Soil is composed of rock that has been weathered by atmospheric forces of mild acid, water and wind until it is broken down into fine grains. Fertile soil is soil that is infused with decomposed and decomposing plant, insect, fish, animal and other life forms, creating a layer which will support plant growth and other visible and microscopic life forms. Fertile soil is friable or loose enough to allow seeds to gestate and burrowing animals, worms, beetles and other underground life to move easily through it.
Another component of fertile soil is the root structures of trees and plants that serve to hold it together. While not in the soil, the canopy that is produced by plants and trees protects the soil from harsh sun and from erosion by heavy rains.
Then there is the riparian structure of waterways, aquifers, rain levels and running water in the form of rivulets, creeks, streams, ponds, tidal zones, brackish zones, lakes and rivers. As the water either feeds roots from underneath or flows and falls from above, water is a major component of the fertile soil layer.
In general summary, the forces that weather rock, layers of weathered rock, living and decomposing life forms, botanical roots and canopies and water all play a role in the structure of soil.
Underneath the fertile layer, there may be sand, bedrock, oil fields, porous rock layers, underground caves, magma that comes closer to the surface than normal, or basalt, among other structures, caves underground aquifers and waterways and rock formations.
Soil that lies under the larger bodies of water, such as lakes, oceans and seas is called the Benthic zone, and can consist of sand, silt, decomposed life and other components that have settled into the bottom and layered out. The chemical composition of Benthic zone soil is affected by the depth, the light level and the chemicals and dead life material of the water, especially when the water is saline, brackish or mixed salt and fresh water, or fresh water.
In shallower areas, the benthic zone can support plant and coral formations and vast arrays of other life. As the light lowers through the twilight and midnight zones, plant life goes away and a desert like soil composition results, with lots of life, but life that can live under increasing pressure, and that does not requires plants or a lot of oxygen.
Oil spills are a component of benthic zone soil composition, whether they occur naturally or as with the BP Oil Spill. Natural oil seepage and release contributes a lot of additional chemicals and substances to the soil when they occur.
The movement of the water will determine the ever changing composition of the benthic zone soil. A rapidly flowing river will primarily have larger rocks, with smaller grains being constantly washed away or collecting in protected areas.
Intertidal coastal and tidal zones will have soil that is constantly bashed and moved around, creating a finer mix of weathered rock, decomposing aquatic and land life, living creatures that go with the waves and tides and the broken down shells of crustaceans or bones of skeletal animals.
The soil under dairies, animal farms and plantings will have nutrients, fertilizers, decomposing animal and plant material, animal waste and the bacteria, viruses, parasites, insects fungi and molds that go along with animals and plants that attract animals and insects.
The soil under agricultural industrial operations will usually be compressed to the point that it cannot contain plant life. The weight of buildings, tarmac, paved roads, animal hooves, vehicles and other things put plenty of pressure that collapses the spaces between grains of soil. When massive and repetitive weight bears down on gravel, the gravel erodes the soil, creating room for erosion. Then, the industrial, toxic, polluting and other wastes are introduced to the soil in ways that either create a superfund cleanup problem or that cause the soil to wash away, sending the pollution and wastes into the waterways.
Soil that has has water diverted or that has been denuded from timber harvesting, construction or road building, or overgrazed or re-planted without the proper crop rotations will eventually be unable to sustain plant life. When there is no plant life, heavy rains can directly impact and erode the soil, washing away the most fertile soil first, then taking heaver and heaver layers away with the wind until only sterile sand is left. This is the general process of desertification.
Soil that is being ground under glaciers is going through a unique process, as is soil that is being blown up in nuclear tests or in the heat of conventional warfare. The soil under the NASA space shuttle launch platform is undergoing very unique processes. The soil that lies under US Interstate 80 or the German Autobahn have to be different from other soils. The soil under Hoover dam must be unique. The soil under the Empire state building can’t be normal, either.
Over the next millions of years, there will be soil that will be infused with the ejecta and ash from volcanic eruptions. There will be extraterrestrial objects and the effects of their impact. Soil will be melted back into magma, then cooled and re-exposed to atmospheric weathering. There will wind and water and rain and waves and acids to continue the march of rock weathering and soil creation, just as the movement of the tectonic plates and exposures of magma to the atmosphere will create new rocks.
As a result, depending on the type of soil, there are many factors that go into the formation, deformation, maintenance, desertification or the return of soil to its molten state and return to the process of becoming rock all over again.
