Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2013)
F ARMING
October, 1931.
IN SOUTH AFRICA.
Humus and Soil Erosion.
WHEN it rains some of the water soaks
into the ground, some runs off and
some is evaporated. If all the rain
water could be absorbed by the soil
without any being allowed to run off
no soil would be washed away, in other
words, there would be no surface
erosion or sloot formation. The conditions may, however, be of such a
nature as to prevent all the rainwater
from soaking into the ground, in
which case a certain amount would be
lost. If man were not to disturb the
processes of Nature a state of equilibrium would be established between
the soil that is eroded and the soil that
is formed as a result of the crumbling
or mouldering away of rock-formations, in other words, there would be
no loss of soil.
Generally, such a state of equilibrium would persist were it not for the
activities of man, whose ploughing,
stock farming, etc., tend to disturb
the processes of Nature. By these
necessary activities the natural vegetal
covering of the soil is either reducea
or completely destroyed, as a result of
which the soil surface is either
partially or entirely exposed to the
erosive power of the water and,
incidentally, the latter is enhanced as
a result of the increased volume and
velocity of the run-off.
Erosion of Cultivated Lands.
In Missouri experiments, in the
course of which the run-off and its silt
content are carefully measured, are
being conducted. These experiments
are carried out on loam soil with a
gradient of 1 in 27, and observations
over a period of 6 years show (1) that
where the land had been sown continuously to maize 27.38 per cent. of
the rain water, on an average, had
run off, and that the water had carried
away 226 tons of soil, which means
that the surface soil would be removed
to a depth of 7 inches in 56 years'
time; (2) that where the surface was
covered with grass 11.55 per cent. of
the rain water had, on an average,
run off and carried away only 3.6 tons
of soil to the morgen, which means
that the surface would be removed to
a depth of 8 inches in 3,547 years'
time.
The results of these experiments are
very interesting and instructive in that
they demonstrate what is taking place
on cultivated lands, i.e., the serious
loss of water coupled with the gradual
removal of the fertile surface soil
while, in the case of the soil that,had
Dr. J. C. Fick, Lecturer in
Chemistry at the School of Agriculture, Glen, O.F.S., describes
in this article how some of our best
surface soils are washed aW'ay by
storm water, and hoW' clay soils are
being converted into sandy soils,
to be eroded in course of time by
wind. He emphasizes the valufJ of
humus, which binds the soil, and
increases its waterholding capacity,
thus preventing its erosion by wind
01" water.
retained its vegetal covering, little
water had run off and practically no
soil had been washed away. In this
instance the equilibrium between
eroded and newly formed soil is being
maintained.
Value of Soll Humus.
But it is impossible for us to
furnish all our land with a dense
vegetal covering, as we require a proportion, some of which might be more
or less level, for crops. The question
arises, what should be done to prevent
loss of water and the resultant loss of
soil by erosion? Further experiments
in Missouri furnish some information
on this point. These have shown that
the run-off from land that had been
well supplied with humus was fifty per
cent. less than from land that had
been sown continuously to maize without its humus content having been reinforced by humus forming material.
After experiments, spread over a
period of 3 years, had been carried out
it was shown that annually during the
rainy season 6.08 tons had, on an
average, been washed away from the
land that had been supplied with
humus, and double the amount (14.49
tons) from the land of which the
humus content had not been reinforced. This demonstrates at once
this additional advantage of humus in
the soil, i.e., in reducing the loss of
soil by surface erosion.
Knowing, as we do, the functions of
humus, these results are only what
mig-ht have been expected. It is a
well known fact that soil deficient in
humus absorbs less water and absorbs
it more slowly than soil that is rich
in humus and that, consequently, more
water runs off and more soil is eroded
III the former case than in the latter.
The Origin of Sa)Jdy Sol1s.
As the water flows over the soil
surface it carries with it the soil
particles taken up and held in suspen-
260
sion. The greater the velocity of the
flow the more soil particles will be
carried away. But when the flow is
retarded the heavier particles will be
precipitated. The smaller particles,
however, are not so easily precipitated
when once they are held in suspension,
on account of their being so infinitesimally small. Thus the best soil constituents, which are of such vital
importance to our light form of
agriculture, are the first to be washed
away, with the result that our soils are
gradually becoming more sandy and
less fertile. In some districts, especially where the humus in the soil has
been depleted, the position in this
respect is becoming alarming. Lands
that had been sown continuously to
maize for 20 to 30 years and more are
now showing signs of wind erosiona form of erosion which had previously
not been experienced in those parts.
Exhaustion of the humus content of
the soil is the chief cause of this evil,
which is responsible for exte;nsive
damage to some of our best lands.
Many of these lands have for years
been subjected to continuous cropping
without any attempt having been
made to re-inforce the humus content
of the soil.
Put Humus into the Soll.
When impoverished soil is enriched
with the necessary humus the danger
of the finer soil particles being washed
away is immediately reduced, not so
much as a result of less water running
off, but because of the ability of the
clay particles to build up larger particles, which are not so easily taken u:p
and held in suspension by the water,
and should this by any chance occur
the danger of the heavier clay and
sand particles being washed away if>
thus considerably reduced.
One good reason, among many, for
maintaining the humus content in the
soil is that humus is of special importance in the case of lands lying against
a fairly steep slope, where the danger
of the finer soil particles being carried
away by surface erosion is accentuated.
Considerable attention is at preflent
being devoted to the combating of
sloot erosion. The enormous damagedone by this form of erosion is evident,
but surface erosion, like a thief in the
night, does its fell work unseen,
robbing us of millions of tonsof good
soil, and slowly but surely depriving
the fertile surface layer of its valuable
plant foods. By enhancing a;n<,l maintaining the humus content in the soil
these losses will be reduced.