Soil erosion

Soil erosion
Soil erosion occurs when soil is removed through the action
of wind and water at a greater rate than it is formed
SOIL
The soil covering the surface of the earth has taken mil ions of years to form
and we must learn to respect it. Soil is formed at a rate of only 1 cm every 100
to 400 years and it takes 3 000 to 12 000 years to build enough soil to form
productive land. This means that soil is a nonrenewable resource and once
destroyed it is gone forever.
If we disregard this, a time will come when there would not be enough soil
left to sustain life on earth, because the soil is a necessary growth medium
for plants, a home for certain insects and animals, as well as a medium
from which we get minerals, such as gold. It is important therefore to treat
soil, especially topsoil, as a living entity.

---

WHAT IS SOIL EROSION?
? When a raindrop hits soil that is not protected by a cover of vegetation and
where there are no roots to bind the soil, it has the impact of a bullet.
? Soil particles are loosened, washed down the slope of the land and
either end up in the val ey or are washed away out to sea by streams and
rivers.
? Erosion removes the topsoil first. Once this nutrient-rich layer is gone,
few plants will grow in the soil again.
? Without soil and plants the land becomes desertlike and unable to
support life.
CAUSES OF SOIL EROSION
Erosion occurs when farming practices are not compatible with the fact that
soil can be washed away or blown away. These practices are:
? Overstocking and overgrazing
? Inappropriate farming techniques such as deep ploughing land 2 or 3 times
a year to produce annual crops
? Lack of crop rotation
? Planting crops down the contour instead of along it.
Water erosion
Water erosion causes two sets of problems:
? An on-site loss of agricultural potential
? An off-site effect of downstream movement of sediment, causing
flooding and the silting up of reservoirs.
Sheet erosion
? Soil erosion is characterised by the
downslope removal of soil particles
within a thin sheet of water.
? Sheet erosion occurs when the entire
surface of a field is gradually eroded
in more or less uniform way.
— 2 —

---

? It is a gradual process and it is not immediately obvious that
soil is being lost.
Gully erosion (dongas)
Dongas usually occur near the bottom of
slopes and are caused by the removal of
soil and soft rock as a result of concentrated
runoff that forms a deep channel or gully.
On steep land, there is often the danger
of gullies forming. Water running downhill
cuts a channel deep into the soil and where
there is a sudden fall, a gully head forms at the lower end of the channel
and gradually works its way back uphill. As it does so, it deepens and
widens the scar that the gully makes in the hillside. Gully erosion is
related to streambank erosion, in which fast-flowing rivers and streams
increasingly cut down their own banks.
Rill erosion (channel erosion)
Channel erosion can occur on steep land or
on land that slopes more gently. Because
there are always irregularities in a field,
water finds hollows in which to settle and
low-lying channels through which to run.
As the soil from these channels is washed
away, channels or miniature dongas are
formed in the field.
Wind erosion
Wind erosion occurs when the land surface is left bare in regions that are
arid enough, as a result of low rainfall, to allow the soil to dry out, and flat
enough to allow the wind to carry the soil away over several consecutive
days. Land may become susceptible to wind erosion through grazing
animals, which remove the protective plant cover, and whose hooves break
up the soil, especially round watering points. Arable land that has been left
bare is also a major problem.
— 3 —

---

FACTORS dETERmINING SOIL EROSION
There are various factors determining soil erodibility of which the following
are the most important:
Slope
The steeper the slope, the greater the erosion, as a result of the increased
velocity (swiftness) of water-flow. The length of the slope is very important,
because the greater the size of the sloping area, the greater the concentration
of the flooding water.
Soil texture
Soil texture is the size distribution of soil particles. The size of particles
never changes. A sandy soil, therefore, remains sandy and a clayey soil
remains clayey. The three main particles are sand, silt and clay. The more
sandy a soil the easier it will erode.
Soil structure
The term soil structure means the grouping or arrangement of soil particles.
Overcultivation and compaction cause the soil to lose its structure and
cohesion (ability to stick together) and it erodes more easily.
To test to soil type:
Sandy loam
Clayey soil
Roll a sausage between your hands
and try to form a circle
Sandy soil
— 4 —

---

Terrain unit
The crest (top of slope) is usually well drained as soil moisture
moves downhill, leaving air in the pore spaces most of
the time. Over time, the fine (clay) particles are
carried downslope leaving the soil sandy.
Plant roots can penetrate easily
to deep levels and withdraw
enough soil water from there.
These soils have a lower
erosion potential and
Crest
are normally more
stable.
Midslope
Footslope
In the midslope soil moisture moving from the crest starts to dam up as
a result of the clay-rich soil just downhill. The soils are moderately well
drained with a higher erosion potential.
In the footslope the soil has been waterlogged (saturated with water) as a
result of the long-term accumulation of clay which does not allow water to
infiltrate. Plants that grow on these soils are limited to those that can adapt
their root systems to grow laterally above the hard clayey layer. These
imperfectly drained soils have a high erosion potential.
Organic material
Organic material is the “glue” that binds the soil particles together and
plays an important part in preventing soil erosion. Organic matter is the
main source of energy for soil organisms, both plant and animal. It also
influences the infiltration capacity of the soil, therefore reducing runoff.
— 5 —

---

Vegetation cover
The loss of protective vegetation through overgrazing, ploughing and fire
makes soil vulnerable to being swept away by wind and water. Plants
provide protective cover on the land and prevent soil erosion for the
following reasons:
? Plants slow down water as it flows over the land and this allows much of
the rain to soak into the ground.
? Plant roots hold the soil in position and prevent it from being blown or
washed away.
? Plants break the impact of a raindrop before it hits the soil, reducing the
soil’s ability to erode.
? Plants in wetlands and on the banks of rivers are important as they
slow down the flow of the water and their roots bind the soil, preventing
erosion.
Land use
Grass is the best natural soil protector against soil erosion because of its
relatively dense cover. Small grains, such as wheat, offer considerable
obstruction to surface wash. Row crops such as maize and potatoes offer
little cover during the early growth stages and thereby encourage erosion.
Fallowed areas, where no crop is grown and all the residue has been
incorporated into the soil, are most subject to erosion.
PREVENTING SOIL EROSION
Some of the following measures can be implemented to prevent soil
erosion:
? The use of contour ploughing and windbreaks
? Leave unploughed grass strips between ploughed lands (strip cropping)
? Make sure that there are always plants growing on the soil, and that the
soil is rich in humus
? Avoid overgrazing
? Allow indigenous plants to grow along riverbanks
? Conserve wetlands
— 6 —

---

? Cultivate land, using a crop rotation system
? Minimum or no tillage
? Encourage water infiltration and reduce water runoff.
2008
2006 Second print
1999 First print
Compiled by

Directorate Agricultural Information Services, Department of Agriculture
in cooperation with

Directorate Agricultural Land and Resources Management
Printed and published by

Department of Agriculture
Otainable from

Resource Centre, Directorate Agricultural Information Services

Private Bag X144, Pretoria, 0001 South Africa

and on the web at: www.nda.agric.za/publications
— 7 —

---