concern		Soil erosion and degradation are so severe world-wide, that it threatens our agricultural base.
degradation		Soil can degrade without actually eroding. It can lose its nutrients and soil biota. It can get damaged by waterlogging and compaction. Erosion is the visible part of degradation, where soil particles are transported down-hill by the forces of gravity, water flow or wind. Erosionand degradation are world-wide problems.
damage		Soil erosion affects farming in detrimental ways. Physical damage is the most visible form of soil loss, and most likely to be remedied.
gravity		Gravity pulls constantly at soil, nudging it down hill, causing soil slips, earth clips, cracks, creep and slumps.

-- home -- soil index -- environment -- issues -- Rev 20001115,20011029,20030307,20041106,20051120,20070718,

This map shows the result of 200 years of farming in the USA. Most of the land has suffered 25-75% loss of its topsoil, and a substantial area has lost even more, to the point of becoming unproductive altogether. From the map one can see that the problem is not experienced as much in the coastal flatlands and areas with moderate rainfall. Areas with high rainfall, severe droughts and steep terrain, are always worst affected. Whereas this map shows the situation after 200 years of farming, the next map shows the sediment delivery as it is today, in 1997.

Although the exact values in ton/ha are missing, the erodible areas are marked in colour, ranging from high to none (most red areas are in excess of 10t/ha). Forested mountainous areas experience no erosion problems, since these are natural ecosystems, but the heavily farmed areas, particularly those with large-scale operations, are most affected. The USA is a good example of the difficulty of controlling erosion, because it applies large-scale intensive agriculture and has been doing so for some 200 years. Already since 1930 this country has had a network of erosion experiment stations (complete with small watersheds)  that have provided a wealth of information on all kinds of soil, climate and topography. Soil formation is roughly an inch (25 mm) in 1000 years or 600 t/ha. 1 ton/ha per year is common for natural ecosystems but under cultivation, soil formation increases somewhat. 'Sustainable' agriculture without ploughing achieves annual soil losses of around 10t/ha for flat land, to 40t/ha for land with 10% slope, which are still remarkably higher than natural replenishment. Yet, due to the use of fertilisers, overall productivity is measured to decline no more than 7% in 100 years! Yet other authors have measured a decline of about 50% in organic carbon, and nitrogen from virgin Great Plains soils (clay loam to silt loam) in 30-40 years of cropping (Haas, Evans and Miles, 1957, USDA).

The USA has about 170 Mha in cropland, 62 Mha potential cropland and 138 Mha unsuitable. Per capita this amounts to 0.68 ha, compared to 0.23 ha per person worldwide (year 2000).
World land degradation is about 7-10 Mha/yr on a total arable area of 1500 Mha. In the history of world farming as much as 2000 Mha may have been rendered unproductive. Erosion alone has destroyed some 430 Mha. Worldwide natural erosion is estimated at 10 Gt/yr but human-induced erosion is more than 2.5 times higher, 26 Gt/yr.
Soil loss in China averages at 40t/ha/yr. New Zealand lost 30 mm in 100 years of farming.
 
 

Soil loss is not the same each year, but happens as the table shows, mainly during exceptional rain storms when the land is most vulnerable. Here a comparison is made between no-till farming of corn, where the soil biota are allowed to compose their own soil structure, and conventional tilling where the soil structure is made by ploughing. In the no-till situation, the soil is capable of absorbing most rain water, whereas ploughed soil is not. Since 1982 conventional tilling in this area was discontinued, when it was returned to meadow production to help control the severe erosion experienced.

Annual precipitation and runoff  for continuous corn watersheds, no-till and conventional tillage, 1979-1988 year precipi tation mm runoff no-till mm runoff conventional mm 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1124 1175 1057 889 1027 909 929 966 841 854 3.81 4.90 0.14 0.00 0.00 2.31 0.01 9.23 0.15 0.03 140.2 312.8 142.2 - - - - - - - Source: W M Edwards: Soil structure: process and management in Lal & Pierce: Soil management for sustainability, 1991.

In the table below, the five most common reasons for soil degradation are given for several areas, and the world. Actual erosion of the soil often follows the initial period of degradation.
 
