Rangeland Sheet 6

Soil Quality Information Sheet

Rangeland Soil Quality—Organic Matter

USDA, Natural Resources Conservation Service May 2001

Table 1.—Soil organic matter

 Component Rate of Primary function

 decay

Light fraction Weeks to • Serves as food for soil

months organisms

• Stores and provides plant

nutrients

Physically Decades • Enhances soil structure,

protected porosity, and the water-holding

capacity

Chemically Hundreds to • Holds nutrients

stable thousands • Stabilizes microaggregates

of years

What is soil organic matter?

Soil organic matter is carbon-rich material that includes

plant, animal, and microbial residue in various stages of

decomposition. Live soil organisms and plant roots are part of

the carbon pool in soil but are not considered soil organic matter

until they die and begin to decay.

The quantity and composition of soil organic matter vary

significantly among major ecosystems. Soil in arid, semiarid,

and hot, humid regions commonly has less organic matter than

soil in other environments. The total content of organic matter

ranges from less than 0.5 to more than 8 percent in the surface

layer of rangeland soils.

Soil organic matter includes three main components (table 1).

The light fraction is more biologically active than the other two

and includes relatively fresh plant fragments. Physically

protected organic matter is locked within aggregates of mineral

particles, where it is protected from microbial decomposition.

Chemically stable organic matter gives soil its dark color and is

generally the largest pool of organic matter in soil. Physically

protected organic matter may also be chemically stable.

Why is organic matter important?

Soil organic matter enhances soil functions and

environmental quality because it:

• binds soil particles together into stable aggregates, thus

improving porosity, infiltration, and root penetration and

reducing runoff and erosion;

• enhances soil fertility and plant productivity by improving

the ability of the soil to store and supply nutrients, water,

and air;

• provides habitat and food for soil organisms;

• sequesters carbon from the atmosphere;

• reduces mineral crust formation and runoff; and

• reduces the negative water quality and environmental

effects of pesticides, heavy metals, and other pollutants by

actively trapping or transforming them.

What affects soil organic matter?

The amount of organic matter in the soil is a balance between

additions of plant and animal materials and losses through

decomposition and erosion.

Environmental factors interacting over time affect the

amount of organic matter in soil. Rainfall and temperature affect

plant productivity and the rate of organic matter decomposition.

Increasing levels of organic matter promote a higher waterholding capacity, which results in increased plant growth and

thus an increased amount of organic matter and plant nutrients.

Roots are the primary source of organic matter. Dead roots

and gelatinous materials exuded by plant roots as they grow

through the soil are decomposed by soil organisms and

converted into organic matter. Since much of what is produced

above ground is lost through photo-oxidation, the amount of

Organic mattter darkens and stabilizes the surface layer in soils.
 For more information, check the following: http://soils.usda.gov/sqi and http://www.ftw.nrcs.usda.gov/glti

(Prepared by the Soil Quality Institute, Grazing Lands Technology Institute, and National Soil Survey Center, Natural Resources Conservation Service,

USDA; the Jornada Experimental Range, Agricultural Research Service, USDA; and Bureau of Land Management, USDI)

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root production is very important. Every year, about 25 percent

of the total root biomass in areas of tall prairie grasses dies and

becomes available for incorporation into the soil as organic

matter. In the drier areas, such as areas of short prairie grasses,

about 50 percent of the root biomass becomes available, but the

total amount is less than that in the areas of tall grasses.

Plant composition and distribution control the distribution

of organic matter. The horizontal and depth distribution of roots,

the distribution of plants across the landscape, and the

susceptibility of roots to decay vary among species. The roots of

forbs and shrubs generally contribute less organic matter to the

surface layer of the soil than the roots of grasses. Changes in the

composition of plant species, especially from grasses to shrubs,

affect the contribution of roots to soil organic matter. The

organic matter is enhanced by litter beneath shrubs in areas of

arid and semiarid rangeland. Fire initially reduces the amount of

plant residue added to the soil. If the fire results in a shift from

shrubs to grasses, however, the long-term effect can be an

increase in soil stability and organic matter.

Soil organisms break down litter, dead roots, and organic

matter into smaller fragments and compounds. As they

decompose organic matter, they convert nutrients into plantavailable forms and release carbon dioxide into the atmosphere.

Warm, moist soil supports higher decomposition rates than

waterlogged, dry, or cool soil.

Wind erosion and water erosion increase losses of organic

matter. Erosion breaks down soil aggregates, exposing

physically protected organic matter to decomposition and loss.

Organic-rich soil from the surface layer is carried away by

runoff or wind. Litter redistribution by wind or water from or to

surrounding rangeland also affects the content of organic matter.

Grazing can change plant composition and distribution and

increase or decrease the amount of organic matter in the soil.

Grazing can increase the rate of root turnover, but overgrazing

reduces the amount of plant energy available for the growth of

new roots. Trampling by livestock can help to incorporate the

plant material above the ground into the soil. In arid ecosystems,

however, little plant material is available for incorporation.

Trampling also breaks up soil aggregates, exposing organic

matter to decomposition and loss through erosion.

Management strategies

The following strategies can help to maintain the optimum

content of organic matter in rangeland soils:

• Increase or maintain plant production.

• Promote the growth of species with high root production

and promote a mix of species with different rooting depths

and patterns.

• Promote the incorporation of above-ground plant material

in moist plant communities with large amounts of standing

plant material (e.g., areas of tall prairie grasses).

• Protect the soil from erosion by maintaining or increasing

the plant cover and reducing the amount of bare soil.

• Properly manage grazing, fire, and vehicle use and thus

promote the desired plant community and protect the soil

from erosion.