Environmental Engineering Reference
In-Depth Information
Figure 5.1
A generalized soil profi le. Individual horizons vary in thickness, and some may be locally
absent. The A-horizon is dominated by mineral grains in various stages of decay, releasing
plant nutrients in the process. Water and air are present between the mineral grains. The
B-horizon is enriched in either red hematite (iron oxide) in humid climates or calcite
(calcium carbonate in dry climates).
Soil performs many important roles that are essential for human
survival, especially the topsoil (A-horizon) (fi gure 5.1). It gives plants a
foothold so they can root, nourishes them, provides homes for the
myriad of small creatures whose presence is necessary for healthy crops,
and fi lters rainwater.
But excessive cultivation, monoculture, erosion, and exposure have
led to the degradation of much of America's cropland. The Department
of Agriculture reports that 54 percent of the cropland is eroding above
soil loss tolerance rates and that about 1.8 billion tons of soil are lost
from cropland each year because of improper management, deforesta-
tion, overgrazing, and industrial activity.
3
Approximately 40 percent of
the mud in U.S. streams comes directly from cultivated land, and 26
percent comes from the erosion of stream banks. Much of the stream
