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Soil erosion is the removal of soil by water, by wind, or both. Surface horizons almost always contain more organic matter than subsurface horizons.
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The degree of soil erosion is judged by the amount of original surface material remaining in the upper 8 inches of soil. The three classes of soil erosion are illustrated in the figure to the left.
Soil erosion classes are determined from the plow layer (Ap), or upper 8 inches if the Ap is less than 8 inches:
1. None to slight: This layer contains more than 75% of original surface material.
2. Moderate: This layer contains 25% to 75% of original surface material.
3. Severe: This layer contains less than 25% of original surface material.
“Original surface material” means soil material that was once the A and E horizons (above B horizons). A soil that has never been plowed, such as in a woods, is considered to have slight erosion.