Soil Quality, as a general concept, can be thought of as the ability of a soil to function, in either natural or managed ecosystems, to sustain plant and animal life, and maintain or enhance air and water quality.
For agricultural ecosystems, we may consider Soil Quality as the ability of a soil to produce safe and nutritious crops in a sustained manner over the long-term, without impairing the resource base or harming the environment.
Notice that this premise seems to be contrary to the currently accepted commercial agricultural model in the developed countries as well as those currently becoming developed countries.
Machinery use is one of the catch 22’s of the modern agricultural model.
Another is the relatively new “package” where seed
is matched to herbicides and sold with discounts to entice farmers to purchase these products.
Soil Quality has the potential for many different interpretations.
Quality is dependent upon factors such as land use, soil management practices, ecosystem and environmental interactions, and the priorities of human societies.
When considering Soil Quality in any specific case, it is necessary to identify the major issues of concern with respect to that soil’s function.
Soil that is great for holding up buildings is not particularly good for raising any crops.
Whatever definition of the term Soil Quality is deemed appropriate for a specific use, it should
relate to the capacity of the soil to function effectively with regard to productivity; environmental quality; and plant, animal, and human health now and in the future.
Since the majority of food and fiber needs of the human population are met by crops grown in managed agricultural ecosystems, the focus of government agencies is on those systems.
However, the basic principles presented should be applicable to soils in other ecosystems, both natural and managed.
Some soil properties can be relatively easy to observe, measure, and monitor over time:
Soil properties used as indicators of soil quality.
Texture and aggregation
Aeration and infiltration
Organic matter content
Acidity - alkalinity (pH)
Soil Respiration (CO2)
Major factors which lead to reductions in soil quality, land degradation, and soil erosion:
Lands that are improperly managed (e.g., improper tillage) lose their topsoil.
Either in large chunks during extreme erosive events, or little by little over an extended period of time, the soil disappears from the land resulting in reduced productivity and a degraded condition.
Results from the accumulation of salts in improperly irrigated soils, most frequently in arid regions.
Occurs on cultivated soils when repeated harvests are made from land without returning organic residues and mineral nutrients to the soil.
Exposure of soil to toxic substances, as a result of industrial processes or chemical spills, can severely damage the ability of a soil to perform its ecosystem function.
Cultural and environmental factors which enhance or degrade soil quality:
Soil Quality Enhancing
organic material additions
cool, humid climate
fibrous root systems of plants
minimal tillage operations
Soil Quality Degrading
hot, arid climate
For plant growth, the topsoil is the richest and most valuable part of the soil.
Topsoil formation is a very slow process (in nature, (we are developing methods that enhance the speed of natural topsoil production)), which makes it a non-renewable (within the current standard thinking), (but re-usable) resource in terms of human lifespans.
Keeping the soil in place while it is used for construction or crops is one of the greatest challenges faced by engineers and land managers.
Unfortunately the current engineering manifest does not take soil condition (other than what is best for their use) into account.
Soil erosion losses are greatest when the soil surface is exposed to intense rainfall, resulting in gulley formation.
Natural soil fertility is largely contained in the remains of formerly living things, also known as organic matter.
Continuous removal of plant material for food or forage leads to gradual depletion of natural soil fertility.
Soil properties can vary greatly from one location to the next, even within distances of a few meters.
These same soil properties can also exhibit similar characteristics over broad regional areas of like climate and vegetation.
The most general level of classification in the USDA system of Soil Taxonomy is the Soil Order.
All of the soils in the world can be assigned to one of 12 orders.
By surveying soil properties of color, texture, and structure; thickness of horizons; parent materials; drainage characteristics; and landscape position, soil scientists have mapped and classified nearly the entire contiguous United States and much of the rest of the world.
Soil Orders and General Descriptions
Entisols Little, if any horizon development
Inceptisols Beginning of horizon development
Aridisols Soils located in arid climates
Mollisols Soft, grassland soils
Alfisols Deciduous forest soils
Spodosols Acidic, coniferous forest soils
Ultisols Extensively weathered soils
Oxisols Extremely weathered, tropical soils
Gelisols Soils containing permafrost
Histosols Soils formed in organic material
Andisols Soil formed in volcanic material
Vertisols Shrinking and swelling clay soils
Descriptions of the 12 soil orders
are a very diverse group of soils with one thing in common, little profile (horizon) development.
Includes the soils of unstable environments, such as floodplains, sand dunes, or those found on steep slopes.
Entisols are commonly found at the site of recently deposited materials (e.g., alluvium), or in parent materials resistant to weathering (e.g. sand).
Entisol soils also occur in areas where a very dry or cold climate limits soil profile development.
Productivity potential of entisols varies widely, from very productive alluvial soils found on floodplains, to low fertility/productivity soils found on steep slopes or in sandy areas.
are dry soils with CaCO3 (lime) accumulations, common in desert regions.
The extent of aridisol occurrence throughout the world is widespread, second in total ice-free land area only to the entisols.
Extensive areas of aridisols occur in the major deserts of the world, as well as in southwestern north america , Australia , and many Middle Eastern locations.
Aridisols are commonly light in color, and low in organic matter content. Lime and salt accumulations are common in the subsurface horizons.
Some Aridisols have an argillic (clay accumulation) B horizon, likely formed during a period with a wetter climate.
Water deficiency is the dominant characteristic of Aridisols with adequate moisture for plant growth present for no more than 90 days at a time.
Crops cannot be grown in these soils without irrigation. Productivity of Aridisols is generally low, and there is potential for land degradation due to overgrazing by livestock.
If irrigation water is available, Aridisols can be made productive through use of fertilizers and proper management.
are found in cool to hot humid areas, and in the semiarid tropics; they are formed mostly under forest vegetation, but also under grass savanna.
