314 CHAPTER 12 Mineral and Soil Resources
crop rotation. When the same crop is grown over and
over in one place, pests for that crop accumulate to de-
structive levels, and the essential
nutrient minerals for that crop
are depleted in greater amounts.
This makes the soil more prone
to erosion, and it makes the crops
less productive as well. Crop ro-
tation is effective in decreasing insect damage and dis-
ease, reducing soil erosion, and maintaining soil fertility
(Figure 12.17c also shows crop rotation).
A typical crop rotation would be corn l soybeans l
oats l alfalfa. Soybeans and alfalfa, both members of the
legume family, increase soil fertility through their asso-
ciation with bacteria that fix atmospheric nitrogen into
the soil. Thus, soybeans and alfalfa provide nutrients for
the grain crops they alternate with in crop rotation.Contour Plowing, Strip Cropping, and Terrac-
ing Hilly terrain must be cultivated with care because
it is more prone than flat land to soil erosion. Contour
plowing, strip cropping, and terracing help control
ero sion of farmland with variable
topography. In contour plowing,
furrows run around hills rather
than in straight rows. Strip crop-
ping, a special type of contour
plowing, produces alternating strips of different crops
along natural contours (see Figure 3.1a). For example,
alternating a row crop such as corn with a closely sown
crop such as wheat reduces soil erosion. Even more ef-
fective control of soil erosion is achieved when strip crop-
ping is practiced in conjunction with conservation tillage.
Farming is undesirable on steep slopes, but if it
must be done, terracing can be used to level areas and
thereby reduce soil erosion due to gravity or water runoff
(Figure 12.17d). Nutrient minerals and soil are retained
on the horizontal platforms instead of being washed away.Soil Reclamation Badly eroded land can be re-
claimed by (1) stabilizing the land to prevent further ero-
sion and (2) restoring the soil to its former fertility. To
stabilize the land, the bare ground is seeded with plants
that eventually grow to cover the soil, holding it in place.
The plants start to improve the quality of the soil almost
immediately, as dead material decays into humus. The
humus holds nutri ent minerals in place, releas ing them a
little at a time; it also improves the water-holding capacitySoil Conservation and Regeneration
Only 11 percent of the world’s soil is suitable for agricul-
ture (Figure 12.17a, b). We therefore need to protect
the soil we use for agriculture. Although agriculture may
cause or accelerate soil degradation, good soil conserva-
tion practices promote sustainable soil use. Conservation
tillage, crop rotation, contour plowing, strip cropping,
terracing, and shelterbelts minimize erosion and min eral
depletion of the soil. Badly eroded and depleted land can
be restored, but restoration is costly and time-consuming.
Conservation Tillage and Crop Rotation Con-
ventional methods of tillage, or working the land, in clude
spring plowing, in which the soil is cut and turned in prep-
aration for planting seeds. Although conventional tillage
prepares the land for crops, it greatly increases the likeli-
hood of soil erosion because it removes all plant cover.
Conventionally tilled fields contain less organic material
and generally hold less wa ter than undisturbed soil.
Conservation tillage, which causes little disturbance
of the soil, is one of the fastest-growing trends in U.S. ag-
riculture (Figure 12.17c). Dur-
ing planting, special machines
cut a narrow furrow in the soil for
seeds. Several types of conserva-
tion tillage fit different areas of
the country and different crops.
The most extreme of these, no-
tillage, does not involve any tilling
(that is, no plowing or disking) of
the soil. Almost 40 percent of U.S.
farmland is currently planted us-
ing conservation tillage, compared to less than 7 percent
of farmland worldwide.
In addition to reducing soil ero sion, conservation
tillage increases the organic mate rial in the soil, which
improves the soil’s water-holding capacity. Decomposing
organic matter releases nutri ent minerals more gradually
than when conven tional tillage methods are employed.
However, use of conservation tillage requires new equip-
ment, new techniques, and greater use of herbicides to
control weeds. Research is under way to develop alterna-
tive methods of weed control for use with con servation
tillage. (Chapter 14 discusses sustainable agriculture, which
includes conservation till age and the other soil conserva-
tion practices presented in this chapter.)
Farmers who practice effec tive soil conservation mea-
sures often use a combination of conservation tillage and
conservation
tillage A method
of cultivation in
which residues from
previous crops are
left in the soil, par-
tially covering it and
helping to hold it in
place until the newly
planted seeds are
established.crop rotation The
planting of a series
of different crops in
the same field over a
period of years.contour plowing
Plowing that matches
the natural contour of
the land.