Soil Erosion Mechanisms and the Threat to Nonrenewable Land

The Critical Importance of Soil

Soil is the most valuable natural resource of a country, rightly described as the "bridge between the inanimate and the living." It is composed of weathered and decomposed rock material, water, air, organic matter (consisting of plant and animal decomposition), and thousands of different forms of life, mainly microorganisms and insects. All these elements play a role in maintaining the complex ecology of healthy soil.

Soil Formation: A Nonrenewable Resource

In the humid tropics, 200 years are sufficient for the formation of soil from a sandy base. However, the process is usually much longer. In most cases, soil is formed only at a rate of 1 cm every 100–400 years, requiring 3,000 to 12,000 years for the ground to constitute productive land.

This means that soil is a nonrenewable resource: once destroyed, it is gone forever. While erosion is a natural phenomenon, it is typically a slow process. Humanity has accelerated the rate of natural erosion by at least two and a half times, and it is estimated that over the centuries, this has destroyed some 2,000 hectares of land. There is clear evidence that the destruction of past civilizations in the Mediterranean and Central America was due to soil erosion caused by logging in steep areas and other destructive practices.

The Mechanism of Soil Erosion

Soil erosion occurs mainly when the soil is exposed to the wind and rain. Without the protection of vegetation cover and the fixing action of the roots, every drop of water hits the bare soil like a bullet. The soil particles are detached, and the water creeps down the slope into the valley or even the sea, transported by rivers and streams.

Types of Soil Erosion

Sheet Erosion: The Insidious Threat

The most insidious form of erosion is sheet erosion, which occurs when the entire land surface gradually erodes, more or less uniformly. This process is insidious because the loss of land is not immediately obvious. The only apparent indications of sheet erosion are:

• The bottom of poles and fences becoming exposed.
• The roots of trees and crops becoming increasingly visible.

When the farmer notices these signs, they have probably already lost tens of tons of soil per hectare. In a typical field, a farmer who loses 1.5 cm of topsoil—just enough to be noticed—has lost about 190 tons of soil per hectare.

Rill Erosion: Grooves and Furrows

Rill erosion can occur on steep slopes or gentler slopes. Ground irregularities often create depressions where water is deposited, forming small cracks or grooves through which water runs. The floor of these grooves is swept away, creating many small furrows or rills. Even without the formation of deep gullies, the soil losses caused by rill erosion are significant.

Gully Formation: Deep Scars on the Landscape

In hilly terrain, there is often danger of gully formation. The water that runs down the slope opens a deep cleft in the ground when the slope is steep. At the lower end, it forms a step, which gradually progresses uphill, deepening and expanding the gully—an open scar on the hillside. What began as a trickle of water running along a path used by humans or animals can become a fissure tens of meters deep and hundreds of meters wide. In India, gully erosion causes a loss of...