Soil is the earth’s fragile skin that anchors all life on Earth. It is comprised of countless species that create a dynamic and complex ecosystem and is among the most precious resources to humans. 
Human activities that significantly reduce soil cover (e.g., tillage and clear-cutting) and/or intensify wind or water movement (e.g., the removal of windbreaks and channelization of streams) often result in accelerated erosion that exceeds geologic erosion rates by several orders of magnitude. 
Generating three centimeters of top soil takes 1,000 years. If current rates of degradation continue all of the world’s top soil could be gone within 60 years, a senior UN official said. …. about a third of the world’s soil has already been degraded. 
The total land area of the world exceeds 13 billion hectares, but less than half can be used for agriculture, including grazing. A much smaller fraction – about 1.4 billion hectares – is presently suitable for growing crops. The rest of the land is either too wet or too dry, too shallow or too rocky. Some is toxic or deficient in the nutrients that plants require and some is permanently frozen.
While a growing portion of anthropogenic earth moving involves deliberate engineering, about 70% of the geomorphic activity tallied by Hooke (2000) is the unintentional side effect of land-use practices that accelerate soil erosion.  Large-scale farming erode(s lowland agricultural fields at rates comparable to glaciers and rivers in the most tectonically active mountain belts. Humans are playing a significant role in speeding erosion in low lying areas. These low-altitude areas do not have the same rate of tectonic uplift, so the land is being denuded at an unsustainable rate. 
What keeps soil in a natural state from eroding is vegetation. Undisturbed by man, soil is usually covered by a canopy of shrubs and trees, by dead and decaying leaves or by a thick mat of grass. … Whether the plant cover is disturbed by cultivation, grazing, burning, or bulldozing, once the soil is laid bare to the erosive action of wind and water, the slow rate of natural erosion is greatly accelerated.
Erosion accelerates when sloping land is ploughed and when grass is removed from semi-arid land to begin dryland farming. It accelerates when cattle, sheep and goats are allowed to overgraze and when hillside forests are felled or cut indiscriminately. … Man, in the majority of instances, degrades the soil when he begins agricultural operations. 
The soil washed (or blown away) is usually the most fertile, containing most of the nutrients and organic matter required for normal plant growth.
Organic matter in soil can absorb and store much more water than can inorganic fractions. It acts like a sponge, taking up water and releasing it as required by plants. It also helps bind soil particles into larger aggregates, or crumbs. Soils with this kind of structure are very resistant to erosion. Conversely, nearly all soils containing little or no organic matter are very susceptible to erosion.
Besides absorbing water readily, a good cropland soil should be able to dry out or warm up quickly when the rain is over. It should hold enough moisture to supply the needs of a crop between rains, yet permit water to pass through the soil. A good soil will not stay too wet or too dry. 
Another factor in erosion from water is the crop that is being grown in the soil and the way that crop is being managed. … Many soils can be planted with maize without much erosion risk if the maize crop is rotated with legumes and small grains. If maize is planted year after year, however, soil losses begin to mount.
Damage from water erosion is not limited to the loss of productivity on the land where it occurs. The bulk of eroded soil from a hillside comes to rest a short distance away, at the foot of the slope or on a nearby flood plain, where it may bury crops or lower the fertility of bottomlands. A portion of the eroded soil is deposited in local drainage or irrigation ditches or runs into ponds, reservoirs, or tributary streams and rivers. Wherever it is deposited, it is unwelcome. Sediment-filled ditches have to be dug out again; ponds, lakes, and reservoirs either have to be dredged out or abandoned. Locally, sediment is an expensive nuisance .
Damage also occurs downstream, sometimes at great distances from the farmland that originally contributed the sediment. Carried along by a river, sediment is dropped out as the waterway reaches flatter, lower reaches. The sediment deposits raise the level of the riverbed and reduce the capacity of the channel to hold water. Riverbanks overtop more frequently, and valuable bottomland, often extremely productive, is damaged by flooding.
Windborne topsoil may be transported over very long distances and, like soil eroded by water, it is usually deposited where it is not wanted. Farmlands, fences, machinery, and buildings can be severely damaged by wind erosion, and sometimes they can be buried completely. Costs of rehabilitation can run so high that the land is abandoned.
The physical causes of wind erosion are clearly different from those which allow soil to wash, except for one factor that is constant in all man-made soil erosion – the absence of vegetation to hold and cover the soil. It is when trees, bushes, grasses, and other plants are removed from land that erosion occurs. 
References: 1. https://www.worldwildlife.org/threats/soil-erosion-and-degradation
2. http://www.nature.com/scitable/knowledge/library/prediction-prevention-and-remediation-of-soil-degradation-113130829 (2013)
3. http://www.scientificamerican.com/article/only-60-years-of-farming-left-if-soil-degradation-continues/ (December 5, 2014)
4. http://www.fao.org/docrep/t0389e/t0389e02.htm (1990)
5. http://www.livescience.com/7870-farming-erosion-rivers-glaciers.html (September 02, 2009)