© 2007 Donald G. McGahan (aka soilman) All Rights Reserved
These pages are meant to be a terse outline for College Level introductory survey courses. The target is upper division (advanced) coursework.
Wait, what is soil?
Soil is a naturally occurring covering on the surface of the Earth, capable of supporting life. Soil is not bedrock. A soil body often contains rocks, but is not typically directly considered part of the geosphere.
Soil also contains air and is connected to the above ground air, but soil itself not always accounted for and not considered atmosphere in a classical sense.
Soil contains varying amounts of water, but soil is not classically considered to be hydrosphere.
One distinguishing property of soil is that it often contains within its body many organisms and supports plant life. Much of the action in the overall cycle of life happens within, or in direct contact with soil. Still, soil is not classically considered biosphere.
The Formal Definition of Soil
The Soil Science Society of America (SSSA) changed the definition of soil to:
“The layer(s) of generally loose mineral and/or organic material that are affected by physical, chemical, and/or biological processes at or near the planetary surface and usually hold liquids, gases, and biota and support plants.” (H. van Es, 2017).
While the above definition is a succinct, it is one that requires expansion and clarification for popular science. The SSSA further offers the following explanations and guidance of the definition:
The proposed definition is inclusive of diverse soil environments, including urban, hydric, desert, and even extraterrestrial soils, and combines important concepts from the previous definitions: (i) the composition of soil and (ii) the processes that affect(ed) it.
We use the term “layer(s)” to include both the natural layered soil and non-layered soil, like disturbed soil.
We use the term “generally loose” instead of “unconsolidated” to allow for a wider range of particle arrangements.
The expression “are affected by” defines soil as being actively impacted by “physical, chemical, and/or biological processes,” which is more direct and less theoretical than the model of soil forming processes.
The phrasing “usually hold liquids, gases, and biota and support plants” explicitly brings in the multi-phase dimension of soil but allows for flexibility. It stresses the importance of these attributes, but it does not make them required features; e.g., soils in extreme deserts do not require plants, and life is not essential for a soil on Mars.
sphere of It’s Own
It is often presented that there are four spheres of earth: geosphere, hydrosphere, biosphere, and atmosphere. Adsorbing the content offered in the soil reader will transform the learners knowledge to recognizing soil as a natural body and as a sphere in its own right: pedosphere. This pedosphere is unity with the geosphere, hydrosphere, biosphere, and atmosphere.
Further, the pedosphere is a thermodynamic and biogeochemical “hot-spot”, a true dynamo, when considering it in a comparison amongst the other four “sphere's”.
A Thumbnail for Soil
Soil is a natural body and as such should be called soil only when it is in its natural setting and is interacting with its natural setting. Soil interacts with the atmosphere, hydrosphere, biosphere, and geosphere.
Take a part of the soil away from the body of soil and that part is no longer soil. This should not be a complicated concept. For instance, if we remove a part of the human body we would not use the term human, alone, for that removed part. However, we might preface the name of the part with human, i.e. human ear, to help us reference what organism the part originated from. Dig up soil and put it in a vessel or vehicle and it becomes not-soil. Dirt maybe. Unconsolidated material certainly.
Rock is not soil. Unconsolidated material is not necessarily soil. Though it is not hard rock, fill dirt, road base, volcanic ash, fresh loess, or fluvial depositions are not soil. However, once these unconsolidated materials become altered by processes acting in concert with a set of environmental factors that we will call the soil forming factors these materials may become soil. A pivotal turning point in the transition between not-soil to soil is the alteration of the material by biological organisms.
Therefore, a fresh blanket of ash, or blankets of sediments deposited from flooding events, are not soil when deposited, but once plants colonize these materials and begin to significantly alter the physical and chemical properties the ash or sediment deposition becomes soil.