if one hectare of land is left without green cover , the amount of fertile top soil taken away by the wind and water every year is
Answers
Soil is a complex mixture
Soil covers most of the land surface of the earth in a thin layer, ranging from a few centimetres to several metres deep. It is composed of rock and mineral particles of many sizes mixed with water, air, and living things, both plant and animal, and their remains.
Soil teems with life
All soil is full of life, and good soils are teeming with it. Plants and animals help keep the soil fertile. Plant roots tunnel through the soil and break it up, and decaying plants form humus. Burrowing animals mix the soil; the excrete of animals contribute nutrients and improve soil structure.
Only a fraction of land is arable
Man's chief interest in soil is for agriculture, but not all soils are suitable for farming. 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 .
Europe, Central America, and North America have the highest proportion of soils suitable for farming, although a number of the more developed countries seem intent on paving over much of their best farmland with roads and buildings. The lowest proportions of arable soils are in North and Central Asia, South America, and Australia. The single most serious drawback to farming additional land is lack of water.
Erosion destroyed civilizations
Civilizations began where farming was most productive. When farm productivity declined, usually as a result of soil mismanagement, civilizations also declined - and occasionally vanished entirely.
The worst threat is erosion
The most serious form of soil degradation is from accelerated erosion. Erosion is the washing or blowing away of surface soil, sometimes down to bedrock. While some erosion takes place without the influence of man, the soil is lost so slowly that it is usually replaced through natural processes of decay and regeneration. Soil loss and soil creation of new soil stay in balance.
Bad farming encourages soil loss
Unfortunately, many bad farming and forestry operations encourage erosion. 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. While there are isolated instances of deserts being reclaimed by irrigation or of new forests being planted, man, in the majority of instances, degrades the soil when he begins agricultural operations.
The mechanics of soil erosion are fairly well understood today by conservationists and by many farmers. Erosion from water proceeds in three steps: (1) soil particles are loosened by the bomb-like impact of raindrops or the scouring action of runoff water; (2 ) the detached particles are moved down the slopes by flowing water; and (3) the soil particles are deposited at new locations, either on top of other soil at the bottom of the slope or in ponds or waterways.
Rainfall energy varies
Still another factor in soil erosion from water is the erosivity of the rain its intensity and duration. In many parts of Europe, where rains are relatively gentle, erosion is rarely severe. In most tropical countries and in parts of the United States, however, rains are much more intense and occasionally torrential. Much more rain falls per hour, and as rainfall intensity increases, the size of individual raindrops also increases. A tropical raindrop strikes unprotected soil with more force than raindrops in Europe, dislodging more soil. The flow of water down a slope is also greater, and the net result is more soil eroded and moved downhill.
Why some soils erode easily
Another factor in water erosion is the character of the soil itself. Some soils tend to erode easily from the action of rain and runoff; others are remarkably resistant, even in heavy downpours. The susceptibility of different kinds of soils to erosion under cultivation varies widely. Perhaps the most important factor is the relative ability of the soil to absorb rainfall rapidly. Certain soils of the tropics absorb rainfall so rapidly that there is little erosion, even on steep slopes.
Organic soils soak up water
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.