

overall biological activity because it increases 

plant growth and organic matter inputs to the 

soil. 

Genetically modified (GMO) crops 

Each type of GMO is likely to have a different 

effect on soil biology. For example, Bt crops 

seem to have little direct effect on the 

composition of the soil biological community, 

yet decomposition rates of the crop residue 

differ from that of other corn varieties – perhaps 

because of changes in plant lignin composition, 

which may indirectly affect soil organisms via 

changes in food resources. (Lignin is a plant 

compound that is highly resistant to microbial 

attack.) However, soil type and crop variety

seem to be more important than the presence of 

the Bt gene in determining decomposition rates 

(Saxena and Stotzky, 2001a, b). 

Drainage 

Improved water drainage tends to improve 

microbial activity by increasing oxygen 

availability. Poorly drained soils have a high 

level of anaerobic microsites and therefore a 

higher rate of denitrification (conversion of 

nitrate to gaseous nitrogen) compared to welldrained soils. 

Irrigation and salt build-up 

Where irrigation increases plant yield, it 

increases biomass production and soil organic 

matter, and therefore tends to increase biological 

activity and to alter the biological community

structure. However, irrigation water can contain 

salts. To prevent salt accumulation that can 

reduce biological activity and crop yield, 

additional water must be applied to leach these 

salts from the root zone. Some irrigation 

techniques, such as furrow irrigation, require 

extreme soil disturbance that is detrimental to 

biological habitat. When the disturbance is a 

one-time event (i.e., intense but infrequent), as 

with installation of subterranean irrigation pipes, 

the disturbance is less likely to do lasting 

damage. 

Soil inoculants and compost tea 

Some commercially available inocula are 

intended to increase populations of specific soil 

organisms. Some products have a long track 

record of effectiveness, including nitrogenfixing bacteria and some pest predators, such as 

bacteria, nematodes, or insects. Some products 

are unproven or unpredictable. 

Inoculants will have little effect or only a 

temporary effect if the organisms cannot 

compete in their new environment. Because they

must have supplies of organic matter as a food 

source, soils low in organic matter will not see 

long-lasting results unless a recurring supply of 

organic matter is added to the system. 

Furthermore, many functions performed by soil 

organisms result from the interactions of 

organisms, not from a few individual species. 

When considering using inoculants, ask the 

following questions: 

Do you have assurances that the 

organisms claimed to be in the product 

are viable? 

• 

• 

• 

Will the organisms survive in your soil 

environment long enough to have the 

desired effect?

If you achieve positive results, was the 

change caused by the inoculated 

organisms or by associated management 

Soil Biology and Land Management Page 11 of 20 
practices, such as changes in tillage or 

added organic matter? 

Before committing a whole farm to a new 

product, test it on a small area and compare 

results to a control strip managed identically but 

without application of the product. Monitor both 

short- and long-term effects. 

Land leveling 

Land leveling may have effects similar to 

erosion and sedimentation because biologically

active surface soil is removed from some areas 

and deposited in other parts of the field. It may

also expose subsoil with a less desirable texture 

or lower organic matter levels. This effect can be 

reduced by intense soil building practices after 

land leveling or by removing and stockpiling the 

topsoil just prior to land leveling and then 

spreading it over the newly leveled surface. 

Terraces and grassed waterways 

Permanent vegetative structures add diversity to 

a landscape and thus can enhance the 

biodiversity of the area. They serve as a refuge 

for larger soil organisms such as arthropods and 

pest predators. However, like land leveling, the 

soil movement involved during construction of 

terraces can significantly disrupt soil biological 

habitat. 

Tillage and no-till 

Tillage enhances bacterial growth in the shortterm by aerating the soil and by breaking apart 

soil aggregates to expose organic matter that had 

been protected from microbial decay. The 

bacterial activity increases the loss of carbon 

respired as CO2, and triggers population 

explosions of bacterial predators such as 

protozoa. Ultimately, recurrent tillage reduces 

the amount of soil organic matter that fuels the 

soil food web. 

The mechanical action of tillage tends to 

temporarily reduce populations of fungi, 

earthworms, nematodes, and arthropods. Over 

the long term with repeated tillage, these 

populations are likely to decline because of the 

lack of surface residue rather than because of the 

mechanical action of tillage. 

