Environmental Engineering Reference
In-Depth Information
bank mud probably came from farmland, which supplies about half of
the sediment we see suspended in streams and heading for ocean depths.
4
The rate of soil loss is greatly increased by farming because tilling the
soil loosens it, making it more easily moved. Two-thirds of America's
soil degradation results from crop farming and nearly all the rest from
overgrazing.
5
Furthermore, tilling rips from the soil last season's plant
roots, and roots hold the soil in place. In the Upper Mississippi Valley,
agricultural activities have boosted erosion rates between 200 and 700
percent.
6
We have lost about one-third of the nation's topsoil since
farming started about 350 years ago.
7
As the Dalai Lama wrote,
The threat of nuclear weapons and man's ability to destroy the environment are
really alarming. And yet there are other almost imperceptible changes—I am
thinking of the exhaustion of our natural resources, and especially of soil ero-
sion—and these are perhaps more dangerous still, because once we begin to feel
their repercussions it will be too late.
8
Soil loss from cropland can be reduced in several ways.
Leaving land unplowed every seven years is a biblical directive that
both reduces erosion and allows the land to rejuvenate its nourishing
capabilities. It is sound advice and was practiced from antiquity until the
industrialization of agriculture in the 1950s. Today fallowing is regarded
as uneconomical, and in the short run, it is. Unfortunately, consideration
of only the short run is the guiding light of most of agriculture in the
United States today.
Crop rotations are effective in controlling erosion and were used by
American farmers from the founding of the country until the advent of
industrial monocultures following World War II. Changing the crop
from year to year can reduce erosion rates by as much as 90 percent,
depending on the crops chosen.
9
Another effective way to reduce erosion is no-till cultivation.
10
Whereas
traditional plowing overturns up to 8 inches of soil, no-till reduces the
depth of overturning by 75 percent, keeps crop and plant residue on the
surface longer, and can reduce soil erosion by 90 to 95 percent. No-till
works best in sandy soils. If the soil contains too much clay, traditional
plowing is required to stir the soil suffi ciently for adequate crop yields.
The downside to no-till is an increase in weeds. No-till cultivation in
planted cropland grew from 6 percent in 1990 to 16 percent in 2000 to
22 percent in 2008.
11
If crop residue such as corn stalks is left on the fi eld all winter
after harvesting, erosion is reduced and soil fertility is increased. The