 

In every continent the main causes of soil degradation differ: Europe suffers most from deforestation although agriculture is a close contender. In Africa, its cause is overwhelmingly overgrazing, whereas in North America it comes with agriculture. The figures for Oceania are dominated by Australia and New Zealand where overgrazing is by far the largest contributor. World-wide these figures average out. Degradation due to fuelwood cutting, is large in the poor continents, while absent in the rich continents, where fossil fuel has taken its place. Here is a description of each cause:

• Deforestation: forest soils contain much organic matter, indeed often more than can be converted by the soil organisms. When a forest is cleared, the trees are burnt, which leads to an immediate loss in organic matter, but above the soil. Some of the organic matter in the soil is burnt too. But in the years following, soil organisms become starved of a carbon source and burn the remaining organic soil content. It all leads to massive emissions of carbondioxide. In the wet tropics, forest soils do not contain much fertility. The tropical rains make farming a nightmare.
• Fuelwood: cutting forest for fuelwood is another form of deforestation. Fuelwood is usually converted to charcoal, which burns cleanly. In the process, all hydrogen and oxygen are removed, so that carbon remains. Humans need enough fuelwood for cooking, to be problematic. In arid regions, even the last tree and shrub is used, leaving the landscape barren.
• Overgrazing: when insufficient amounts of grass litter are left for the soil, the soil organisms die and the soil loses fertility. Sparse cover lets raindrops erode the surface. It is a common practice that leads to desertification.
• Agriculture: most agricultural practices are harmful to the soil.
• Industrialisation: industries can pollute soils, mining operations do.

• natural factors
• heavy rains on weak soil: rain drops loosen soil particles and water transports them down hill.
• vegetation depleted by drought: rain drops are free to hit the soil, causing erosion during rainfall. Winds blow away the fine particles during droughts.
• steep slopes: gravity 'pulls harder': water flows faster; soil creeps, slips or slumps downhill.
• sudden climate change
• rainfall: erosion increases unexpectedly rapidly as rainstorms become more severe.
• drought: water dries up and the soil becomes a playball of winds. Soil biota die. 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 fertiliser 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.

The devastation of Greece had two principal causes: deforestation for the sake of fuel and lumber, and overgrazing and overbrowsing by a multitude of uncontrolled goats, who thrived on seedling trees. Semiarid lands are especially vulnerable to such destruction. What happened to Greece and most of the lands bordering the Mediterranean is now happening in parts of the United States. In 1981 a committee of the Council on Environment Quality concluded that "about 225 million acres of land in the US are undergoing severe desertification - an area roughly the size of the 13 original states."
(Source: David Sheridan: Desertification of the United States. Washington DC. Superintendent of Documents, 1981)

Damage From a period of 33 years (1965-1998) of presidential disaster declarations in the USA (Maps courtesy of Baker: www.bakerprojects.com/fema/), it can be seen that flooding, severe storms and tornados with floods are by far the major causes of disaster. From the previous subchapter, it is now easy to understand why rains can so suddenly become disasters. When water with mud moves, it becomes as destructive as a hurricane, affecting large areas, swelling rivers and causing down-stream damage as well.

This map shows the relative occurrence of disasters in a large continent like North America. Disaster areas appear to be clustered, covering the areas shown in the erosion maps at the beginning of this page. Amazingly, California ('where it never rains') appears to be worst affected. As usual, rainstorms cause most damage in arid areas ('when it rains, it pours'). Areas with intensive farming seem to be more prone to disaster than those without.

The kinds of damage caused by rainstorms and flooding appear as pictures in the newspapers, although some remain invisible:

• loss of land: the top layer of productive land is washed away.
• loss of crop/pasture: crops and pasture are destroyed by either being washed out or by being covered in mud.
• reduced yields: flooded fields may take a long time to recover; fertiliser may have been washed out.
• damage to structures: roads, fences, bridges, trees, houses and more may get damaged, needing costly repair, often when cashflow is at its lowest (in winter).
• down-stream damage: neighbouring fields may receive an unwelcome load of soil and mud. Rivers silt up. River banks erode.
• flooding: floods over cropland destroy crops, kill animals, damage houses. Fields become waterlogged with fine silt.
• coastal marine damage: silt and mud settle on the bottom, killing bottom-living organisms. Dirty water suffocates filterfeeders like sponges, seasquirts and clams. Nutrients from mud and fertilisers cause excessive plankton blooms which turn poisonous, killing fish and upsetting mariculture of mussels, oysters and scallops. Seaweeds die through lack of light.

Because of the enormous variability in field data, soil losses are difficult to quantify. The graph on the right shows how crop land erosion increases with slope. 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 has doubled again. But slopes over 20% appear to be less affected, and the reasons for this could be that 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, the 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 (blue line), but in sufficient quantity to worry about. Although the graphs are rather puzzling, the main message they bring is that soil slope has a considerable, and unintuitively large effect on erosion.

As far as sustainability is concerned, any land steeper than 5% should not be ploughed, but returned to perennial crops like viticulture, horticulture or grassland. Slopes above 10% for trees. If it is accepted that only land sloping less than 5 degrees can be cropped sustainably, then over 70% of arable cropland appears unsustainable.
 

