Page 96 - Soil Degradation, Conservation and Remediation

3.4 Control of Water Erosion 83

3.3.10 Soil Loss Tolerance Value ( T Value)

The soil loss tolerance value has been defined as an indication of how much erosion
should be tolerated. The T value is the maximum soil erosion loss that does not

cause significant loss in productivity. It depends on soil characteristics. For example,
shallow soils over hard bedrock have small T values. More erosion loss can be toler-
ated for thick permeable soils on permeable unconsolidated parent materials. The T
value is the maximum average annual soil loss that allows continuous cropping and
maintains soil productivity without requiring additional management inputs. Many
soils that have developed from thick sediments of loess are agriculturally produc-
tive. Where subsoils have physical properties unsuitable for rooting, erosion results
in reductions in soil productivity that cannot be overcome with only fertilizer appli-
cation. Such soils have low tolerance levels (small T value). The average annual rate
−1

of soil erosion on cropland in the USA is 11 t ha . T values in the USA range from
−1
2 to 11 t ha (Foth 1990 ).
3.4 Control of Water Erosion

Control of soil erosion by water is based on the following principles:

• Reducing raindrop impact: This can be achieved by providing a cover on the soil
during the rainy season. Dense forest canopy, close-growing crops such as cover
crops and mulches on the bare or cropped soils can provide necessary protection
against raindrop impact.
• Stabilizing soil aggregates: Stable soil aggregates are obtained in soils supplied

with sufficient organic matter. Aggregation improves porosity and infi ltration
and reduces runoff.

• Increasing infiltration and reducing runoff: Infiltration can be increased by mulch-

ing and by modification of the slope. Organic mulches soak water and allow water

more time to infiltrate. Level lands have more infiltration capacity than sloping
soils.

• Reducing velocity of runoff: Velocity of runoff can be reduced by modifying the
degree and length of slope through terracing and contouring. Contour cropping,
strip cropping, and contour strip cropping effectively reduce runoff velocity.
When velocity of runoff is reduced, rate of infi ltration increases.

• Minimum disturbance of soil: Tillage makes the soil more erodible. Conservation
tillage systems, including no-tillage, minimum tillage, and subsoil tillage are
efficient soil conservation practices.

• Preventing concentration of runoff water in channels: Leveling previously
developed rills, growing crops closely, and keeping crop residues in fi eld prevent
concentration of runoff water.

• Carrying runoff water safely out of fi eld. Runoff water can be driven safely out
of the field by grassed waterways.