Soil Quality in Vineyards - Soils for Fine Wines - page 195

Soil and Water Quality Problems 7.5

Erosion
The term erosion describes the transport of materials, usually from the soil sur-
face, by water and wind. As indicated in chapter 1, past phases of geologic ero-
sion have led to the formation of sedimentary deposits that are the parent mate-
rials of many present-day soils. In the short term, accelerated erosion resulting
from human activities, especially in agriculture, is of greater concern. Erosion and
deposition are complementary processes because soil removed from the steeper
slopes is deposited to a variable extent on flatter, lower slopes and floodplains.
Similarly, soil eroded from one area by wind is usually deposited in another part
of the landscape (unless blown out to sea).
Erosion is very damaging to soil fertility because mainly the nutrient-rich sur-
face soil is removed, and of that, predominantly the fine and light fractions—clay
and organic matter—leaving behind the more inert sand and gravel. The light
fractions remain in suspension longer in air or water and so are carried farther.
Suspended stream sediment is eventually deposited in lakes, reservoirs, or river es-
tuaries, where it may cause water quality problems.

7.5.1

Erosion by Water
Soil loss by water erosion depends on the potential of rain to erode, called rain-
fall erosivity , and the susceptibility of soil to erosion, the soil erodibility . We dis-
cuss these two factors separately, although they can interact in the field.

7.5.2

Rainfall Erosivity
Erosivity depends on the physical characteristics of rainfall, primarily the kinetic
energy ( KE ) available to break down aggregates and dislodge soil particles. Soil
particles are detached as part of raindrop splash (fig. 7.13). During splash, the
particles are transported a few centimeters. If it occurs on a slope, more of the
splash lands downslope than upslope, so there is progressive movement of soil
downslope. If water ponds on the surface, lateral movement can occur. Such runoff
achieves a greater velocity and hence erosive capacity on steeper slopes. Erosion
by water is therefore a two-step process, involving the detachment of soil parti-

7.5.2.1

Figure 7.13 Raindrop splash at a smooth soil

surface (White 1997, courtesy of
D. Payne). Reproduced with
permission of Blackwell Science
Ltd.