The environment for soil organisms can differ 

significantly in no-till compared to 

conventionally tilled soils. For example, because 

the surface soil structure is not regularly

disrupted, no-till soils are more likely to have: 

• Anaerobic micro-environments, 

• 

• 

• 

• 

• 

Cooler spring soil temperatures because 

of greater surface cover, 

More macropores to facilitate 

infiltration, 

Greater soil moisture and carbon near 

the surface, and 

Uneven distribution of organic matter 

throughout the topsoil. 

In addition, surface compaction may be 

a problem if compaction was present 

before the conversion to no-till, and if 

biomass inputs are low or traffic 

patterns are not controlled. 

Organic matter decomposition rates are lower in 

no-till vs. conventionally tilled soils because of 

the lower level of soil disturbance. The lack of 

disturbance and the presence of surface residue 

encourage fungi and large organisms such as 

arthropods and earthworms. No-till soils 

generally have a higher ratio of fungi to bacteria. 

Fallow periods 

Because microbes concentrate around living 

roots, fallow periods of even a few weeks at the 

beginning or end of a growing season reduce an 

important microbial habitat. During long fallow 

periods, most arthropods will emigrate or die of 

starvation. Some organisms can form cysts, 

allowing them to lie dormant until conditions 

become more favorable. Mycorrhizal fungi also 

“starve” during a fallow period and take time to 

recover after the fallow period ends. Growing 

non-mycorrhizal plants is equivalent to a fallow 

year from the perspective of mycorrhizal fungi. 

Plants that do not support mycorrhizae include 

brassicas (mustard, broccoli, canola) and 

chenopods (beets, lamb’s-quarters, chard, 

spinach). (See section on mutualists, page 4, for 

more information about mycorrhizae.)

Page 12 of 20 Soil Biology and Land Management 
Forestland 

Forest soils have high ratios of fungi relative to 

bacteria, especially under coniferous forests. The 

fungi are predominately ectomycorrhizae that 

infect tree roots and then extend their hyphae 

into the soil. This greatly increases the tree’s 

effective root zone, allowing access to a greater 

area of soil from which to extract water and 

nutrients. The mantle created by mycorrhizae 

around the root also prevents pathogenic fungi 

and bacteria from attacking the root system. 

Arthropods can be quite numerous in forests 

because the soil is rarely disturbed. Earthworms

are common in deciduous forests, but rare 

among conifers. Where non-native earthworms 

(e.g., fishing bait) have been introduced into 

deciduous forests, significant changes in 

understory vegetation have been observed 

(Minnesota DNR, 2004).

Tree harvesting 

Tree harvesting removes nutrients from the area, 

reduces the total uptake of nutrients by plants, 

and can accelerate biological activity and 

decomposition. Tree harvest can change the 

activity, amount, and diversity of the microbial 

community. The degree of site disturbance 

during harvesting and the amount of slash 

remaining after harvest determines the effects on 

soil organisms. Techniques that minimize soil 

disturbance and compaction will have the least 

detrimental effect on soil organisms. Tractors, 

wheeled skidders, and mechanized harvesting 

equipment disturb the soil surface and can cause 

compaction that restricts biological activity. 

Cable, helicopter, and horse logging produce the 

least disturbance. Soil surface displacement and 

mixing of soil, duff, and slash temporarily

increase microbial activity and may interfere 

with arthropod activity. 

Clear-cutting generally has a negative effect on 

soil biological activity. The large number of 

roads, skid trails, and landings compact soil and 

are particularly susceptible to erosion. When 

erosion and loss of mycorrhizal host plants is 

severe mycorrhizal fungi may be lost from the 

ecosystem. Compared to soil under trees, 

bacteria and actinomycetes, such as 

streptomyces, are more prominent in clear-cut 

areas, between trees in thinned areas, and 

between intact forest patches. When reclaiming 

clear-cut forests it is beneficial to inoculate new 

tree seedlings with mycorrhizal fungi, forest 

soils, or litter containing these fungi. Retaining 

patches of intact forests among clear-cut areas 

provides a source of soil organisms to reinoculate the harvested areas.

