Geoscience Reference
In-Depth Information
With annual, monotonous production of only wheat or corn coupled with intense
tillage and summer fallowing, it is a heartbreaking tragedy not to realize that soil
structure shall eventually be nearly or completely wiped out. But, it is also encour-
aging to realize that conservation tillage coupled with realistic extensions of no-
tillage techniques during numerous years contributes to the conservation of soil
structure, especially if crop rotation including pulse crops such as lentils and fi eld
pea along with small grain cereal crops is adopted. High harvests are accompanied
by the increase of plant remnants, and thus a good base is laid for the increased
production of glomalin and for increased aggregate stability.
Next, we mention steps aimed at the direct protection of soil against erosion.
Crop residue cover should be kept until the crop canopy is closed, and thus the soil
surface is protected against direct action of raindrops. Moreover, the presence of
crop residues makes surface runoff more diffi cult owing to its increased fl ux resis-
tance. Here, the word crop is meant in the broadest way as, e.g., the alternation of
summer crops with winter crops and perennial crops.
Contour plowing, contour farming, and contour strip cropping are each very effi -
cient to protect soil against erosion. The procedure described as contour plowing
means that crops are planted nearly on contour lines. The ruts made by the plow run
perpendicular rather than parallel to slopes, generally resulting in furrows that curve
around the land and are level. These ridges along contour lines create a water-break
that reduces the direct downhill slope fl ux and avoids the formation of rills as well
as subsequent gullies during times of heavy water runoff. This practice is mostly
effective on moderate slopes in ranges from 2 to 10 % when crops are planted in
tilled soil with ridge heights of about 5-8 cm (2-3 in.). If the system is applied
without plowing on moderate slopes, it is denoted as contour farming. It still reduces
erosion, especially if plant residues cover the slope in horizontal strips.
Contour strip cropping involves strips of high-residue cover or perennial crops
alternating with strips of low-residue cover. The strips having widths usually
between 20 and 35 m (approximately 75 and 120 ft) should be positioned on the
contour lines or close to them. Even though it is not always possible in rolling land-
scapes where the strips are interrupted by an abrupt obstacle, the technique is very
useful. Soil that erodes from the bare or low-residue strips is deposited in strips with
high residue or dense vegetation because runoff velocity is decreased. This practice
is most useful if the soil is tilled or if the soil is left bare during part of the year in
no-till. Soil eroding from bare or low-residue strips is deposited in strips with high-
residue or dense vegetation because runoff velocity is decreased. The perennial con-
tour strips build up a greater and more stable permeability thanks to the uninhibited
activity of vegetation and fauna, so they are better at soaking up surface runoff and
trapping sediment. This practice is most useful if the soil is tilled or if the soil is left
bare during part of the year in no-till.
When both slope length and its steepness increase, runoff and soil loss also
increase. Effective erosion protection can be best achieved if both characteristics are
reduced. The slope steepness can be changed by the construction of terraces as is
frequently done in southeastern Asia and around the Mediterranean. Because the
terraces also shorten the length of slopes, they are a very effective contra-erosion
