Originally written by Lee Amoroso, September 2000
Engineering/Environmental:all unconsolidated materials above bedrock.
Agricultural: good
or bad for growing crops (water retention, resistance to erosion, nutrient
content)
Some just call it dirt.
Availability of soil data:
County soil surveys (USDA),
geotechnical and environmental reports
Why study soils?
Geologists, geomorphologists, and Quaternary scientists are not simply interested in classification of soil types but also want to understand the processes that took place to create the soil and what record of past climate and geography may have been preserved.
Soils are characterized by horizons - recognizable zones of weathering (accumulation or depletion of material) that are created by physical, chemical, and biological processes. Soil horizons approximately parallel the land surface.
Principal soil horizons
O - uppermost, fresh to partly decomposed organic material
A - mineral soil mixed with decomposed organic material (dark color). From 10 to 150cm in depth. Dominated by weathered mineral matter but has sufficient organic material to
E - zone of maximum leaching of clay, iron, aluminum, and leaving behind residual minerals (lighter than A horizon). Not present everywhere
B - zone of accumulation of clay, and iron/aluminum compounds. Soil structure development (prismatic, granular, blocky). Obliteration of rock structure. More intense colors than E or C horizons. Bt, is a reddish horizon seen in paleosols in the southwestern U.S. Of all the layers, as a zone of accumulation, B tends to preserve the best
C - relatively unweathered bedrock rubble. Saprolite: C
Climate
Organisms
Topography
Time
Soil properties vary with parent material
Soil properties vary with climate
Soil properties vary with time
Soil properties vary with organisms
Laterization - high rainfall and temperature, intense leaching and oxidation, especially but not exclusively tropical regions.
Laterites are a good paleoclimate indicator. Australian bauxites are Paleocene age (57-66 Ma) when the climate was warm and moist.
Calcification
- sub-humid to arid climates. The stage of development (of near-surface
pedogenic carbonates) is an indicator of the age of the soil. This is discussed
more fully later.
Weathering rinds - formed by oxidation of the exposed part of a clast or outcrop
Carbonate rinds - accumulation of CaCO3 on buried clasts
Surface roughness of rocks - usually increases with time
Microscopic etching of minerals - measure depth of etching
Depletion
of chemical species - compare geochemical analysis of weathered vs. un-weathered
materials
Relict soils, buried soils, and exhumed soils
Relict - remained on the surface since formation
Buried - soils buried by subsequent deposition (i.e. no further pedogenesis)
Exhumed
- buried and later uncovered
Used to develop a model to understand the depositional history and soil development of a deposit or series of deposits in a region.
(Vreeken, 1975) identified four kinds of chronosequences:
1. Post-incisive - soils develop on deposits of different ages
2. Pre-incisive - a soil that develops on a deposit are buried by subsequent deposition
3. Time-transgressive without historical overlay - vertical stacking of sediments and buried soils
The buried soils indicate times of non-deposition
4. Time-transgressive with historical overlay - a combination of the first three
This is just a preview of an interesting and detailed subject.
A soil classification packet has been assembled from references that I (Lee) use when describing soils in the field. Also necessary are a hand lens, water, Munsell soil color charts, shovel or spade, and a trowel. A sieve set is useful to estimate the gravel percentage. We will learn how to use these tools in the laboratory session.
Aridisols
and Entisols are the dominant soil orders on the basin floor in the Phoenix
area. Aridisols are mineral soils with at least one diagnostic pedogenic
or soil forming horizon, which have low concentrations of organic matter,
and are dry for more than 6 months of the year. Aridisols are divided into
Orthids that have little textural change with depth and have calcium carbonate
throughout. Many Orthids have calcic horizons Argids are Aridisols
with accumulations of clay that was eluviated or transported from an upper
horizon. Entisols are mineral soils with little or no evidence of development
of soil horizons. Entisols are divided into Fluvents (recently deposited
alluvium in floodplains and stream channels), Orthents (thin soils developed
on steep slopes), and Psamments (sand or sandy loam soils) (Birkeland,
1984 ; Hendricks, 1985).
Carbonate cemented river gravels in Mesa AZ:
Carbonate coatings on clasts:
"to do Caliche is to do dust":
Professor T. L. Pe'we':
See SCIENCE Ghost town reprint
Dorn, R. I., 1994, The role of climatic change in alluvial fan development. In ?Geomorphology of Desert Environments.? (A. D. Abrahams, and A. J. Parsons, Eds.), pp. 593-615. Chapman & Hall, London, U.K.
Gerrard, J., 1992, Soil Geomorphology, Chapman & Hall, London.
Gile, L. H., Hawley, J. W., and Grossman, R. B., 1981, Soils and Geomorphology in the Basin and Range Area of Southern New Mexico - Guidebook to the Desert Project, pp. 222 p. New Mexico Institute of Mining and Technology, Socorro, NM.
Hendricks, D. M., 1985, Arizona Soils, University of Arizona, Tucson, AZ.
Jenny, H., 1941, Factors of Soil Formation, Dover Publications, Inc, Mineola, NY.
Kraus, M. J., and Bown, T. M., 1986, Paleosols and time resolution in alluvial stratigraphy. In ?Paleosols - Their Recognition and Interpretation.? (V. P. Wright, Ed.), pp. 180-201. Princeton University Press, Princeton.
Machette, M. N., 1985, Calcic soils of the southwestern United States. In ?Soils and Quaternary geology of the southwestern United States.? (D. L. Weide, Ed.), pp. 1-22. Geological Society of America, Boulder, CO.
McAuliffe, J. R., 1994, Landscape evolution, soil formation, and ecological patterns and processes in Sonoran Desert bajadas. Ecological Monographs 64, 111-148.
McFadden, L. D., and Tinsley, J. C. (1985), Rate and depth of pedogenic-carbonate accumulation in soils. In ?Soils and Quaternary Geology of the Southwestern United States.? (D. L. Weide, Ed.), pp. 23-41. Geological Society of America, Boulder, CO.
Morrison, R. B., 1978, Quaternary soil stratigraphy - concepts, methods, and problems. In ?York University Symposium on Soils.? (W. C. Mahaney, Ed.), pp. 77-108. Geo Abstracts, Ltd., Norwich, England.
Ollier,
C., and Pain, C., 1996, Regolith, Soils, and Landforms, John Wiley and
Sons, New York.
Péwé, T. L., Péwé, E. A., Péwé, R. H., Journaux, A., and Slatt, R. M.,1981, Desert Dust: Characteristics and rates of deposition in central Arizona. In ?Desert Dust: Origin, Characteristics, and Effects on Man.? (T. L. Pewe, Ed.), pp. 169-190. Geological Society of America, Boulder, CO.
Ruhe, R.V., 1967, Geomorphic surfaces and surficial deposits in southern New Mexico, State Bureau of Mines and Mineral Resources, Socorro, NM, 66 p.
Selby, M. J., 1993, ?Hillslope Materials and Processes.? Oxford University Press, New York.
Watson, A., 1992, Desert Soils. In ?Weathering, Soils, and Paleosols.? (I. P. Martini, and W. Chesworth, Eds.), pp. 225-260. Elsevier, Amsterdam.