120 / Basics of Environmental Science
In fact, soil erosion is an entirely natural process. Unconsolidated surface material is transported by
wind and water from the moment it is exposed, whether the land is cultivated or not, and this transport
has been continuing throughout the history of our planet. Many sedimentary rocks are made from
eroded soil, after all, but the Earth is still blanketed by soil, which is constantly being formed.
Erosion need not become a cause for concern until its rate exceeds the rate of soil formation; after
that threshold is crossed, soil is actually being lost.
Under natural conditions, soil probably forms at an average rate of about 8 mm per century.
Ploughing the land aerates the soil and increases the rate of leaching. This accelerates soil
formation to perhaps 80 mm per century (HUDSON, 1971, p. 36). The planned reclamation of
spoil heaps from mining can produce several centimetres of organically enriched surface soil
within 5 years, but soil formation is considered to commence when substances leached from the
A horizons start accumulating in the B horizons and the soil begins to acquire its layered structure.
This takes much longer.
On cultivated land, therefore, soil is ordinarily forming at a rate of about 2 t ha-1 yr-1, which is the
weight represented by the 0.8 mm forming each year. A loss greater than this indicates a net loss by
erosion, but just how undesirable this is considered to be depends on the soil itself. If the soil is no
more than a thin layer overlying the bedrock, for example, its erosion is much less acceptable than a
similar rate of erosion from a very deep, fertile soil.
Whether a particular rate of erosion is acceptable is a matter of judgement, but the susceptibility
of a field to erosion under different farming regimes can be estimated and the erosion rate
predicted. The technique requires a number of factors to be calculated. The erosivity of the
rainfall (R) is calculated by measuring the amount and type of rainfall, converting this into an
index number, and reading R from a scale. The credibility of the soil (K) is a number representing
the liability of a particular soil to erosion. The length factor of the field (L) is the ratio of the
length of the field to the length of a standard field (of 22.6 m). The slope factor (S) is the ratio
of the soil lost to the amount lost from a field with a 9 per cent gradient. The crop management
factor (C) is the ratio of soil loss to that from a field under cultivated bare fallow. The conservation
practice factor (P) is a ratio of soil loss to that from a field where no care is taken to prevent
erosion. The amount of soil lost from that field each year (A) is then calculated from the universal
soil-loss equation:
A=R×K×L×S×C×PWind erosion can be spectacular and sometimes frightening, but water is probably the more
important agent. Quite ordinary rain will initiate erosion, known as ‘splash erosion’, if it falls
on bare soil. Raindrops fall at about 9 m s-1, giving them a kinetic energy 13.6 times their own
weight (DONAHUE ET AL., 1958, pp. 323–325). This is sufficient to detach soil particles, in
the case of fine sands and silt splashing them to a height of up to 60 cm and a distance of up to
1.5 m. Larger particles do not move so far and clay particles are held together by their strong
cohesive attraction, but even these will travel some distance. Splashing does not remove soil
directly, but moves particles around and the water carries them into every tiny opening. At the
same time, the pounding of the rain packs the surface particles together. When the rain ceases
the surface dries as a tough, impermeable crust sealing the soil beneath. The next time it rains,
water is unable to drain vertically and must flow across the surface, carrying soil particles with
it in a thin mud, down the slope and away.
The effects of splash erosion can sometimes be seen on banks beside roads or ditches, where tree
roots or a large rock have sheltered the soil. These are partly exposed, because the soil has been
washed away from around them. This is known as ‘pedestal erosion’.