1. Overview of the Underpinnings and Building Blocks of Soil Taxonomy
Soil Forming Processes
Overview of the Underpinnings and Building Blocks of Soil Taxonomy

2. Why is this topic important?
Associating soil morphology with pedogenic process will make you a better pedologist
An appreciation of these scientific underpinnings is required in order to understand rationale for the concepts in Soil Taxonomy

3. Objectives List Simonson’s 4 general processes of soil formation
Understand purpose of the diagnostic horizons in Soil Taxonomy
Know the difference between “diagnostic horizons” and “horizon designations (e.g., A-Bt-Bk-C)”

4. Generalized Theory of Soil Genesis
(Simonson, 1959)
Processes
There are many possible physical, chemical, and biological reactions and interactions that occur in soils. These are categorized as soil-forming processes.
The possible number of process events, their combinations, and the interactions among them is impressive.
Processes of soil formation occur in unique proportions, intensities, and sequences.
The horizons of a soil profile reflect the relative strength of these processes and the degree to which they offset each other.
Parent Material
(Phys, Chem., Biological)
“biogeochemical”
Today
Time ‘0’

5. Additions Removals Transfers Transformations
Physical, chemical and biological reactions and interactions are categorized as soil-forming processes
Additions
Removals
Transfers
Transformations
ASK THE CLASS FOR EXAMPLES OF EACH OF THE 4 PROCESSES
Possible answers:
Additions
- OM accumulation, CaCO3 and sodium salt dust, groundwater recharge of bases, etc.
Removals
- Soil erosion, Leaching of bases and soluble salts, OM oxidation, etc.
Transfers
- Eluviation/illuviation (e.g. clay, AL, Fe, humified OM, silica, CaCO3), base cycling, etc.
Transformations
- Mineral weathering (e.g. muscovite > vermiculite, vermiculite > illite, feldspar > montmorillonite, vitric volcanic glass > cristobalite), Fe+3 > Fe+2, raw OM to humified OM, etc.

6. (Bockheim and Gennadiyev, 2000)
Bockheim’s processes
Simonson’s general processes
Defined
Base cation leaching
removals
eluviation of Ca, Mg, K, and Na from the solum under extreme leaching conditions
Argilluviation
transfers
the movement of clay in the solum
Paludization
additions, transformations
the deep ( >40 cm ) accumulation of organic soil materials
Gleization
transformations
presence of reduced matrix or redoximorphic features such as concentrations, depletions, strong gleying, etc.
Bockheim’s processes Taxa in Soil Taxonomy displaying process
Base cation leaching Spodosols; Dystric great groups of Inceptisols and Vertisols,
Dystric subgroups of Entisols; Ultisols; Ultic subgroups of Alfisols,
Andisols, Entisols, Inceptisols, Mollisols, and Spodosols
Argilluviation Alfisols; Ultisols; Argids suborder and Argic great groups of Aridisols;
Albolls suborder and Argic great groups of Mollisols;
Kandic great groups of Oxisols;
Alfic subgroups of Spodosols; Argiorthels great group of Gelisols
Paludization Histisols; Histels suborder and Histic great groups of Gelisols
Gleization Aquic suborders of most orders (except Aridisols, Gelisols, and Histosols);
Aquic great groups of Aridisols and Gelisols
(Bockheim and Gennadiyev, 2000)

7. Objectives List Simonson’s 4 general processes of soil formation
Understand purpose of the diagnostic horizons in Soil Taxonomy
Know the difference between “diagnostic horizons” and “horizon designations”

8. Where Jenny’s soil forming factors (clorpt) are the same:
(Guy Smith, 1963)
Where Jenny’s soil forming factors (clorpt) are the same:
genetic expression is similar [result of soil forming processes]
the soils are the same due to pedogenesis
Similar soils respond equally to the same management.
Soil genesis is that part of soil science that deals with factors and processes of soil development; the science of evolution of soils. Soil genesis is a discipline that has survived the fragmentation of soil science into soil chemistry, physics, mineralogy, and fertility. It can be thought of as a system of bridges that connect eight islands called chemistry, physics, geology, biology, climatology, geography, anthropology, and agriculture.
We conduct soil surveys to help us understand how best to manage soils and predict behavior from their properties.

