Soil Quality Resource Concerns: Compaction

USDA Natural Resources Conservation Service April 1996

How can compacted soils be identified?

- platy or weak structure, or a massive condition,

- greater penetration resistance,

- higher bulk density,

- restricted plant rooting,

- flattened, turned, or stubby plant roots.

The significance of bulk density depends on the soil

texture. Rough guidelines for the minimum bulk density

at which a root restricting condition will occur for

various soil textures are (g/cc stands for grams per cubic

centimeter):

Bulk Density

 Texture (g/cc)

Coarse, medium, and fine sand and loamy

 sands other than loamy very fine sand 1.80

Very fine sand, loamy very fine sand 1.77

Sandy loams 1.75

Loam, sandy clay loam 1.70

Clay loam 1.65

Sandy clay 1.60

Silt, silt loam 1.55

Silty clay loam 1.50

Silty clay 1.45

Clay 1.40

What is compaction?

Soil compaction occurs when soil particles are pressed

together, reducing the pore space between them. This

increases the weight of solids per unit volume of soil

(bulk density). Soil compaction occurs in response to

pressure (weight per unit area) exerted by field machinery or animals. The risk for compaction is greatest when

soils are wet.

Why is compaction a problem?

Compaction restricts rooting depth, which reduces the

uptake of water and nutrients by plants. It decreases

pore size, increases the proportion of water-filled pore

space at field moisture, and decreases soil temperature.

This affects the activity of soil organisms by decreasing

the rate of decomposition of soil organic matter and

subsequent release of nutrients.

Compaction decreases infiltration and thus increases

runoff and the hazard of water erosion.
What causes soil compaction?

Soil compaction is caused by tilling, harvesting, or

grazing when the soils are wet.

Soil water content influences compaction. A dry soil is

much more resistant to compaction than a moist or wet

soil.

Other factors affecting compaction include the texture,

pressure exerted, composition (texture, organic matter,

plus clay content and type), and the number of passes by

vehicle traffic and machinery. Sandy loam, loam, and

sandy clay loam soils compact more easily than silt, silt

loam, silty clay loam, silty clay, or clay soils.

Compaction may extend to 20 inches. Deep compaction

affects smaller areas than shallow compaction, but it

persists because shrinking and swelling and freezing and

thawing affect it less. Machinery that has axle loads of

more than 10 tons may cause compaction below 12

inches. Grazing by large animals can cause compaction

because their hooves have a relatively small area and

therefore exert a high pressure.

How long will compaction last?

The persistence of soil compaction is determined by the

depth at which it occurs, the shrink-swell potential of the

soil, and the climate. As the depth increases, the more

persistent the condition. The type and percentage of

clay determine the shrink-swell potential. The greater

the shrink-swell potential and number of wet/dry cycles,

the lower is the duration of compaction at a particular

depth. Freeze/thaw cycles also help decrease nearsurface compaction.

How do organic matter and compaction interact?

Soil organic matter promotes aggregation of soil particles. This increases porosity and reduces bulk density

(i.e., compaction). It also increases permeability and

may increase plant available water.

Addition of manure, compost, or other organic materials

including newspaper, woodchips, and municipal sludge

can improve soil structure, helping to resist compaction

Thick layers of forest litter reduce the impact of machin

ery, thus reducing compaction.

How can compaction be reduced?

- Reduce the number of trips across the area.

- Till or harvest when the soils are not wet.

- Reduce the pressure of equipment.

- Maintain or increase organic matter in the soil.

- Harvest timber on frozen soil or snow.

(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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Literature Cited

Vorhees, W.B. 1992. Wheel-induced soil physical limitations to root growth. In i and B.O.. Stewart eds. Adv. Soil Sci

19:73-92.