Soil erosion keeps happening all the time, but at such a slow pace that it takes decades for us to take notice and corrective action. But this rate of loss is many times that of soil formation. Artificial fertilizers help compensate the loss temporarily to some extent.
Why should I be aware of this?
Approximately 1” of soil is formed in 1000 years, which converts to 600 t/ha. In natural ecosystems, soil formation of 1 ton/ha per year is common. It increases to some extent when land is under cultivation. There is an annual loss of around 10t/ha for flat 'sustainable' agriculture land and 10% for land with slope. As we can see, the loss is far higher than natural replenishment. But because of use of fertilizers the average net loss is estimated at around 7 percent in 100 years.
World land degradation on a total arable area of 1500 Mha is about 7-10 Mha/yr. It is estimated that natural erosion worldwide is 10 Gt/yr, but human-induced erosion is more than 2.5 times higher.
All about soild erosion
Causes of soil erosion
- Natural factors
- Heavy rains on weak soil: rain drops loosen soil particles which get transported downhill by water
- Vegetation depleted by drought: rain drops on soil causes erosion. Winds blow away the fine particles during droughts.
- Steep slopes: water flows faster; soil creeps, slips or slumps downhill.
Sudden climate change
- Rainfall: erosion increases with unexpected rapidity with severe rainstorms
- Drought: water dries up and the soil becomes a playfield of the winds. Soil biota dies. A sudden rain causes enormous damage.
- Changing winds: areas previously sheltered, become exposed.
- Human-induced factors
- Change of land (deforestation): the land loses its cover, then its soil biota, porosity and moisture.
- Intensive farming: the plough, excessive fertilizer and irrigation damage the land, often permanently.
- Housing development: soil is bared; massive earthworks to landscape the subdivision; soil is on the loose.
- Road construction: roads are cut; massive earthworks, leaving scars behind. Not enough attention paid to rainwater flow and maintenance of road sides.
Deforestation, grassland cultivation and overgrazing are the main causes of human induced soil erosion. Soil erosion results in water loss, induced frequent flood and drought, soil loss induced by aggravated ecocatastrophes, and soil nutrient loss induced by soil leanness, salinization and the eutrophication of surface water. Cattle and sheep industries contribute to large levels of soil erosion. Cattle and sheep compact the soil and remove vegetation.
The eroded 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.
Erosion by water
- Rainfall intensity and runoff
The impact of raindrops on the soil surface can break down soil aggregates and disperse the aggregate material. Lighter aggregate materials such as very fine sand, silt, clay and organic matter can be easily removed by the raindrop splash and runoff water. Greater raindrop energy or runoff amounts might be required to move the larger sand and gravel particles.
High-intensity thunderstorms cause the maximum soil erosion. Long-lasting and less-intense storms may not cause as much erosion as thunderstorm, but the amount of soil loss can be significant over time. When excess water on a slope cannot be absorbed into the soil it results in run off which increases if infiltration is reduced due to soil compaction, crusting or freezing.
- Soil erosion factors
Generally, soils with faster infiltration rates, higher levels of organic matter and improved soil structure have a greater resistance to erosion. Sand, sandy loam and loam textured soils tend to erode less than silt, very fine sand, and certain clay textured soils.
Tillage and cropping practices which lower soil organic matter levels, cause poor soil structure, and result in soil erosion. Decreased infiltration and increased runoff can be a result of compacted subsurface soil layers. A decrease in infiltration can also be caused by a formation of a soil crust, which tends to "seal" the surface. On some sites, a soil crust might decrease the amount of soil loss from sheet or rain splash erosion. However, a corresponding increase in the amount of runoff water can contribute to greater rill erosion problems.
- Slope gradient and length
Soil erosion by water also goes up as the slope length increases due to the greater accumulation of runoff. Consolidation of small fields into larger ones often results in longer slope lengths with increased erosion potential, due to increased velocity of water which permits a greater degree of scouring (carrying capacity for sediment).
Very little or no vegetative cover of plants and/or crop residues increases the chances of soil erosion. Plants and residue cover protect the soil from the impact of raindrop, tend to slow down the movement of surface runoff and allow excess surface water to infiltrate.
Erosion by wind
The rate and magnitude of soil erosion by wind is controlled by the following factors:
- Erodibility of soil
Very fine particles can be suspended by the wind and then transported to great distances. Fine and medium size particles can be lifted and deposited, while coarse particles can be blown along the surface (commonly known as the saltation effect). The abrasion that results can reduce soil particle size and further increase soil erodibility.
