14 DAY TRIAL // 1. Soil is the fragile and loosened material covering most of the ground surface and supporting life. At an overall human population of 6 billion, 2.5 hectares of ground are corresponded to each person.
But 70 % of the ground surface is of no use for humans: barren, frozen or snow covered areas, steep mountains, sand and stone deserts, or too thin soils. Of the remaining surface, two thirds is made of grasslands. Thus only 10 % of the Earth's surface is suitable for agriculture. The average cultivable surface per human was 0.3 hectares in 1987. By 2000, this was of 0.24 hectares. But 0.5-0.7 hectares per human would be necessary so that each person will eat the same amount of food as a westerner.
2. Soil is a mix of mineral (coming from the grinding of the mother rock found below) and organic (rotten remains of plants and animals) matters, to which living organisms are added: invertebrates (worms), bacteria and fungi, all involved in breaking down organic and mineral matters. The organic matter acts like a binding element for the ground mineral particles.
About 40 % of the soil is made of air and water. The soil thickness varies between several cm to several meters, but the upper layer, the most productive, is no thicker than 25 cm (10 in).
3. Many plants prefer specific soils: wheat flourish in fertile clay soils, the barley in light sandy soils, while potato grows best in deep humus soils, rich in organic matters. Farmers can modify the soil composition through organic or chemical fertilization.
4. Soil quality is also influenced by the slope: on a steep slope, soil is easily washed out once the vegetation is removed, and with this the roots fixing the soil's grains. Agricultural machines are hard to use in such terrains.
The best soil is flat, deep, fertile and located in places protected from natural disasters. One way to achieve this is the digging of a series of levels called terraces. The terrace slows down erosion of the soil, but this is a costly method.
High quality soils are usually used for cereal crops, as they bring more profit. When soil quality decreases too much, plant cultivation is not economical, and the terrain is turned into pasture or forest.
5. Soils form in hundreds of years, but can be destroyed in several years, if incorrectly managed. The intensive use of pesticides decreases the ability of bacteria and other soil microbes to decompose the organic matter, delivering nutrients for the plants. Nuclear accidents, like Cernobyl, contaminate the soil, too. Industrial wastes contaminate the soil as well, introducing toxic metals in it, which kill the bacteria. Aluminum is a major component of the clay mineral, where it is encountered in a non-toxic form. But acid rains release it from the clay minerals, turning it toxic for the microbes and plants. The acid rain itself can contain chemicals coming from industries.
6. Human-induced erosion of the soil is caused by incorrect management. Natural erosion is slow, and the soil can recover. The period of natural erosion and removal of a square meter of soil is about 30,000 years. But the incorrect use of the soil can degrade it in a few decades. The first American colonists exploited so badly the soil, that in 100 years 20 % of the cultivable surface was severely damaged. In some developing countries, like Columbia, Lesotho, Malawi or Swaziland, misusage destroyed 75 % of the agricultural surface.
Soil erosion occurs in terrains intensively cultivated or grazed. Soil exhaustion appears when plants are cultivated annually but fertilizers or nutrients are not re-introduced into the soil. This decreases soil's fertility and crop production. Soil cohesion is also diminished, decreasing its resistance to erosion. Too many animals on pastures consume vegetation in a rhythm that overcomes vegetation's natural ability to regenerate, leaving barren soil behind. Goats are considered soil exterminators, as they eat all types of plants, and even the subterranean roots, leaving the place devoid of any plant trace.
Erosion has two stages: first, the soil clods break down, then the small particles are removed by flowing water or wind. In areas with abundant rainfall, rain droplets hit the soil with considerable force. Normally, plants absorb most of this power, but, when vegetation misses, the rain droplets hit the uncovered soil. Dust particles are raised into the air up to 1.5 m (5 ft). The force of the rain droplets hardens the soil forming a compact crust on its surface. The crust impedes the infiltration of the rain water into the soil and basal rock, that's why most of the water flows on the ground's surface. The surface flow carries the soil grains into the creeks and rivers. The resulting sediments fill the river beds, decreasing their depth and causing floods. In the end, huge amounts of fertile soil are deposed on the bottom of the oceans.
Surface erosions can occur due to the diffuse flow of the waters on slopes, washing a thin layer of soil. Initially, this erosion is not visible. But in time, productivity decreases, as soils turn less fertile. Deep erosion takes place in inclined terrains, when the surface flows gather in currents that, following a powerful rainfall, turn into a torrent that cuts deep grooves into the soil.
In dry areas, wind is the main erosion factor. Unfixed soil is gathered by the wind in backs and dunes. Particles transported by the wind are further broken, resulting a fine powder risen into the air and carried at great distances. This way, a great part of the soil of the US Great Plains ends on the bottom of the Atlantic Ocean. The area is called the Dust Bowl.
In the '50s, Sahel, the southern fringe of Sahara, experienced a period of abundant rains. People from the densely inhabited southern savannas migrated into the Sahel. Their animals devastated the pastures, while people cut the trees for firewood. In the '70-'80s, long droughts hit the Sahel, and the vegetation could not recover. Strong winds carried out the dry soil, turning the area into a barren desert. People and animals experienced famine.