Environmental Engineering Reference
In-Depth Information
Another problem with irrigation is
waterlogging
(Figure 10-13). Farmers often apply large amounts
of irrigation water to leach salts deeper into the soil.
Without adequate drainage, water may accumulate
underground and gradually raise the water table.
Saline water then envelops the deep roots of plants,
lowering their productivity and killing them after pro-
longed exposure. At least one-tenth of the world's irri-
gated land suffers from waterlogging, and the problem
is getting worse.
up and down (Figure 10-16b). Each row acts as a small
dam to help hold soil and to slow water runoff.
Strip cropping
(Figure 10-16b) involves planting
alternating strips of a row crop (such as corn or cotton)
and another crop that completely covers the soil (such
as a grass or a grass-legume mixture). The cover crop
traps soil that erodes from the row crop, catches and
reduces water runoff, and helps prevent the spread of
pests and plant diseases.
Another way to reduce erosion is to leave crop
residues on the land after the crops are harvested.
Farmers can also plant
cover crops
such as alfalfa,
clover, or rye immediately after harvest to help protect
and hold the soil.
Another method for slowing erosion is
alley crop-
ping
or
agroforestry,
in which one or more crops are
planted together in strips or alleys between trees
and shrubs that can provide fruit or fuelwood (Fig-
ure 10-16c). The trees or shrubs provide shade (which
reduces water loss by evaporation) and help retain
and slowly release soil moisture. They also can pro-
vide fruit, fuelwood, and trimmings that can be used
as mulch (green manure) for the crops and as fodder
for livestock.
Some farmers establish
windbreaks,
or
shelter-
belts,
of trees (Figure 10-16d) to reduce wind erosion,
help retain soil moisture, supply wood for fuel, and
provide habitats for birds, pest-eating and pollinating
insects, and other animals.
10-3 SUSTAINABLE AGRICULTURE
THROUGH SOIL CONSERVATION
Science: Conservation Tillage
Modern farm machinery can plant crops without
disturbing the soil.
Soil conservation
involves using a variety of ways to
reduce soil erosion and restore soil fertility, mostly by
keeping the soil covered with vegetation.
Eliminating plowing and tilling is the key to re-
ducing erosion and restoring healthy soil. Many U.S.
farmers use
conservation-tillage farming,
which dis-
turbs the soil as little as possible while planting crops.
With
minimum-tillage farming,
the soil is not dis-
turbed over the winter. At planting time, special tillers
break up and loosen the subsurface soil without turn-
ing over the topsoil, previous crop residues, or any
cover vegetation. In
no-till farming,
special planting
machines inject seeds, fertilizers, and weed killers (her-
bicides) into thin slits made in the unplowed soil and
then smooth over the cut.
In 2004, farmers used conservation tillage on
about 45% of U.S. cropland. The U.S. Department of
Agriculture (USDA) estimates that using conservation
tillage on 80% of U.S. cropland would reduce soil ero-
sion by at least half. Conservation tillage also has great
potential to reduce soil erosion and raise crop yields in
the Middle East and in Africa.
Science: Organic Fertilizers
Soil conservation can reduce the loss of soil nutrients,
and applying inorganic and organic fertilizers can
help restore lost nutrients.
The best way to maintain soil fertility is through soil
conservation. The next best option is to restore some of
the plant nutrients that have been washed, blown, or
leached out of soil or removed by repeated crop har-
vesting.
Fertilizers are used to partially restore lost plant
nutrients. Farmers can use
organic fertilizer
from
plant and animal materials or
commercial inorganic
fertilizer
produced from various minerals.
Several types of
organic fertilizers
are available.
One is
animal manure:
the dung and urine of cattle,
horses, poultry, and other farm animals. It improves
soil structure, adds organic nitrogen, and stimulates
beneficial soil bacteria and fungi.
A second type of organic fertilizer called
green
manure
consists of freshly cut or growing green vege-
tation that is plowed into the soil to increase the or-
ganic matter and humus available to the next crop. A
third type is
compost,
produced when microorganisms
in soil break down organic matter such as leaves, food
wastes, paper, and wood in the presence of oxygen.
Science: Other Methods for Reducing
Soil Erosion
Farmers have developed a number of ways to grow
crops that reduce soil erosion.
Figure 10-16 shows some of the methods farmers have
used to reduce soil erosion.
Terracing
can reduce soil
erosion on steep slopes by converting the land into a
series of broad, nearly level terraces that run across the
land's contours (Figure 10-16a). This practice retains
water for crops at each level and reduces soil erosion
by controlling runoff.
Contour farming
involves plowing and planting
crops in rows across the slope of the land rather than
