intensive tillage, soil erosion,
and insufficient added residues
aggregates break down
soil organic matter decreases
increased erosion by wind and water
surface becomes compacted, crust forms
less soil water storage, less diversity of
more soil organic matter is lost
soil organisms, fewer nutrients for plants
crop yields are reduced
hunger and malnutrition result
Figure 1.1. The downward spiral of soil degradation. Modified from Topp et al. (1995).
A high-quality soil is free of chemicals that might
years of poor management and turn into one with poor
harm the plant. These can occur naturally, such as
health, although it generally takes a lot of mistreatment
soluble aluminum in very acid soils or excess salts and
to reach that point. On the other hand, an innately chal-
sodium in arid soils. Potentially harmful chemicals also
lenging soil may be very “unforgiving” of poor manage-
are introduced by human activity, such as fuel oil spills
ment and quickly become even worse. For example, a
or application of sewage sludge with high concentrations
heavy clay loam soil can be easily compacted and turn
of toxic elements.
into a dense mass. Both naturally good and poor soils
A high-quality soil should resist being degraded. It
can be productive if they are managed well. However,
also should be resilient, recovering quickly after unfa-
they will probably never reach parity, because some
vorable changes like compaction.
limitations simply cannot be completely overcome. The
key idea is the same that we wish for our children—we
THE NATURE AND NURTURE OF SOIlS
want our soils to reach their fullest potential.
Some soils are exceptionally good for growing crops, and
others are inherently unsuitable; most are in between.
HOW DO SOIlS bEcOME DEgRADED?
Many soils also have limitations, such as low organic mat-
Although we want to emphasize healthy, high-quality
ter content, texture extremes (coarse sand or heavy clay),
soils because of their ability to produce high yields of
poor drainage, or layers that restrict root growth. Iowa’s
crops, it is also crucial to recognize that many soils in
loess-derived prairie soils are naturally blessed with a
the U.S. and around the world have become degraded—
combination of silt loam texture and high organic matter
they have become what many used to call “worn-out”
content. By every standard for assessing soil health, these
soils. Degradation most commonly occurs when ero-
soils—in their virgin state—would rate very high.
sion and decreased soil organic matter levels initiate a
The way we care for, or nurture, a soil modifies its
downward spiral resulting in poor crop production (figure
inherent nature. A good soil can be abused through
1.1). Soils become compact, making it hard for water to
5
Building SoilS for Better CropS: SuStainaBle Soil ManageMent