Soil Problems and Conservation 313Jim Richardson/NG Image CollectionRichard R. Hansen/Science Source
Sufficient plant cover limits soil erosion. Leaves and
stems cushion the impact of rainfall, and roots help to
hold soil in place. Although soil erosion is a natural pro-
cess, the abundant plant cover in many natural ecosys-
tems makes it negligible.Soil Pollution
Soil pollution is any physical or chemical change in soil
that adversely affects the health of plants and other organ-
isms living in or on the soil. Soil pollu tion is important
not only in its own right but because many soil pollutants
tend to also pollute surface water, groundwater, and the
atmosphere. For example, sele nium, an extremely toxic
natural element found in many western soils, leaches off
irrigated farmlands and poisons nearby lakes, ponds, and
rivers. This has caused death and deformity in thousands
of migratory birds and other organisms. Most soil pollut-
ants originate as agricultural chemicals such as fertilizers
and pesti cides (see Chapter 14). Other soil pollutants in-
clude salts, petroleum products, and heavy metals.
Irrigation of agricultural fields often results in their
becoming increasingly saline, an occurrence known as
salinization (Figure 12.16). In time, salt can become so
concentrated in soil that plants are poisoned.-ÊiÀÃÊV>ÕÃi`ÊLÞÊÜ>ÌiÀÊUÊ}ÕÀiÊ£Ó°£x
Water erosion in a field in San Simeon, California. The branching
gullies will continue to grow unless checked by some type of
erosion control.Soil Erosion
Water, wind, ice, and other agents promote soil erosion, a
natural process often accelerated by human activities. Wa-
ter and wind are particularly effec-
tive in moving soil from one place
to another. Rainfall loosens soil
particles, which are transported
by moving water (Figure 12.15).
Wind loosens soil and blows it away, particularly if the soil
is barren and dry.
Ero sion reduces the amount of soil in an area and
therefore limits the growth of plants. Erosion causes a
loss of soil fertility because essential nutrient minerals
and organic matter in the soil are removed. As a result of
these losses, the productivity of eroded agricultural soil
drops, and more fertilizer must be used to replace the nu-
trient minerals lost to erosion. Humans often accelerate
soil erosion with poor soil management. Poor agricultural
practices are partly to blame, as are the removal of natural
plant communities during road and building construc-
tion, and unsound logging practices such as clear-cutting.
Soil erosion has an impact on other natural resources as
well. Sediment that gets into streams, rivers, and lakes af-
fects water qual ity and fish habitats (see Chapter 10). If
the sediments contain pesticide and fertilizer residues,
they further pol lute the water.soil erosion The
wearing away or
removal of soil from
the land.Cut-away view of salinized soil
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Irrigation water contains small amounts of dissolved salts.
Over time, the salt accumulates in the soil. This irrigated soil
in Colorado has become too salty for plants to tolerate.