Thinning and fuel management 

Thinning of forests has a positive effect on soil 

biological diversity, especially in single-age 

stands. Thinning increases the diversity of 

understory vegetation and thus creates more 

diversity of habitats and food sources for soil 

organisms. (This is an example of the 

intermediate disturbance hypothesis described 

on page 6.) However, excessive compaction and 

soil displacement can have negative effects. 

Roads, trails, and landings 

Compaction created under roads, skid trails, and 

landings compresses soil particles together and 

reduces pore sizes. This restricts the habitat for 

soil organisms, especially the nematodes and 

larger arthropods. The use of designated skid 

trails and restoration of landings after harvest 

minimizes the amount of forestland affected. 

Slash piling and woody debris 

Machine or hand piling of slash concentrates 

nutrient-rich branches, foliage, and sometimes 

topsoil. This increases soil organism populations 

and activity locally. Excessive nutrient leaching 

can occur in areas where microbial activity

increases. Windrowing of slash has severe 

detrimental effects on areas between windrows 

because topsoil rich in soil organisms is scarified 

and placed in the windrows. Runoff may also 

erode nutrient-rich surface soil and organic 

matter from slash areas, potentially degrading 

water quality of nearby streams and lakes. 

Wind damage increases the amount of dead 

wood available for soil organisms. Woody

debris can enhance biological function because 

Soil Biology and Land Management Page 13 of 20 
Page 14 of 20 Soil Biology and Land Management 

dead wood mitigates environmental extremes, 

such as heat and cold, in the microclimate of a 

disturbed area. In forests, downed logs serve as 

centers for biological activity including 

mycorrhizal hyphae, nitrogen-fixing bacteria, 

other microbes, arthropods, and even small 

mammals. In some systems, fungi extract water 

from large rotting logs and supply the water to 

growing trees during times of moisture stress. 

Downed logs also serve as natural dams to help 

reduce erosion and increase infiltration and thus 

improve recolonization by desirable species in 

highly disturbed areas. 

Fire 

Stand replacement fires – Stand replacement 

fires are common in single age stands of fireadapted tree species such as lodgepole pine, jack 

pine, longleaf pine and black spruce. Fire is an 

integral part of the ecology of these forests. The 

microbial community tends to drop back to its 

previous level of activity after an initial flush of 

activity after a fire. 

Cool or patchy fires – Frequent ground fires are 

an integral part of some forest ecosystems, such 

as ponderosa pine stands. These fires are not hot 

enough to burn the overstory trees, but the 

understory trees and brush are killed. 

A short, cool fire rarely eliminates any group of 

organisms. If the fire is cool, nitrogen will not be 

volatilized, and the nitrogen in ash may 

stimulate plant growth and diversity of species. 

Arthropods will repopulate readily after a patchy 

fire. 

Catastrophic wildfire – Hot, long-duration fires 

will kill most organisms, including microbes at 

or near the soil surface. The ignition of litter and 

duff as well as erosion after the fire reduces food 

available to soil organisms. The mineralization 

and subsequent leaching of nitrogen can 

significantly decrease soil fertility. Hydrophobic 

layers formed during hot fires can restrict the 

penetration of water into the soil for several 

years. This restricts plant and root growth thus 

reducing the food supply for soil organisms. 

Insects and disease 

The reduction of fire in some forests, such as 

Ponderosa pine forests, has led to an increase in 

insect infestations and disease. Diseased or dead 

trees increase the amount of woody debris that 

serves as fuel for hotter-burning, potentially 

catastrophic fires. 

An invasion or increase of less disease-resistant 

tree species will make a forest stand more 

susceptible to insect infestation and disease. 
Rangeland 

Soil biological activity in rangelands may vary

greatly over short distances. Activity may be 

high under shrubs and grasses and almost none 

in the bare spaces between plants. 

Some seemingly bare spots are actually

encrusted with soil organisms, such as 

cyanobacteria. Biological soil crusts are 

important for nutrient and water cycling, 

particularly in arid and semi-arid environments.

When plant assemblages change dramatically

over time, for example, from grass- to shrubdominated, the character of the soil biology may

change to the extent that it may be difficult to 

convert the system back to the original plant 

assemblage with its associated soil community. 

