Soil Quality Resource Concerns:

Sediment Deposition on Cropland

USDA Natural Resources Conservation Service April 1996

What is sediment deposition?

Sediment is solid material that is or has been transported

from its site of origin by air, water, gravity, or ice to a

field or low landscape position. Deposition occurs when

the amount of sediment becomes greater than the

carrying capacity of the force that is moving it.

How is soil quality affected?

Sediment can either improve or degrade the soils upon

which it is deposited. The impact of sediment deposition depends on the characteristics of the original soil,

rate of deposition, type of material, and depth of deposition.

Fine-grained soil particles deposited on sandy soils

generally improve soil quality, but if coarser material is

deposited on fine-textured soils there is a more delicate

balance. Soil quality may improve over a short period,

but coarser material generally results in degraded soil

structure and physical characteristics and decreased

fertility.

Deposits of infertile sand on a highly productive silt

loam that is high in organic matter and nutrients can

significantly decrease the quality of the silt loam.

However, soil quality would change little if similar

deposits occurred on a sandy soil that had a low content

of organic matter, and low levels of nitrogen, phosphorus, and potash.

The rate of deposition also affects soil quality. If an inch

of sand is deposited on a fertile soil every year for 16

years, the effects would be much less than if eight inches

of sand were deposited in one year. Incremental deposits become incorporated with the surface layer and

improve with organic matter accumulation.

How is sediment deposition

identified?

Modern deposits of sediment have different physical

characteristics than the older, buried soils upon which

they were deposited. The buried soil is generally darker

and more uniform in color. The sediment deposits are

generally less dense, with a wider range in grain sizes.

Sediment deposits often show distinct stratification or

layering.
What can be done about sediment

deposition?

Management response to sediment deposition is generally determined by the depth of deposition and the

quality of the underlying soil. Generally, as the depth of

sediment deposition increases, less mixing is possible.

Potential management practices include the one-time use

of:

- moldboard plowing, which generally turns 6 to 8

inches of soil over but causes a minimum amount of

mixing between the surface and subsurface layers.

- chisel plowing, which causes a greater degree of

mixing but generally disturbs the soil to a shallower

depth of only 4 to 6 inches.

- deep chiseling, which disturbs the soil to the greatest

depth (12 to 24 inches) but generally results in a

minimal amount of mixing.

The best method for addressing sedimentation is prevention, since soil quality generally decreases as the

depth of sediment deposition increases.

Prevent soil erosion in upstream landscape positions by

maintaining plant or crop residue cover, high infiltration

rates, and minimal runoff.

Conservation practices on upstream watersheds reduce

the risk of high volume flooding and damaging sediment

deposition. Dikes, levees, and intercepting channels are

used to provide local protection from some flooding and

sediment deposition.

Relationships between the depth and type of sediment

deposit and damage to soils on flood plains relative to

crop yield are shown in the following table. An estimate

of the amount of recovery and the length of time required are made with the assumption that the flooding

was a one-time event and would not reoccur.

 Damage

Recovery Remaining

Depth and Texture Damage Period After Recovery

 Pct Yrs Pct

4 - 8" fine sand and silt 20 5 0

coarse sand and silt

4 - 8" medium sand 40 10 10

coarse sand

8 - 12" fine sand 40 10 10

coarse sand

12 - 14" coarse sand 60 20 30

12 - 24" coarse sand and 90 30 50

gravel

(from Technical Release No. 17, Geologic Investigations for Watershed Planning, USDA, SCS, 1966)

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

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