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INTRODUCTION

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/hectare/year.

In North, Central, and South America, loss rates average 10-20 tons/hectare/year

Losses are highest in Asia, averaging 30 tons/hectare/year

Note: A hectare is a unit of land measure equal to 100 ares or 10,000 square meters; equivalent to 2.471 acres.

The processes of erosion are fairly straightforward, but being able to counteract the effects of excessive erosion is immensely more difficult.  With this in mind, it becomes important to work towards erosion prevention/reduction before erosion becomes a problem.

In short, erosion is the movement of soil particles by wind and water. Agricultural runoff contains pollutants including nutrients, sediment, animal wastes, salts, and pesticides.

The amount of erosion that occurs is dependent on two factors:  size of soil particle (it is easier to move a grain of sand than a boulder) and speed of erosive force (wind/water). 

Animal waste, including manure and urinary waste can enter streams directly when cattle wade in and around the water. Since we have no control on the size of soil particle, this factor is measured and accepted as a characteristic of the area.  With this soil size data, effort can be focused on controlling the speed of the erosive force. 
The presence of smaller soil particles (silt) dictates the need to further slow the erosive force.  One of the easiest ways of doing this is by minimizing the occurrence of bare ground.  In general, the presence of plants will cause both water and wind to slow, thus reducing erosion. Stream crossings provide a controlled crossing or watering access point for livestock.

 In some cases, however, just the presence of live vegetation is not enough.  In these instances, cement barriers may be essential to prevent excessive erosion.

WHAT ARE SOME CONSEQUENCES
OF EXCESSIVE EROSION?

Top soil (because it is on top!) is first to erode, and with the top soil goes most of the organic matter and nutrients.

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!

soil loss in the US

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

US erosion of farmland 1997 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.

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 productivity for corn.

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