Grazing and vegetation management 

Grazing and vegetation management are the 

most important tools for maintaining the benefits 

of the soil food web. Timely grazing, the proper 

frequency of grazing, and control of the amount 

of vegetation removed will maintain or enhance 

plant production and the supply of organic 

matter. Both overgrazing and non-grazing 

reduces root growth and thus the amount of 

organic matter inputs to the soil. Grazing 

stimulates root growth and production of root 

exudates, but overgrazing reduces the amount of 

leaf surface for photosynthesizing. (This is 

another example of the intermediate disturbance 

hypothesis. See page 6.) With reduced food 

supplies, biological activity decreases along with 

important soil functions, including nutrient 

cycling, water infiltration, and water storage. As 

these functions decline, the ability of the plant 

and soil biological communities to replenish soil 

organic matter also declines. Heavy grazing can 

reduce the abundance of nitrogen-fixing plants, 

causing a decrease in the nitrogen supply for the 

entire plant community. Where biological crusts 

provide important functions such as protection 

from erosion, the timing and intensity of grazing 

should be managed to minimize damage to 

crusts. Ensuring even manure distribution is 

another mechanism by which good grazing 

management enhances soil biological activity. 

Fire 

Prescribed burning in grassland communities 

generally produces cool temperature fires that 

have little or no direct effect on soil organisms. 

There can be short-term losses of habitat or food 

sources, but patchy fires leave refuges for larger 

soil organisms in adjacent unburned litter and 

grass. 

Absence of fire or an increased length of time 

between fires commonly promotes vegetation 

shifts from grass-dominated to shrub- or treedominated plant communities. Such vegetation 

shifts affect the soil organisms through the 

change in residue composition and the depth of 

root zones that contribute food sources. Woody

residue and roots will increase fungal 

populations. Bare soil between shrubs provides 

little food and will result in a decline in soil 

organism populations. 

Catastrophic fire is likely to occur in dense 

shrub- or tree-dominated plant communities. 

The resinous wood and longer burn times 

promote hot fires that can effectively scorch the 

upper few inches of the soil. This will kill some

soil organisms and reduce their food supply

while also increasing the availability of some

nutrients. 

Invasive weeds 

Invasive weeds can cause a shift in the types of 

soil organisms present because the quantity and 

quality of plant residue and root exudates will 

change. Weeds that cause increased litter 

buildup tend to promote more fungal dominance 

in soil. The encroachment of annuals into 

perennial plant systems will cause changes in 

organism community composition because soil 

biological activity corresponds with plant 

growth stages and periods of litter fall and root 

die-off. 

Shrub removal 

Removing shrubs by chaining, thinning, or 

applying herbicides promotes a shift to a 

different plant community and affects food 

sources for soil organisms. Less woody material 

Soil Biology and Land Management Page 15 of 20 
Page 16 of 20 Soil Biology and Land Management 

and more herbaceous material will promote 

bacterial increases compared to fungi. Arthropod 

species will also change to those supported by 

the new plant community. As with tillage, the 

soil displacement and mixing that occurs during 

chaining will enhance bacterial decomposition 

of soil organic matter and residues. Compaction 

and herbicide effects, as described in “General 

Management Principles” above, may also be an 

issue during shrub removal. 

Plowing and seeding 

Plowing and seeding disturbances are related to 

the degree of soil mixing. Tillage that 

completely mixes or inverts the topsoil will 

result in a sudden, drastic change in habitat, 

increased organic matter decomposition rates, 

and thus reduced food sources for soil 

organisms. It also destroys larger pores and 

some macrohabitats. Use of a seed drill 

minimizes soil mixing and is much less of a 

disturbance to soil biology. Changes in the 

amount of residue either from plowing or the 

establishment of the seeded species will alter the 

food sources for soil organisms. Loss of residue 

can reduce or eliminate habitats for larger 

organisms in the food web, such as insects and 

arthropods. 

Compaction from traffic 

Grazing animals and vehicles may cause 

compaction, especially when traffic is 

concentrated in small areas or soil is too wet. 

See “General management strategies” for a 

description of the implications of compaction on 

soil biology. 

