Soil Quality Indicators: Aggregate Stability

USDA Natural Resources Conservation Service April 1996

What are soil aggregates?

Soil aggregates are groups of soil particles that bind to

each other more strongly than to adjacent particles. The

space between the aggregates provide pore space for

retention and exchange of air and water.

What is aggregate stability?

Aggregate stability refers to the ability of soil aggregates

to resist disruption when outside forces (usually associated with water) are applied.

Aggregate stability is not the same as dry aggregate

stability, which is used for wind erosion prediction. The

latter term is a size evaluation.

Why is aggregate stability important?

Aggregation affects erosion, movement of water, and

plant root growth. Desirable aggregates are stable

against rainfall and water movement. Aggregates that

break down in water or fall apart when struck by raindrops release individual soil particles that can seal the

soil surface and clog pores. This breakdown creates

crusts that close pores and other pathways for water and

air entry into a soil and also restrict emergence of

seedlings from a soil.

Optimum conditions have a large range in pore size

distribution. This includes large pores between the

aggregates and smaller pores within the aggregates. The

pore space between aggregates is essential for water and

air entry and exchange. This pore space provides zones

of weakness through which plant roots can grow. If the

soil mass has a low bulk density or large pore spaces,

aggregation is less important. For example, sandy soils

have low aggregation, but roots and water can move

readily.

How is aggregate stability measured?

Numerous methods measure aggregate stability. The

standard method of the NRCS Soil Survey Laboratory

can be used in a field office or in a simple laboratory.

This procedure involves repeated agitation of the aggregates in distilled water.
What influences aggregate stability?

The stability of aggregates is affected by soil texture, the

predominant type of clay, extractable iron, and extractable cations, the amount and type of organic matter

present, and the type and size of the microbial population.

Some clays expand like an accordion as they absorb

water. Expansion and contraction of clay particles can

shift and crack the soil mass and create or break apart

aggregates.

Calcium ions associated with clay generally promote

aggregation, whereas sodium ions promote dispersion.

Soils with over about five percent iron oxides, expressed

as elemental iron, tend to have greater aggregate stability.

Soils that have a high content of organic matter have

greater aggregate stability. Additions of organic matter

increase aggregate stability, primarily after decomposition begins and microorganisms have produced chemical

breakdown products or mycelia have formed.

Soil microorganisms produce many different kinds of

organic compounds, some of which help to hold the

aggregates together. The type and species of microorganisms are important. Fungal mycelial growth binds

soil particles together more effectively than smaller

organisms, such as bacteria.

Aggregate stability declines rapidly in soil planted to a

clean-tilled crop. It increases while the soil is in sod and

crops, such as alfalfa.

(Prepared by the National Soil Survey Center in cooperation with the Soil Quality Institute, NRCS, USDA, and the National Soil

Tilth Laboratory, Agricultural Research Service, USDA)

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require weighing. The measurements are made on

air-dry soil that has passed through a sieve with 2-

millimeter mesh and retained by a sieve with a 1-

millimeter mesh. A quantity of these 2-1 millimeter

aggregates is placed in a small open container with

a fine screen at the bottom. This container is placed

in distilled water. After a period of time, the container is removed from the water and its contents

are allowed to dry. The content is then removed

and visually examined for the breakdown from the

original aggregate size. Those materials that have

the least change from the original aggregates have

the greatest aggregate stability.

Soils that have a high percentage of silt often show

lower aggregate stability if measured air-dry than

the field behavior would suggest, because water

entry destroys the aggregate structure.