- Soil surface roughness
Soil surfaces that are not rough or ridged offer little resistance to the wind. However, over time, ridges can be filled in and the roughness broken down by abrasion to produce a smoother surface susceptible to the wind.
The speed and duration of the wind have direct relationship on soil erosion. Soil moisture levels can be very low at the surface of excessively drained soils or during periods of drought, thus releasing the particles for transport by wind. This effect also occurs in freeze drying of the surface during winter months.
- Unsheltered distance
The lack of windbreaks (trees, shrubs, residue, etc.) allows the wind to put soil particles into motion for greater distances, thus increasing the abrasion and soil erosion. Knolls are usually exposed and suffer the most.
- Vegetative cover
The lack of permanent vegetation cover in certain locations has resulted in extensive erosion by wind. Loose, dry, bare soil is the most susceptible. However, crops that produce low levels of residue also may not provide enough resistance. Crops that produce a lot of residue also may not protect the soil in severe cases.
The most effective vegetative cover for protection should include an adequate network of living windbreaks combined with good tillage, residue management, and crop selection.
- Soil Degradation
Soil degradation takes place due to farming practices. The soil loses organic matter during ploughing and changes its composition. Valuable soil organisms are lost. With irrigation soil can accumulate salts (salinization) and eventually become unproductive.
Gravity and compaction
Gravity is the force that pushes both land and water downhill. Steeper the soil, the more it is pushed downhill and the faster the water runs.
Because of the enormous variability in field data, soil losses are difficult to quantify. Flat land is very stable (losing 2-5 times natural replenishment) but soil losses increase rapidly with land sloping 2-5%. Land with a 10% slope has 8 times higher erosion, which makes it impossible to farm by ploughing, but perennial crops may be sustainable. At 15% slope, soil erosion doubles. But slopes over 20% are less affected, probably because they are higher uphill, less prone to receive the water from a field higher up, and the run from hillcrest to valley floor is shorter. Their fields are shorter too.
As can be expected, loss in productivity follows the erosion curve, reducing flat land by 18% in a millennium and climbing to 100% for slopes of 10%. Fortunately, the amount of steep cropland is much less than flat cropland, but nevertheless in sufficient quantity. Overall, however, it emerges that soil slope has a considerable effect on erosion.
Effects of erosion
- Soil erosion removes valuable top soil, resulting in lower yields and higher costs of production.
- The sub-soils left after the topsoil has been removed are often unable to support agriculture because advanced erosion causes rills and gullies that make cultivation paddocks unworkable.
Downstream effects include:
- damage to roads and railway lines
- siltation of watercourses and water storages
- overall reduction in water quality of creeks, rivers and coastal areas.
Eroded soil (which can contain nutrients, fertilizers or pesticides) may not directly travel to the streams—it can be deposited where there is a reduction in the slope of the land, in sediment traps, along contour banks, or in grassed waterways, dams, or wetlands.
Heavier soil particles will be the first to be deposited, while finer colloidal clay particles may remain in suspension. Soil removed by gully erosion, especially finer colloidal clay, may be transported directly to a creek or river.
What can I do?
Certain conservation measures can reduce soil erosion by both water and wind. Tillage and cropping practices, as well a land management practices, directly affect the overall soil erosion problems on a farm. When crop rotations or changing tillage practices are not enough to control erosion on a field, a combination of approaches or more extreme measures might be necessary. For example, contour plowing, strip cropping, or terracing may be considered.
Soil erosion can be stopped in so many ways. Depending on the environment one of these methods can be used or all the methods can be used in combination:
- Plant more trees.
- Avoid deforestation
- Avoid soil, air pollutants in destruction of plant habitat
- Use mulch
- Avoid felling of trees. Stop using products in your daily life which will encourage erosion.
- Use recycled paper, instead of regular paper.
- Avoid overgrazing by farm animals
- Encourage Soil coverage to protect soil erosion by winds
- Preserve temperate evergreen forests
- Annual soil loss in South Africa is estimated at 300 - 400 million tonnes, nearly three tonnes for each hectare of land.
- Replacing the soil nutrients carried out to sea by our rivers each year, with fertilizer, would cost R1000 million.
- For every tonne of maize, wheat, sugar or other agricultural crop produced, South Africa loses an average of 20 tonnes of soil.
- The FAO (Food and Agriculture Organization, a branch of United Nations) estimates that the global loss of productive land through erosion is 5-7 million ha/year.