|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.|
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|Reasons for concern
Because soil degradation and erosion happen so slowly, they seldom give rise to immediate action. In fact, it is hardly noticed during one generation of say, 30 years. Soil appears to be here forever. It is nearly impossible to imagine that this unnoticeable rate of loss, is many times that of natural formation. Had artificial fertilisers not been available, soil degradation would have been noticed much earlier, but these miracle cures appear to be able to compensate for declining fertility. Only by looking at the larger picture, does the severity of soil loss become clear.
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.
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.||
no-till and conventional tillage, 1979-1988
Soils can degrade without any loss of soil particles, but always due to farming practices. When ploughing, the soil loses organic matter and changes its composition. Valuable soil organisms are lost. When irrigating soils, they can accumulate salts (salinisation), eventually becoming unproductive. For a complete overview of degradation and erosion, see soil classification/degradation.
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:
|[In earlier days] Attica yielded far more abundant produce.
In comparison of what then was, there are remaining only the bones of the
wasted body; all the richer and softer parts of the soil having fallen
away, and the mere skeleton of the land being left.
But in the primitive state of the country, the mountains were high hills covered with soil, and plains were full of rich earth, and there was abundance of wood in the mountains. Of this last traces still remain, for although some of the mountains now only afford sustenance to bees, not so very long ago there were still to be seen roofs of timber cut from trees growing there, which were of such a size sufficient to cover the largest houses; and there were many other high trees, cultivated by man and bearing abundance of food for cattle.
Moreover, the land reaped the benefit of the annual rainfall, not as now losing the water which flows off the bare earth into the sea, but, having an abundant supply in all places, and receiving it into herself and treasuring it up in the close clay soil, it let off into the hollows the streams which it absorbed from the heights, providing everywhere abundant fountains and rivers, of which there may still be observed sacred memorials in places where fountains once existed; and this proves the truth of what I am saying. (Plato)
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)
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:
|Gravity and compaction
Gravity is the force that pushes both land and water down-hill. Ironically, gravity also keeps soil in its place. The steeper the soil, the more it is pushed down-hill and the faster the water runs. The following graph shows how quickly erosion accelerates.
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.
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.
|Erosion types caused mainly by the force
of gravity are:
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.
An entirely different effect of gravity is compaction. Fields are compacted by heavy machinery or draught animals. Cattle and sheep that are left to range their meadows, trample the ground, particularly where they camp. In wet times when the soil's resistance to compaction is least, farm animals can cause tremendous damage to soils by pugging it. Compaction causes the soil to lose its porosity and its ability to absorb and to drain water, resulting in water logging.
Soil compaction is not easily noticeable but preventive measures can be taken:
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