Erosion 

Off-road vehicle traffic or heavily used trails can 

create ruts that compact soil and channel water. 

The resulting accelerated erosion, rills, and 

gullies can strip or bury topsoil and have a 

negative effect on soil organisms. Erosion 

associated with vegetation shifts often results in 

the redistribution of topsoil, organic matter, 

resources, and habitat across short distances. At 

the shrub-intershrub scale, bare areas between 

shrubs provide the least habitat and resources for 

soil organisms. Areas of grass, shrubs, or trees 

have more diversity of soil organisms. 
Assessment and monitoring 

In contrast to soil physical and chemical 

parameters, there are few specific guidelines for 

managing soil biological properties. Thus, it is 

especially important that land managers track 

changes in soil biological functions over time to 

monitor the effects of management choices. 

Monitoring is the identification of trends by

systematically collecting quantitative data over 

time from permanently marked locations. 

Objectives for monitoring soil biological 

function include: 

• Evaluation and documentation of the 

progress toward management goals, 

• Detection of changes that may be an 

early warning of future degradation, and 

• Determination of the trend for areas in 

desired condition, at risk, or with 

potential for recovery. 

Assessment is the estimation of the current 

functional status of soil biological processes. It 

requires a standard for comparison. Objectives 

of assessments can be: 

• Selection of sites for monitoring, 

• Gathering of inventory data, 

• Identification of areas at risk of 

degradation, and 

• Targeting management inputs. 

Techniques for measuring soil biological 

properties range from informal, qualitative 

observations to quantitative laboratory

techniques. These techniques can be useful for 

learning about organisms’ resource requirements 

and functions. However, soil biological tests can 

be difficult to interpret, and thus provide limited 

support for making specific management 

decisions. More information may be gleaned by

assessing and monitoring properties affected by

soil biological activity including soil surface 

stability (aggregate stability and slake tests), 

water infiltration rates (ring infiltrometer and 

rainfall simulation tests), decomposition rates, 

pest activity, and soil nitrate and carbon levels 

(microbial biomass and total organic carbon 

tests). These measures of soil properties assess 

the soil functions of interest to a land manager. 

However, they are likely to change more slowly

than biological measures and thus are more 

delayed indicators. When deciding what to 

assess or monitor, keep in mind the objectives 

and the time and resources available. 

For more about planning appropriate soil quality

assessments see “Guidelines for Soil Quality

Assessment in Conservation Planning” (NRCS, 

2001c). 

Types of tests 

As discussed in the Soil Biology Primer, a 

variety of approaches can be used to describe the 

soil community, including 1) counting soil 

organisms or measuring biomass, 2) measuring 

their activity, or 3) measuring diversity, such as 

diversity of functions (e.g., biolog plates) or 

diversity of chemical structure (e.g. cell 

components or DNA). Each approach provides 

different information. 

Methods for measuring biomass identify either 

the total number of organisms or only those that 

are active. A pitfall trap or Berlese funnel 

(NRCS, 2001b) can be used to collect larger 

organisms living in litter from a forest floor, 

pasture, or cropland. 

Activity measurements provide a better 

understanding of soil biological function than do 

biomass measurements. One measure of 

biological activity is testing for various 

microbial enzymes (Dick, 1997). By choosing 

the appropriate enzyme, an enzyme assay can be 

used to assess the rate of carbon, nitrogen, or 

phosphorus cycling, or overall microbial 

activity. Enzyme assays have potential as a 

useful indicator and can be done with the 

equipment found in typical soil analysis labs, but 

most labs do not yet offer these tests. 

Respiration, or the amount of CO2 produced 

from the soil, is another measure of biological 

activity. The test can be done in the field 

(NRCS, 1998), but results are difficult to 

interpret. Respiration rates are extremely

variable hourly, seasonally, and by region and 

Soil Biology and Land Management Page 17 of 20 
soil type, so baseline or reference data are nearly

meaningless. Furthermore, high respiration may

indicate a healthy and active biological 

community, or it may indicate recent 

disturbance, such as tillage, that has triggered a 

flush of activity. High respiration represents a 

loss of soil carbon to the atmosphere, which is 

counter to the goal of carbon sequestration. Yet 

there are no guidelines for determining how 

much is too much. For these reasons, soil 

respiration tests can be useful in side-by-side 

demonstrations, but are of limited value as a soil 

biological indicator. 

