Geoscience Reference
In-Depth Information
D I S C O V E R . . .
SOIL CONSERVATION ON STEEP SLOPES
Soils on steep slopes are prone to erosion because flowing
water has a lot of energy and thus moves sediment. You
can tell that hillslope erosion has been a problem in places
where conservation terraces are found. Farmers build these
features to reduce the effects of soil erosion by creating a
series of stair-steps on steep hillslopes. These steps pro-
duce a series of essentially horizontal surfaces on which
farmers plant their crops. Each of the terraces is separated
by a built-up ridge of dirt, called a berm, which reduces the
velocity of water and traps sediment. Although some soil
erosion still occurs where terraces exist, the amount is sig-
nificantly reduced. Perhaps the most impressive conserva-
tion terraces on Earth are the rice paddies shown here in
the mountainous area of Yunnan province, China. Note that
each of these surfaces can hold a great deal of water, which
falls as precipitation during the summer monsoon.
Water
Crops
Berms
Sediments collect at the
base of berms instead
of washing away
So how does the textural triangle work? Imagine, for
example, that a soil contains 40% sand, 30% silt, and 30%
clay. To determine the textural class, simply follow the rela-
tive percentages of each variable to where the lines intersect
within the triangle. For percent sand, use the lines slanting
from the bottom of the triangle up to the left; for percent silt,
use the lines slanting from the right side down to the left; for
percent clay, use the horizontal lines. In this particular case,
the soil would be texturally classified as
clay loam
. Now
imagine that a soil has the relative percentages of 80% sand,
10% clay, and 10% silt. This soil would be texturally classi-
fied as
loamy sand
.
You may wonder why it matters whether a soil is clay
loam, loamy sand, or any other textural class? These kinds of
classifications may seem mind-numbing rather than informa-
tive, but in the case of soils individual textural classifications
are important because they provide a rough measure of how
well water flows through a soil or how appropriate the soil is
for agriculture, just to name two examples. Regarding water
movement, soils that are sandy generally drain better—that is,
water flows through them faster—than clay-rich soils. Thus,
soil texture has significant implications for the soil-water
budget in any given locality. Although clay-rich soils hold
more water than sandy soils, they are frequently difficult to
work with because clays stick together so well. In general,
the best agricultural soils are the loamy soils because they are
nicely balanced. On the one hand, they contain some sand,
which allows water to drain through. On
the other hand, they
contain some clay, which holds the soil together and prevents
water from draining through the soil too fast. These clays are
also important with respect to overall soil fertility, as you will
learn later. A high percentage of silt allows the soil to be easily
mixed and aerated.
3.
Structure
Soil structure
refers to the way in which
soil particles naturally clump together in
peds
,
which
are natural soil aggregates. Another way of looking at
structure is that it is related to how clumps of soil break
apart and their resulting shape, size, and arrangement.
Soil structure is a difficult concept to illustrate; never-
theless, four primary types of structure are recognized
(Figure 11.16).
Soil structure
The way soil aggregates clump to form distinct
physical characteristics.
