Erosion caused by agriculture is the second major cause of land degradation that we will examine. It can result from:
cultivating land that is inappropriate for cultivation (e.g. steep slopes and thin soils)
or using inappropriate techniques -- planting row crops and lack of contouring on sloping land, for example.
Either can result in excessive erosion (erosion above the rate of soil formation). (Soil scientists calculate soil "T-values," or soil loss tolerance levels, which take into account not only the soil replacement rate but also soil depth. More losses can be tolerated on deeper soils, for example.)
See information on erosion rates in Oregon and other states here.
While changed farming practices have led to diminished rates of soil erosion in some areas of the world (as we'll discuss more when we talk about sustainable agricultural practices later), in general, erosion rates on agricultural lands over much of the world have increased in recent decades. For example, farmers are increasingly planting row crops instead of hay and pasture crops, which, particularly on sloping ground, results in more soil erosion because, unless conservation tillage is being practiced, row crops provide less complete cover of the ground, exposing it to more erosive forces (as we saw for rangelands ).
This is another place where good numbers are hard to come by, particularly for lesser developed regions of the world. Erosion rates are sometimes estimated from sediment loads in rivers, but it isn't always clear where all the sediment came from. It is very difficult to estimate erosion losses by measuring soil depth. For example, a field can lost 6 tons of soil ha but that loss will only decrease soil depth by about a mm! (It is easy, incidentally, to lose that much soil by sheet erosion in one storm.) Difficulties in quantifying erosion rates add to the difficulties in perceiving it as a crucial problem: "It nickles and dimes you to death." Nevertheless, some reasonably reliable estimates are available.
Excessive erosion is estimated to affect more than 1/3 of the global total of cropland, outside of the humid regions
In Africa, Europe, and Australia soil loss rates average 5 - 10 tons/ha/yr
In North, Central, and South America, loss rates average 10-20 tons/ha/yr
Losses are highest in Asia, averaging 30 tons/ha/yr
To put these numbers into some perspective, please consider that average rates of natural soil formation are on the order of 0.5 - 1.0 tons/ha/yr! (FEE 2008 6(3)).
(Some data suggest even higher rates of loss; I've tried to use fairly conservative numbers here. For higher estimates, see Pimentel et al. 1995. "Environmental and economic costs of soil erosion and conservation benefits." Science 267: 1117-1123.)
During the 1980's, it was estimated that the world lost 240 billion tons of topsoil in excess of new formation; that is more than half the amount found on all current US croplands combined. (State of the World 1990, pg 60; WW Norton publisher).
When we lose soil, we are losing a resource that is, for practical purposes and human timespans, essentially non-renewable. An inch of soil takes between 200 - 1000 years to form, yet it can be swept away in a few seasons.
In fact, it is estimated that about 1/2 of the fertilizer applied to US farmland is necessary just to replace nutrients that are lost with soil erosion! For China in 1995, it was estimated that 30% of the N and 22% of the K applied simply went to replace nutrients lost with erosion.
Erosion also degrades the soil's structure , diminishes its water holding capacity, and increases its tendency to become compacted (in part through loss of organic matter with the top soil). As a consequence of these changes, runoff of water (and soil!) increases. Increased runoff and decreased water holding capacity in soils results in crops having less moisture available to them, and hence aggravates drought stress. In fact, lack of water storage capacity in eroded systems is often erosion's most damaging effect, in drier regions where water is often the main limiting factor
Much of this eroded sand and soil ends up in rivers and finally in the seas. Sediment loads in rivers can make some forms of life unable to live there, silt up spawning beds used by fish (such as salmon here in the Pacific Northwest), degrade water quality for humans, and cause silting of productive estuaries and reservoirs. Believe it or not, there is a debate in the scientific literature about whether or not soil erosion is good for the global carbon budget! One side claims that when eroded soil is deposited and buried by more sediment, it doesn't decompose as fast as it did when on the surface, so that soil erosion is actually a "sink" for CO2 (we'll talk about sources and sinks for atmospheric CO2 when we discuss golobal climate change later). Another side claims that this isn't so....Neither argue FOR soil erosion, however, believing that its net costs outweigh any possible climate change mitigation possibilities....(Science 22 Feb 08).
Thus, erosion has consequences not only where the soil is lost from but also where it is deposited.
As you can imagine, excessive erosion has consequences for agricultural productivity, resulting largely from lost nutrients and water storage capacity in soils. In some areas, erosion-caused productivity losses have already been demonstrated. For example, badly eroded areas in Illinois and Indiana are estimated to have lost as much as 24% of their inherent initial productivity for corn.
Finally, intact soils DO store a great deal of carbon -- about 45 times more than is stored in all plants -- and so they are very important in terms of the global carbon budget, functioning largely as a net sink for CO2.
In the US, there has been concern about soil erosion losses since the time of George Washington, but these concerns weren't brought to the public attention in any important way until the 1930's "dust bowl." The dust bowl led to the establishment of the Soil Conservation Service in 1935.
(The dust bowl was precipitated by drought and bad land use practices plowing up the prairie. Wendell Berry, "We plowed the prairie and never knew what we were doing because we did not know what we were undoing." There had been droughts before the dust bowl without resulting dust bowls; it took plowing the prairie in company with drought to really do it.)
However, erosion is still a very serious problem in the US. We can theoretically sustain a certain amount of erosion without harming productivity -- when losses are less than the rate of formation. Yet it is estimated that erosion losses exceed the amounts that could be sustained without harming productivity indefinately on more than 1/3 of the major cropland area in the US . (Pimentel et al. in the 1995 Science article referenced above claim that this is true for 90% of US cropland!)
We currently lose more topsoil each year in the US than we did during the dust bowl!
Iowa prairie soils 150 years ago had about 12-16" of topsoil; now they have only about 6-8" of topsoil. That is, they have lost half their topsoil since they began being cultivated, and loss there continues at about 30 tons/ha/yr. This loss is a result of erosion and of the speeding of decomposition that is cause by tillage.
When we discuss sustainable agriculture a bit later, we'll talk about some promising signs in the US re. controls over excessive soil loss
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