Soil-forming processes and soil taxonomy

In 1964, Supreme Court Justice Potter Stewart, making a ruling about indecent or obscene materials, said something along the lines of pornography being hard to define, but you know it when you see it. Soils are a lot like pornography: you know em when you see em, but everyone has a hard time agreeing on a definition.

Soil taxonomy is intimately tied to the genetic processes that drive soil formation. These processes are:

- Parent Material: what the soil has weathered from plays an important role, both in physical properties of grains (i.e., grain size and sorting) as well as the chemical properties of the subsequent soil.
- Biotic Processes: plant and animal activity can have a strong influence on soils. This can include moving sediment and aerating the soil through the actions of burrowing and tunneling animals (which can act to disrupt soil horizons), or aiding the transmission or removal of material through the action of plant community root networks.
- Topography: the local configuration of the earth-surface has a strong influence on soil formation, controlling the amount of solar energy hitting the surface, influencing drainage patterns for groundwater, and affecting the types of plants and animals that can live there.
- Climate: the long term temperature and precipitation trends in a region strongly influence soil formation, both directly through temperature and precip, as well as indirectly by controlling plant and animal communities.
- Time: soil formation requires time, and the development of soil horizons and soil zones is related to how much time the various soil forming processes have had to act on the soil material.

These soil-forming processes were first described by Hans Jenny, considered to be the father of modern pedology, in a 1941 book entitled The Factors of Soil Formation (the link takes you to a pdf of the 1991 reprint of the classic). Soil science, like much of the natural sciences, was hit pretty hard with the old reductionism, and that influence is evident in Jenny’s book, where he posits that quantification of these processes is the key to understanding soils. We won’t get into it too much here, but obviously, it’s a bit more complicated than all that.

What should be obvious from the list of soil forming processes above is their interrelatedness. Only Time, in its abstract sense, is truly a dependant variable here (and even then, one could make the argument that the time available for exposure and soil formation is related to the rate and magnitude of sedimentation events, which can be controlled by local topography, climate, and parent material). Anyway, the point is that because of these complexly interacting variables, defining discrete soil taxa is a very difficult thing. Often, arbitrary cut-offs between some attribute of a soil (such as base saturation) are employed to divide soil orders.

And because of this difficulty, there are numerous soil classification schemes employed by different people across the world. The Food and Agriculture Organization of the UN has its own soil classification scheme, differentiated from one another by the relative importance of those processes listed above. Below is the world soil map, taken from this site, according to the FAO:


The USDA system, employed in America, has its own classification system, and includes the following 12 soil orders:

- Alfisols: aluminum and iron enriched soils, often associated with hardwood forests
- Andisols: soils formed in volcanic material (ash)
- Aridisols: soils with low productivity, and associated with arid climates
- Entisols: incipient soils with little or no horizonation
- Gelisols: soils associated with permafrost
- Histosols: organic-rich soils
- Inceptisols: weakly developed soils
- Mollisols: organic-rich soils associated with grasslands
- Oxisols: soils contained highly weathered materials
- Spodisols: soils with a highly weathered zone, often associated with pine forests
- Ultisols: heavily-weathered soils, specifically with no calcareous matieral and low base saturation
- Vertisols: soils with large amounts of expanding clay minerals

Broadly, the orders represent the influences of the different soil forming factors of Jenny; operational definitions of these, however, often rely on detailed mineralogical and chemical analyses, association with known biotic communities and climates, and delicate fabric and textural interpretations from soil pits. As such, extending this classification into the rock record can be tricky in many cases (such as interpreting soil slickensides in paleovertisols), and impossible in some (such as being able to interpret moisture content in a paleosol from the Triassic).

Next time, we’ll dive into the PALEOSOL classification schemes!