Soil Quality Indicators

Soil Crusts

Structural soil crusts are relatively thin, dense, somewhat

continuous layers of non-aggregated soil particles on the

surface of tilled and exposed soils. Structural crusts

develop when a sealed-over soil surface dries out after

rainfall or irrigation. Water droplets striking soil aggregates

and water flowing across soil breaks aggregates into

individual soil particles. Fine soil particles wash, settle into

and block surface pores causing the soil surface to seal

over and preventing water from soaking into the soil. As

the muddy soil surface dries out, it crusts over.

Structural crusts range from a few tenths to as thick as two

inches. A surface crust is much more compact, hard and

brittle when dry than the soil immediately beneath it,

which may be loose and friable. Crusts can be described by

their strength, or air-dry rupture resistance.

Soil crusting is also associated with biological and

chemical factors. A biological crust is a living community

of lichen, cyanobacteria, algae, and moss growing on the

soil surface that bind the soil together. A precipitated,

chemical crust can develop on soils with high salt content.

Factors Affecting

Inherent - Soil crusting is related to soil texture,

organic matter, and sodium content. Surface crusts are

more common on fine-textured soils, such as silts, loams

and clays. In combination with the splashing effect of

raindrops, increased runoff and erosion of fine-textured

soil increases the likelihood that a crust will develop.

Crusts are usually thin and weak if present on coarsetextured, sandy soils.

Low organic matter results in poor soil structure, reduced

pore space, and weak, unstable aggregates that fall apart

when raindrops hit them. Rainfall and runoff disperses the

soil into individual particles that clog soil pores and seal its

surface with a soil crust.

Soils with high sodium content are more likely to develop

surface crusts since these soils are more readily dispersed

with rainfall and irrigation.

Dynamic - Management activities that deplete soil

organic matter and leave soil bare, smooth and exposed to

the direct impact of water droplets increase soil dispersion,

surface sealing, runoff, erosion, and crusting. Excessive

tillage tends to break up soil clods into smaller sizes more

susceptible to breakdown, bury most plant residue, and

accelerate decomposition of organic matter. Harvest

methods that remove most or all of aboveground biomass

also prevent or reduce organic matter buildup and

protection of the soil surface. Until the crop and its

protective canopy is established, residue removal also

exposes soil to direct sunlight, which increases soil

temperature and accelerates drying of the soil surface into

a hard crust.

Relationship to Soil Function

A surface crust indicates poor infiltration, a problematical

seedbed, and reduced air exchange between the soil and

atmosphere. It can also indicate that a soil has a high

sodium content that increases soil dispersion when it is

wetted by rainfall or irrigation.

Problems with Poor Function

Because they are hard and relatively difficult to break,

crusts restrict seedling emergence, especially in non-grass

USDA Natural Resources Conservation Service

Left: Note the surface crust on this soil. The field was in tall fescue

sod for 11 years. It was cleared and plowed using conventional

tillage methods. Photo courtesy Bobby Brock, USDA NRCS

(retired). Right: Collected from a no-till field in Georgia’s Southern

Piedmont, good structure and aggregation are evident in the soil on

the right. The same soil formed a structural crust under

conventional tillage. Note the sunlight reflectance of the crusted

soil. Photo courtesy James E. Dean, USDA NRCS (retired).

Indicator

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Helping People Help the Land...
crops such as soybeans and alfalfa. Crusts can also reduce

oxygen diffusion into the soil profile by as much as 50% if

the soil crust is wet. Crust development soon after a crop is

planted can result in such poor emergence that the crop

might have to be replanted.

Surface sealing and crusts greatly reduce infiltration, and

increase runoff and erosion. Increased runoff results in less

water available in soil for plant growth.

The sunlight (and energy) reflectance of a surface crust is

higher than that of a non-crusted soil, so soil temperature

may be lower and surface evaporation reduced where a

crust exists (see photo on reverse). This could negatively

effect germination and development of healthy seedlings in

cooler climates.

The relatively smooth surface of a crusted soil initially

increases wind erosion of sandy soils. Loose sand particles

blow across and abrade the smooth surface of the crust.

Roughening of the surface crust eventually reduces wind

erosion. For soils with a small amount of sand, hard crusts

protect the soil surface from wind erosion.

Surface crusts can have other limited benefits. Crusts

decrease water loss because less of their surface area is

exposed to the air compared to a tilled, fluffy soil. In

addition, a crust forms a barrier to evaporation of soil

moisture. Reduced evaporation of soil moisture means

more water remains in the soil for plant use.

Practices that lead to soil crusting include:

• Harvesting, burning, burying, or otherwise removing

plant residues and mulches so as to leave the soil

surface bare for an extended period of time, and

• Soil disturbing activities that destroy organic matter,

soil structure and aggregation, and result in very

smooth seedbeds.

Avoiding Soil Crusting

Practices reducing the development of soil crusts or

minimizing their negative impacts include those that

protect or increase soil structure and organic matter and

provide protective vegetative or residue cover on the soil

surface. No-till or reduced tillage of cropland is the best

way to reduce or eliminate crust formation. If tillage is

necessary, it should only be done to the minimum level

required for good seed germination and emergence. Large

seeded crops do not require the same degree of clod size

reduction or as smooth of a seedbed as do small seeded

crops. Residue intercepts the force of falling raindrops and

is a source of organic matter. Organic matter stabilizes soil

aggregates making them more resistant to the physical

impact of raindrops. Improved aggregation results in lower

bulk density and increased pore space, and improves

infiltration and water movement through soil.

Improved infiltration and water movement through soil

decreases surface ponding and runoff, and helps protect

soil from erosion. Good soil structure and aggregate

stability are vital to supporting healthy, vigorous plants.

Healthy plants provide and conservation tillage methods

manage surface and subsurface plant residues needed to

increase organic matter while maintaining and improving

aggregate stability and soil structure.

To reduce the incidence of surface crusting of soils high in

sodium, irrigation water management prevents sodium

accumulation at the surface, and gypsum (calcium sulfate)

can be applied to promote flocculation and inhibit

dispersion of soil particles.

It may be necessary to break a soil crust with a shallow,

light tillage operation such as with a rotary hoe or row

cultivator, preferably when the soil is still moist. Light

tillage can increase seedling emergence and help control

weeds. Irrigation water can also be used to help with

seedling emergence.

Conservation practices that minimize the development of a

soil crust include:

• Conservation Crop Rotation

• Cover Crop

• Residue and Tillage Management

• Salinity and Sodic Soil Management

Measuring Soil Crusting

Crust air-dry rupture resistance can be measured by taking

a dry piece of the crust about 1⁄2 inch on edge and applying

a force on the edge until the crust breaks. Generally, more

force is required for crusts that are thick and have high

clay content. A penetrometer to measure the penetration

resistance of the crust can be used. Crust thickness can also

be measured.

Reference: Soil Survey Division Staff. 1993. Soil survey

manual. Soil Conservation Service. U.S. Department of

Agriculture Handbook 18. Chapter 3, Part 8. [Online]

http://soils.usda.gov/technical/manual/contents/chapter3d.html

Specialized equipment, shortcuts, tips:

A penetrometer may be needed to measure penetration

resistance of a soil crust.

Time needed: 30 minutes

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