Geoscience Reference
In-Depth Information
(a)
Figure 11.37 Fertilizing soils.
(a) Typical scene of a farmer fer-
tilizing the soil In a farm field. In an effort to boost production,
fertilizers such as potassium, nitrogen and magnesium are added
to a field to maintain and enhance soil fertility. (b) Map of risk
for groundwater contamination by nitrates from the application
of fertilizer in the contiguous United States. Note that the agricul-
tural heartland of the country is particularly at risk for groundwater
contamination.
EXPLANATION
Less than 6 tons nitrogen input per square mile and poorly drained soils
Less than 6 tons nitrogen input per square mile and well-drained soils
More than 6 tons nitrogen input per square mile and poorly drained soils
More than 6 tons nitrogen input per square mile and well-drained soils
(b)
applications have proven very effective in enhancing soil fertil-
ity, increasing yields by as much as 50% in some places. In con-
junction with these commercial fertilizers, chemicals are also
routinely applied to control weeds and pests. Unfortunately,
there is a down side to such heavy application of chemicals to
farms. One major problem, for example, is nitrate contamination
of groundwater supplies. Nitrate comes from nitrogen, which
is a plant nutrient supplied by inorganic fertilizer and manure.
As we will discuss in Chapter 16, groundwater is water stored
below the Earth's surface, within the ground. These supplies are
often important sources of drinking water in many communities.
As you can see in Figure 11.37b, the extent of nitrate ground-
water contamination is very high in California's Central Valley
and throughout the agricultural Midwest. Also note that it is high-
est in well-drained soils, which makes sense given the way that
soil texture influences water infiltration. Given concerns about
nitrate groundwater contamination, many farmers are looking for
other ways to maintain and increase soil fertility without applying
chemical fertilizers. One method employed is the practice of crop
rotation, which involves alternating the kinds of crops grown on
fields over the course of time. Wheat has a tendency to rigor-
ously pull nutrients from soils and thus deplete them, but if such
a field is occasionally converted to legumes such as soybeans,
alfalfa, or peas, nitrogen is naturally added to the soil and fertility
is enhanced. Another successful method used to reduce reliance
on chemical fertilizers is thorough recycling of crop residues fol-
lowing harvest. These stalks and leaves are then plowed into the
soil the next growing season, enhancing soil fertility.
Aside from the obvious significance of soils as the medium in
which our food grows, soils are also important from an engineering
perspective because the foundations of buildings and other struc-
tures are often built within them. As we have seen, a critical vari-
able is the amount and character of the clays that may be contained
within the soil. Vertisols, in particular, contain a high percentage of
expandable clays that swell and shrink during wet and dry periods,
respectively. Structures built on Vertisols often have weak founda-
tions and walls because the soil can be expanding or collapsing
around them. This process can be so strong that it actually causes a
basement wall to collapse or a wall to crack like the one shown in
Figure 11.38, which costs thousands of dollars to repair.
Given the obvious significance of soils to people, it is impor-
tant to understand soils well and care for them. The agency respon-
sible for mapping and managing soils in the United States is the
Natural Resources Conservation Service (NRCS), which is part of
the United States Department of Agriculture (USDA). The origins
of the NRCS can be traced to the infamous
Dust Bowl
era of the
1930s when soils in the central Great Plains were devastated by the
combined impacts of overplowing, drought, and wind erosion. The
impacts of the Dust Bowl will be discussed in Chapter 18, but for
now it is relevant to consider that perhaps 800 million tons of top-
soil were blown from the region because the native (and protective)
grassland had been eliminated by unsustainable farming practices.
Figure 11.38 Cracked outside wall in a home built on clay-rich
soils.
This house was severely damaged because the surround-
ing soil contains expandable clays, which can either expand like
a sponge when wet or lose cohesion and collapse when they dry.
