Food, Soil, and Pest Management - Environmental Science - page 212

with each succeeding rain cut the channels wider and
deeper until they become ditches or gullies. Gully ero-
sion usually happens on steep slopes where all or most
vegetation has been removed.
Soil erosion has two major harmful effects. One is
loss of soil fertility through depletion of plant nutrients
in topsoil. The other occurs when eroded soil ends up
as sediment in nearby surface waters, where it can pol-
lute water, kill fish and shellfish, and clog irrigation
ditches, boat channels, reservoirs, and lakes.
Soil—especially topsoil—is classified as a renew-
able resource because natural processes regenerate it.
If topsoil erodes faster than it forms on a piece of land,
it eventually becomes a nonrenewable resource.

Science: Global Soil Erosion
Soil is eroding faster than it is forming on more than
one-third of the world's cropland.
A 1992 joint survey by the United Nations (UN)
Environment Programme and the World Resources
Institute estimated that topsoil is eroding faster than it
forms on about 38% of the world's cropland (Fig-
ure 10-10). According to a 2000 study by the Consulta-
tive Group on International Agricultural Research, soil
erosion and degradation have reduced food produc-
tion on 16% of the world's cropland. See the Guest
Essay on soil erosion by David Pimentel on the web-
site for this chapter.
Some analysts contend that erosion estimates are
overstated because they underestimate the abilities of
some local farmers to restore degraded land. The UN
Food and Agriculture Organization (FAO) also points
out that much of the eroded topsoil does not go far and

Figure 10-8 Natural capital degradation: erosion of vital top-
soil from irrigated cropland in Arizona.

type of land degradation is soil erosion: the movement
of soil components, especially surface litter and top-
soil, from one place to another. The two main agents of
erosion are flowing water and wind, with water causing
most soil erosion (Figure 10-8).
Some soil erosion is natural; some is caused by hu-
man activities. In undisturbed vegetated ecosystems,
the roots of plants help anchor the soil, and usually
soil is not lost faster than it forms. Soil becomes more
vulnerable to erosion through human activities that
destroy plant cover, including farming, logging, con-
struction, overgrazing by livestock, off-road vehicle
use, and deliberate burning of vegetation.
More severe gully erosion (Figure 10-9) occurs
when rivulets of fast-flowing water join together and

Figure 10-9 Natural capital
degradation: severe gully ero-
sion on cropland in Bolivia after
trees that held the soil in place
were cut.