Basic erosion The diagram gives an idea how erosion derives from gravity. In the top diagram, a man is pulling an object over the flat ground. Had there been no pressure of the object pushing on the ground, there would have been no resistance or friction. Friction not only depends on this pressure but also on the properties of the two materials: object and ground. In the case of soil and its particles, this is not relevant, but a stone on a gravel bed would slide less easily than a stone on a wet mud slide. When placed on a slope, the weight of the object develops both a pressure and a slip component (second figure). Since soil erosion is proportional to the downward pull s, but inversely proportional to the friction, thus pressure p, it follows the tangent of the slope, as plotted in the graph with a blue dotted line.  Erosion rapidly takes off as the slope of the land increases. For small angles it is proportional to the slope, but for steeper land it increases rapidly to infinity. Note however, that this formula does not take into account the erosion caused by the flow of water, or that of raindrop impact. Source: Floor Anthoni, 2001.

Note that slips are the most visible aspects of erosion but not the worst. Where scientists measure soil loss by looking at slips only, they miss out on the slow but widespread erosion by the impact of rain drops, discussed in next subchapter.

A hill side in northern New Zealand showing creeping erosion. Large sections of soil are slowly creeping downhill, leaving deep cracks behind. Notice the difference in soil management between the farm on left, who uses fertiliser and the one on the right side of the fence, who doesn't.

A hill side near the one on left, showing land clipping where sheep and cattle tread. Clips are small drops of soil, leaving bare soil behind.

A slip or land slide on a hill side in northern New Zealand. The bare soil left behind is very vulnerable to erosion from raindrop impact and should be fertilised and re-sown, followed by fertiliser maintenance. Seeding alfalfa, which is a leguminous deep-rooting ground cover, resistant to drought, could hasten the soil's recovery.

A form of creep caused by tunnelling and subsequent collapse of the underground tunnel. Such erosion forms creeping gullies that are very hard to contain. Planting trees uphill preventively and inside the gully helps to contain it and to minimise erosion by water. Retirement would not necessarily be a remedy.

The erodible hillside has been retired from grazing and fenced off. Natural vegetation is allowed to re-establish, pioneered by the leguminous and prickly gorse. Gorse is considered a pest because it infests poor farmland and is hard to eradicate. But for hillslopes like these, it brings natural nitrogen fertiliser, while preparing the soil for the native bush (on left).

A hillside preventively planted in poplar trees, widely spaced in order to let light through. Trees anchor the soil, cycle deep nutrients and provide decomposing litter to feed the soil. Fallen branches and stems slow down sheet wash. Leaf litter covers the soil against raindrop damage. Stems and branches can be used in gullies to stem the flow of water.

A hill side is preventively planted in widely spaced poplar trees. These trees bring many advantages and may make a decisive difference in the sustainability of this grassland. The trees are not intended to be harvested, but need occasional maintenance.

To overcome gully erosion and land slides, the gullies have been planted with poplar trees in dense formation. Poplar trees can be planted as tall posts, reaching over cattle and sheep, so the area does not need to be fenced off.

Soil compaction is not easily noticeable but preventive measures can be taken:

• confining animals: particularly in winter, when the grass grows slowly and the soil is also more vulnerable to compaction, much damage is done by grazing animals. It can be reduced by confining them during the wet or winter months to sheds and providing supplementary feeding. The practice of strip grazing confines animals to a small area, the border of which is guarded by an electric fence, moved outward every day. It prevents stock from treading the feed area and it allocates food better.
• making hay: a supply of hay gives a farmer more options to manage averse and unexpected situations like floods and snowfall. It also allows him to feed animals in confinement, thereby reducing field damage.
• reducing stock: by selling stock before winter, the pressure on pasture is reduced. During spring calves and lambs are born to replenish stock levels. Farmers also practice set-stocking, allowing all animals to roam all fields, thus reducing the grazing and trampling pressure on each. Normally, paddocks (fenced grass fields) are rotated to minimise cyclic infection from parasites.
• paving: animals damage some places more than others: where they pass through gates, find water or camp underneath trees. Such places could be paved with cheap materials like limestone and brown rock.

Particularly where grazing animals camp, the ground becomes compacted and waterlogged, a process called pugging. Pugs with standing water in them, are the result. The soil is bared, and exposed to raindrop impact damage.

Where animals tramp on their accustomed patterns of movement, the soil becomes compacted and pugged. Particularly on dairy farms where the cows walk to and from the milking shed twice daily, this can be a problem. Paving sensitive pathways, helps to protect the soil from eroding.