Botkin Environmental Science Earth as Living Planet 8th txtbk

11.4 Soils 223
A CLOSER LOOK 11.1
The Great American Dust Bowl
Soil erosion became a national issue in the United States in the
1930s, when intense plowing, combined with a major drought,
loosened the soil over large areas. The soil blew away, creating
dust storms that buried automobiles and houses, destroyed many
farms, impoverished many people, and led to a large migration
of farmers from Oklahoma and other western and midwestern
states to California. The human tragedies of the Dust Bowl were
made famous by John Steinbeck’s novel The Grapes of Wrath, later
a popular movie starring Henry Fonda (Figure 11.15).
The land that became the Dust Bowl had been part of
America’s great prairie, where grasses rooted deep, creating a
heavily organic soil a meter or more down. The dense cover
provided by grass stems and the anchoring power of roots protected
the soil from the erosive forces of water and wind. When
the plow turned over those roots, the soil was exposed directly
to sun, rain, and wind, which further loosened the soil.
FIGURE 11.15 The Dust Bowl. Poor agricultural practices and a
major drought created the Dust Bowl, which lasted about ten years
during the 1930s. Heavily plowed lands lacking vegetative cover
blew away easily in the dry winds, creating dust storms and burying
houses.
The development of industrially produced fertilizers,
commonly called “chemical” or “artificial” fertilizers, was
a major factor in greatly increasing crop production in
the 20th century. One of the most important advances
was the invention of industrial processes to convert molecular
nitrogen gas in the atmosphere to nitrate that can
be used directly by plants. Phosphorus, another biologically
important element, is mined, usually from a fossil
source that was biological in origin, such as deposits of
bird guano on islands used for nesting (Figure 11.16). The
scientific-industrial age brought with it mechanized mining
of phosphates and their long-distance transport, which, at
a cost, led to short-term increases in soil fertility. Nitrogen,
phosphorus, and other elements are combined in proportions
appropriate for specific crops in specific locations.
Limiting Factors
Crops require about 20 chemical elements. These
must be available in the right amounts, at the right times,
and in the right proportions to each other. It is customary
to divide these life-important chemical elements into
two groups, macronutrients and micronutrients. A
macronutrient is a chemical element required by all living
things in relatively large amounts. Macronutrients are
sulfur, phosphorus, magnesium, calcium, potassium, nitrogen,
oxygen, carbon, and hydrogen. A micronutrient
is a chemical element required in small amounts—either
in extremely small amounts by all forms of life or in moderate
to small amounts for some forms of life. Micronutrients
are often rarer metals, such as molybdenum, copper,
zinc, manganese, and iron.
High-quality agricultural soil has all the chemical elements
required for plant growth and also has a physical
structure that lets both air and water move freely through
the soil and yet retain water well. The best agricultural
soils have a high organic content and a mixture of sediment
particle sizes. Lowland rice grows in flooded ponds
and requires a heavy, water-saturated soil, while watermelons
grow best in very sandy soil. Soils rarely have everything
a crop needs. The question for a farmer is: What
needs to be added or done to make a soil more productive
for a crop? The traditional answer is that, at any time, just
one factor is limiting. If that limiting factor can be improved,
the soil will be more productive; if that single factor
is not improved, nothing else will make a difference.
The idea that some single factor determines the growth
and therefore the presence of a species is known as Liebig’s
law of the minimum, after Justus von Liebig, a 19th-century
agriculturalist credited with first stating this idea. A general
statement of Liebig’s law is: The growth of a plant is affected
by one limiting factor at a time—the one whose availability
is the least in comparison to the needs of a plant.