Extensive areas of alfisols are found in the Mississippi and Ohio River valleys in the USA, through Central and Northern Europe into Russia, and in the South-central region of South America.
Alfisols generally show extensive profile development, with distinct argillic (clay) accumulations in the subsoil.
Extensive leaching often produces a light-colored E horizon below the topsoil.
Generally fertile and productive, these soils typically have a high concentration of nutrient cations (Ca, Mg, K, and Na) and form in regions with sufficient moisture for plants for at least part of the year.
Natural fertility and productive capacity of alfisols is considered to be greater than that of ultisols, but less than that of mollisols.
are intensely weathered soils of warm and humid climates.
They are typically formed on older geologic locations in parent material that is already extensively weathered.
Ultisols have accumulated clay minerals in the B horizon.
While generally low in natural fertility (basic cations, Ca2+, Mg2+, and K+) and high in soil acidity (H+, Al3+) the clay content of ultisols gives them a nutrient retention capacity greater than that of oxisols, but less than alfisols or mollisols.
Large areas of ultisol are found in the southeastern USA, China, Indonesia, South America, and equatorial regions of Africa.
are soils with permafrost within 2 meters of the surface.
These soils generally have limited profile development.
Most of the soil forming processes in these soils occur near the surface, sometimes resulting in significant accumulation of organic matter.
Large areas of this soil occur in the Northern regions of Russia, Canada, and Alaska.
These areas become boggy wetlands in the summer, and support large numbers of migratory birds and grazing mammals.
The permafrost of gelisols tends to become unstable (melt) if disturbed, leading to a waterlogged soil condition that poses problems for engineering uses.
soils form in volcanic ash
and cinders near or downwind from volcanic activity.
Generally lacking in development, they are not extensively weathered, forming in deposits from geologically recent events.
Usually of high natural fertility, they tend to accumulate organic matter readily and are of a ‘light’ nature (low bulk density) that is easily tilled.
These soils generally have a high productivity potential.
are soils in the beginning stages of soil profile development.
The differences between horizons (layers) are just beginning to appear.
Some color changes may be evident between the emerging horizons, and the beginnings of a B horizon may be seen with the accumulation of small amounts of clay, salts, and organic material.
These soils show more profile development than entisols, but have not developed the horizons or properties that characterize other soil orders.
Inceptisols are commonly found throughout the world, and are prominent in mountainous regions.
The natural productivity of these soils varies widely, and is dependent upon clay and organic matter content, and other edaphic (plant-related) factors.
take their name from the Latin word mollis, meaning soft.
These mineral soils have developed on grasslands, vegetation that has extensive fibrous root systems.
The topsoil of mollisols is characteristically dark and rich with organic matter, giving it a lot of natural fertility.
These soils are typically well saturated with basic cations (Ca2+, Mg2+, Na+, and K+) that are essential plant nutrients.
These characteristics of mollisols place them among the most fertile soils found on Earth. Found in North America from Texas up to Saskatchewan, Canada.
commonly form in sandy parent materials under coniferous forest vegetation.
As a consequence of their coarse texture, they have a high leaching potential.
Spodosols are characterized by high acidity, and have a subsoil accumulation of organic matter, along with aluminum and iron oxides, called a spodic horizon
Typically low in natural fertility (basic cations, Ca2+, Mg2+, and K+) and high in soil acidity (H+, Al3+), these soils require extensive inputs of lime and fertilizers to be agriculturally productive.
Spodosols are most commonly associated with a cool and wet climate, but also occur in warmer climes such as in Florida, USA. Large areas of spodosol are found in northern Europe, Russia, and northeastern North America.
are the most weathered of the 12 soil orders in the USDA soil classification system.
They are composed of the most highly weathered tropical and subtropical soils, and are formed in hot, humid climates that receive a lot of rainfall.
Oxisols are located primarily in equatorial regions.
These soils are extensively leached, and the clay size particles are dominated by oxides of iron and aluminum, which are low in natural fertility (Ca2+, Mg2+, K+) and high in soil acidity (H+, Al3+).
While oxisols are typically physically stable, with low shrink-swell properties and good erosion resistance, these soils require extensive inputs of lime and fertilizers to be agriculturally productive.
are soils without permafrost that are predominately composed of organic materials in various stages of decomposition.
They generally consist of at least half organic materials (by volume).
They are usually saturated with water which creates anaerobic conditions and causes organic matter accumulation at rates faster than that of decomposition.
Little soil profile development is present, due to their saturated and anaerobic condition, however layering of organic materials is common.
Histosols can form in wetland areas of any climate where plants can grow such as bogs, marshes, and swamps, but are most commonly formed in cool climates.
are soils with a high content of clay minerals that shrink and swell as they change water content.
The clay minerals adsorb water and increase in volume (swell) when wet and then shrink as they dry, forming large, deep cracks.
Surface materials fall into these cracks and are incorporated into the lower horizons when the soil becomes wet again.
As this process is repeated, the soil experiences a mixing of surface materials into the subsoil that promotes a more uniform soil profile.
Vertisols are usually very dark in color, with widely variable organic matter content (1 – 6%).
They typically form in Ca and Mg rich materials such as limestone, basalt, or in areas of topographic depressions that collect these elements leached from uplands.
Vertisols are most commonly formed in warm, sub-humid or semi-arid climates, where the natural vegetation is predominantly grass, savanna, open forest, or desert shrub.
Large areas of vertisols are found in Northeastern Africa, India, and Australia, with smaller areas scattered worldwide.
There are color maps available online
to see the Soil Orders by continent, just do a search for "Soil Orders by Continent"
I'll get more of this posted as I have time.