Collecting samples for laboratory

analyses 

A small number of commercial labs will test soil 

biological properties. Typical measures are 

microbial biomass and direct counts of soil 

microbes. When choosing a lab and soil biology

tests, consider the following. 

• What quality control measures does the 

lab use to ensure reliable results?

• What is the significance of each test in 

terms of soil function? 

• How will the test assist in your 

management decisions? Cotton strip tests (NRCS, 1998 and 2001b) and 

a few other techniques measure decomposition 

rates over days or weeks, and therefore are not 

confounded by short term variation as much as 

are respiration tests. However, results are still 

difficult to interpret and require a standard or 

control for comparison. 

• Do interpretations of results consider 

your specific soil type and cropping 

systems? 

The biomass (total amount of organisms) 

changes seasonally, but does not change 

drastically from day to day. However, activity

levels (e.g., respiration) can change quickly, so 

note the time of year and the temperature and 

moisture conditions when sampling, and sample 

under similar conditions for future observations. 

With any of these measures of the soil biological 

community, refer to a specialist for help 

interpreting test results. 

Samples should be placed in sealed bags and 

chilled (but not frozen) immediately. Get to know your community

To gain a general awareness of soil 

organisms and their effects, try these simple 

methods. Choose a few places to take a 

close look at what lives in your soil. Look 

under a shrub, in the woods, along a fence 

line, in a meadow, in a field, etc. Take time 

to examine the litter on the surface and look 

for organisms that move. Look for biotic 

crusts, burrows, fungal hyphae, and other 

evidence of soil organisms. Over the 

seasons, look for birds picking out 

earthworms behind a tillage implement. 

Observe the rate that dung pats decompose

Notice the amount of

.

runoff or ponding after

a rain. 

Page 18 of 20 Soil Biology and Land Management 
Summary

Soil organisms are integral to soil processes, 

including nutrient cycling, energy cycling, water 

cycling, processing of potential pollutants, and 

plant pest dynamics. These processes are 

essential to agriculture and forestry, and for 

protecting the quality of water, air, and habitat. 

Therefore, land managers should consider the 

effects of their actions on the health and function 

of the soil biological community. 

Despite the well-known importance of soil 

biological processes, the development of 

monitoring and management guidelines is in its 

infancy. However, land managers can learn the 

general principles of how their choices affect 

biological processes and can monitor changes in 

soil function. 

Soil biological health generally improves when 

the following management practices are applied: 

• Regularly adding adequate organic 

matter, 

• Diversifying the type of plants across 

the landscape (in all land uses) and 

though time (in cropping systems), 

• Keeping the ground covered with living 

plants and residue, 

• Avoiding excessive levels of 

disturbances including soil mixing or 

tillage, compaction, pesticides, heavy

grazing, and catastrophic wildfires. 

References and further resources 

Atlas, Ronald M. and Richard Bartha. 1981. 

Microbial Ecology, Fundamentals and 

Applications. Addison-Wesley Publishing 

Company, Inc. 

Chapin, F.S.III; P.A. Matson, H.A. Mooney.

2002. Principles of Terrestrial Ecosystem

Ecology. Springer-Verlag, New York. 

Coleman, D.C., D.A. Crossley. 1996. 

Fundamentals of Soil Ecology. Academic 

Press, San Diego. 

Ceuster, T.J.J. de. and H.A.J. Hoitink. 1999. 

Using compost to control plant diseases. 

Biocycle. 40(6):61-64. 

Dick, R.P. 1997. Soil enzyme activities as 

integrative indicators of soil health. In: 

Biological Indicators Of Soil Health. New 

York : CAB International. P. 121-156. 

Gilkeson, L., P. Peirce, M. Smith. 1996. 

Rodale’s Pest and Disease Problem Solver. 

Rodale Press, Emmaus. 

Gliessman, Stephen R. 1998. Agroecology: 

Ecological Processes in Sustainable 

Agriculture. Sleeping Bear Press. 

