SOIL DISTRIBUTION
Scales of Classification
Soils vary from place to place over the earth's surface. As we suggested in the last class, the factors affecting soil at a place are:
parent material (often the bedrock)
climate (mainly the water budget - which may change with time)
vegetation (type and amount)
topography (influencing thickness, moisture, temperature etc)
time
Given all these variables, it would be surprising if any two places had the same soils. And indeed, soils do differ markedly from place to place.
Our concern is to portray soil distribution as realistically as possible. Obviously we must work on various scales to do this properly. We shall deal with 3 scales
- the global scale, mainly using the text
- the state scale (as part of the final class in this section)
- the local scale, using the video.
Global Soils
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On the global scale the concept of a Soil Taxonomy has emerged - using basic descriptions of how the soil looks, or using readily measurable properties to classify soils. This is a hierarchical system, with the highest level divisions being the Soil Orders. These are appropriate for classification on the global scale.
Older systems used classifications which emphasized the creation processes - but did not fit very well because the processes have changed with time.
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Soil degradation is a local problem in that not all soils in any area is degraded. However, the global map clearly shows that some areas are much more prone to it than others - and these must be the areas of major concern. |
The State Scale
We shall deal with this is class23. Here we can just say that the local authorities have solved the 'problem' of portraying soils on this scale by grouping local soils together because they commonly occur together - often with the differences being the result of topographic differences. They have labelled them the 'soil systems' of the state.
Local Soils
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The local soils - the soil actually occuring in your yard - are infinitely variable over the earth. The most detailed level of the soil taxonomy is involved. In the USA this is expressed by identifying the soil type through a locational name. Hence we in the Chapel Hill area may have Georgeville, or Applin, or Cecil, soils. These are strictly defined for a type area.
The video emphasizes how and why changes take place within very short distances. Although that refers to a podsol (spodosol) in Britain, much the same could be said about our soils. |
