Soil that is of sufficient quality to sustain plant life is hard to make since it can take thousands of years for about an inch of potentially potable or fertile soil to be made. An interesting chart of the historical events that happened as one inch of soil was made can be found here.
Much of the Earth’s soil is unusable because it lies under oceans, rivers, lakes and other waterways, is inaccessible or is otherwise set aside as soil that cannot be built upon or lived on, as with protected lands, national preserves and national forests.
The formation of the soil is critical to soil quality
The components of soil include the parent material, or type of rock that was weathered into small enough grains to become soil. There is the weathering process itself, whether by water, wind, man made causes or other causes. There is organic material including living plants and animals, other organisms and microorganisms and decomposing waste or dead material.
Water is a soil component that introduces the moisture and nutrients required to sustain living organisms, the chemicals that affect soil composition and the erosion factors that modify or even wash away the soil.
The location of the soil contributes to its composition. Location determines if more water, less plant life, mineral components and other organic material are present. Location determines whether humans are present to interact with the soil and to modify its composition. There is location at the top, bottom or slope of a hill, which is important to erosion, water drainage and whether people interact with the soil and modify its composition.
Time is important in the formation and development of soil. Over time, soil can be enriched and well formed, can become weathered, will erode or can be broken down until desertification, or removal of all the components that support plant and other life is complete.
Human and animal interaction with the soil will definitely affect quality in shockingly brief periods of time.
Animals, especially heavy ones with hooves, will compress the soil until plants cannot gestate. Large herds that pound across the land or stay fairly stable while they graze can be devastating to the soil of rich grasslands. Over grazing and trampling then becomes the biggest natural and human caused threat to soil quality.
Buildings:
The annual increase in buildings not only renders perfectly fertile and potable soil inaccessible, they compress the soil, are built after critical soil layers are scraped, leveled, compacted by machine or removed and they remain a bulky, soil compressing, difficult to move mass of materials that do not easily, if ever, break down over time.
Chemicals, other substances and water:
Chemicals and substances that are introduced to the soil are a recursive factor. As water is the major transport medium for adding chemical components to large tracts of fertile soil, anything that washes down to change the composition of the water comes back to change the composition of the soil that when he same water is re-applied to it.
When there are underground aquifers, the potential of polluting them affects the quality of well and artesian irrigation processes. Riparian systems involve a waterway, the aquifer and all of the surrounding soil, plants, trees and other life that depends on the waterway, be it an ocean, wetland, river, stream, creek and so on.
The main chemicals that affect water quality are fertilizers, industrial and military waste, human and animal waste, dead organic matter and solids, such as dead plant material and soil that wash down into waterways. Also included in water pollution is the introduction of water that is too cold or too hot for aquatic life to thrive. Temperature pollution comes from dams and industrial waste water.
Denuding of soil through harvesting, water diversions and desertification:
Timber and other over harvesting will denude the soil of plant material that provides not only nutritious organic matter, but root structures that hold the soil together. The plants also provide shade, habitat and cover for organisms that keep the soil friable and with a balance of organic material, as molds, insects, fungi, worms, burrowing animals and other organisms keep the soil from compacting to the point where seeds cannot gestate or roots cannot grow deeply enough to support a decent plant canopy.
Water diversions have caused depletion of all water in places of low annual rainfall and where only the one waterway was the source of moisture. This has led to a severe loss of water for farmlands, croplands, water table dependent woodlands and other parts of riparian systems. With no waterway, the entire system dies.
The plants and trees provide a canopy that protects the lighter elements of the soil from heavier rains that would wash them away. As the top layers wash away, more and more layers of soil elements are exposed and can blow away in the wind until only sterile, heaver grains of soil remain. This is a simple description of the process of desertification.
And it can take a thousand years for another inch of soil to form naturally.
In summary, there are plenty of natural reasons for soil quality to change and there are plenty of anthropogenic reasons for soil quality to be so drastically changed that it would take tens of thousands of years for soil to repair itself through weathering.
The increase in human population obviously leads to needs for more buildings, more plant husbandry and more animal herds for quick production of proteins. Soil protection and protection of the water are two of the most critical needs in the face of such population growth.
http://soil.gsfc.nasa.gov/soilform/parmat.