Liebig, Mark and Tom Moorman. 1999. Soil 

Biology for the Northern Great Plains. 

USDA ARS and NRCS. Mandan, ND and 

Ames, IA. 

Madigan, Michael T., John M. Martinko, Jack 

Parker. 1997. Brock Biology of 

Microorganisms, Eighth edition. Prentice 

Hall, Upper Saddle River, NJ. 

Minnesota DNR. 2004. Invasive Earthworms

[Online]. 

http://www.dnr.state.mn.us/exotics/ 

terrestrialanimals/earthworms/index.html

(verified 20Jan2004). 

NRCS. 1997. Introduction to Microbiotic Crusts. 

http://soils.usda.gov/sqi/ 

NRCS. 1998. Soil Quality Test Kit Guide. 

http://soils.usda.gov/sqi/ 

The NRCS Soil Quality Test Kit Guide 

includes instructions and interpretations for 

tests of biological activity, aggregate 

stability, water infiltration and more. 

NRCS. 2001a. Agricultural management effects 

on earthworm populations. Soil 

Quality─Agronomy Technical Note No. 11. 

http://soils.usda.gov/sqi 

Soil Biology and Land Management Page 19 of 20 
NRCS. 2001b. Soil Biology Classroom

Activities. http://soils.usda.gov/sqi/ 

Describes methods for the pitfall trap, 

Berlese funnel, and cotton strip 

decomposition tests. 

NRCS. 2001c. Guidelines for Soil Quality

Assessment in Conservation Planning. 

http://soils.usda.gov/sqi/ 

NRCS. 2003. Glossary of Soil Biology Terms 

[Online]. 

http://soils.usda.gov/sqi/soil_quality/soil_biolo

gy/sbglossary.html (verified 19Jan2004). 

NRCS. 2003. Conservation Practice Standards 

[Online]. 

http://www.ftw.nrcs.usda.gov/practice_stds.ht

ml (verified 19Jan2004). 

Paul, E.A. and F.E. Clark. 1989. Soil 

Microbiology and Biochemistry. Academic 

Press, Inc., San Diego California. 

Perry, David A. 1994. Forest Ecosystems. The 

Johns Hopkins University Press, Baltimore. 

Reeder, J.D., C.D. Franks, and D.G. Milchunas. 

Root biomass and microbial processes. In: 

Follett, R.F., J.M. Kimble and R. Lal eds. 

The Potential of U.S. Grazing Lands to 

Sequester Carbon and Mitigate the 

Greenhouse Effect. Lewis Publishers. 

Saxena, D., G. Stotzky. 2001a. BT corn has a 

higher lignin content than non-BT corn. 

American Journal of Botany. 88(9): 1704-

1706. 

Saxena, D., G. Stotzky. 2001b. Bacillus 

thuringiensis (Bt) toxin released from root 

exudates and biomass of Bt corn has no 

apparent effect on earthworms, nematodes, 

protozoa, bacteria, and fungi in soil. 

Soil Biology and Biochemistry. 33(9):1225-

1230. 

Swift, M.J., O.W. Heal, and J.M. Anderson. 

1979. Decomposition in terrestrial 

ecosystems. University of California Press, 

Berkeley. 

Tugel, A.J. and A.M. Lewandowski, eds. 1999. 

Soil Biology Primer. NRCS Soil Quality

Institute, Ames, IA. 

The Soil Biology Primer is a colorful and 

easy-to-read introduction to the organisms 

that live in the soil and why they are 

important to agriculture and environmental 

quality. Copies are available through the 

Soil and Water Conservation Society at 

www.swcs.org or 1-800-THE-SOIL. 

Wardle, David A. 2002. Communities and 

Ecosystems: Linking the Aboveground and 

Belowground Components. Princeton 

University Press. Princeton, New Jersey. 

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis 

of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, and marital or family 

status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for 

communication of program information (Braille, large print, audiotape, etc.) should contact USDA's TARGET 

Center at 202-720-2600 (voice and TDD). 

To file a complaint of discrimination write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 

14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964 (voice or TDD). USDA is

an equal opportunity provider and employer. 

Page 20 of 20 Soil Biology and Land Management