Soil erosion is the displacement of soil, earth etc. from an area. Soil erosion can be natural, or accelerated by human activity. Soil erosion is often referred to simply as erosion.
Naturally low rates of soil erosion on Earth occur constantly. Wind erosion, water erosion, and the activity of animals all cause soil to be moved. Wind and water are the main agents of erosion. Large displacements of soil can occur due to events such as floods, glaciation, volcanic eruptions and meteorite strikes.
Soil erosion must not be considered as necessarily bad. Fertile valleys and plains were often rendered fertile by receiving eroded soil from nearby volcanic peaks. However, human accelerated soil erosion is often a cause for very great concern indeed.
Wind and water erosion occur when soil particles are dislodged and carried along by either wind or water. The amount of soil “lost” in this way depends on many factors.
Factors influencing soil erosion:
- Strength of wind or water flow – generally, the stronger the wind or water, the more soil will be removed
- Amount of soil exposed to eroding factor – more exposed soil = more removed soil
- Moisture level of soil exposed to eroding factor – very dry soil is more prone to erosion
- Amount and strength of plant root systems in the soil – plant growth and root systems help to anchor and protect soil, preventing erosion
- Gradient of any slope that the soil is on – the greater the slope – the more risk there is of erosion
- Compaction of the soil – this factor is a little less straightforward. Very loose soil is in danger of eroding, but plants do not grow well in very compacted soil – meaning this, too, is an erosion risk.
- Average soil grain size – the lighter the particles, the easier it is for them to be moved.
Human activities such as agriculture, irrigation and the cattle and sheep industries all contribute to large levels of soil erosion. Conventional agriculture generally involves the soil being bare or ploughed for fairly large amounts of time. Cattle and sheep compact the soil and remove vegetation, and irrigation causes large amounts of water to flow over often very lightly covered soils.
The total effect of all this erosion is the loss of hundreds of tonnes of topsoil every year. Areas are left without the layer of fertile soil they once had, and require the addition of more and more artificial nutrients. The soil generally ends up in the waterways, clogging them, silting up lakes and estuaries. Often the soil is rich in added nutrients – leading to toxic algal blooms.
In order to reduce the amount of soil and fertiliser lost to erosion every year, farming practices need to be reviewed. In Australia, most of the agriculture is dependent on an irrigation system that is itself unsustainable. Australia’s soils are old and not particularly fertile. To try and grow rice in such a dry environment seems like madness – yet we do. The huge amount of irrigation required for these practices causes large amounts of erosion. Farmers need to consider alternative practices such as allowing fields to lie fallow in cycles, thus rejuvenating the soil. There are other alternatives to ploughing that do not break up the soil and leave it exposed to the elements. There are grazing schemes that allow the plant cover to be maintained – protecting the soil from erosion.
I have dealt in this write up with soil erosion on a large scale. The issue of small scale erosion – where farmers lose
paddocks to a channel cut by a waterway, or people lose houses due to the supporting
cliff being eaten away – is a somewhat separate issue. The solutions to such erosion, while they bear some similarity to the options I have mentioned above, are rather different, and really require a separate write up.
On a cheerful note, however, it is important to realise that we should not try and fix every example of shockingly bad erosion. I doubt that anyone would support a move to fill in the Grand Canyon, plant trees and grasses, and add a few large rock walls to slow down the water-course. Soil erosion is often a very good thing.
Acknowledgements:
“Ecology and our Endangered Life Support Systems” by Eugene P. Odum, published Sinauer Associates, 1993.