9. Measure the morphological expression of soil genesis
(Guy Smith, 1963)
If soils w/ similar genetic expression respond the same, our goal is to:
Have a taxonomy that groups soils with similar genesis
But you can’t observe soil genesis, so what’s the next best thing?
Measure the morphological expression of soil genesis
Soil genesis is like an experiment that has already been conducted. The soil scientist is left to observe the result and interpret the pedogenic processes that formed the soil.
Guy Smith - Genesis does not appear in the definitions of the taxa but lies behind them.

10. Spodosol Diagnostic Horizons Processes Morphology Ochric epipedon
- Addition of organic matter
Ochric
epipedon
- Removal of base cations by leaching
Albic horizon
- Transformations to form amorphous alumino-organic complexes
Spodic horizon
- Transfer of alumino-organic constituents and iron from surface to the subsurface

11. Diagnostic Horizons
Taxonomic standards that reflect our understanding of soil genesis
Definitions are quantitative; objective.
Focus is on properties, not theories of genesis
Any competent soil scientist can classify
Based on measurable & tactile soil properties that are present today.
Not presumed for native state
Identified partly by
their own morphology and
how their properties compare to those in the overlying and underlying horizons
Not all mutually exclusive.
2. Diagnostic horizons were developed by the authors of Soil Taxonomy to solve the problem of the subjective nature of horizon designations. There was then and still is today, lack of agreement on what type or degree of genesis a particular horizon designation implies. The strict definitions of the diagnostic horizons allow soils to be classified based on their properties and not on the assumed genetic processes which formed them.

12. Not all mutually exclusive
Mollic epipedon
Argillic horizon
Not all mutually exclusive
Mollic epipedons and argillic horizons are not mutually exclusive and their properties can occur overlap in the same subhorizon. Ochric epipedons and cambic horizons are also not mutually exclusive.
However, mollic epipedons and cambic horizons are mutually exclusive and cannot overlap in the same subhorizon. Ochric epipedons and argillic horizons are also mutually exclusive.

13. Objectives List Simonson’s 4 general processes of soil formation
Understand purpose of the diagnostic horizons in Soil Taxonomy
Know the difference between “diagnostic horizons” and “horizon designations”

14. Terminology Horizon designations
Opportunity for the describer to express their opinion of soil genesis (A, Ap, Bt, Btk, Bk, Bkqm, C, etc.) based on what they see or infer
Horizon designations are subjective, based on the describer’s experience/understanding of soil-forming factors and processes

15. Spodosol Diagnostic Horizons Processes Morphology
- Addition of organic matter A
Ochric epipedon
- Removal of base cations by leaching E
Albic horizon
Same soil profile shown previously with symbol designations for the genetic horizons added.
The A and E horizons constitute the ochric epipedon.
The E horizon has albic materials and constitutes the albic horizon.
The Bhs and Bs horizons constitute the spodic horizon.
Bhs
Spodic horizon
- Transformations to form amorphous Alumino-organic complexes Bs C
- Transfer of alumino-organic constituents and iron from albic to spodic

16. Terminology Horizon designations
Therefore, DO NOT ASSUME horizon designation implies a diagnostic horizon
Some examples:
Oa ≠ histic epipedon
Bk ≠ calcic horizon
Bt ≠ argillic horizon
Bw ≠ cambic horizon
Bx ≠ fragipan

17. Diagnostic Horizons and Horizon Designations
Definitions are measurable/observable
Only 1 correct answer
Horizon Designations
Definitions less objective
Reflects describers theory of genesis

18. Summary
Horizons are morphological expressions of soil-forming processes
Diagnostic horizons are objectively defined taxonomic entities with strict criteria
Horizon designations are subjectively assigned and reflects the describer’s theory of soil genesis