SOILS
Introduction
• True soil science is relatively new
– In the US
•
•
•
•
Less than 100 years
Most of the good information is less than 20-25 years old
Major impetus was the dust-bowl days of 1930-33
Originally conservation oriented
– Europeans
• Began worrying about soils in the late 1800’s
• They ran out of land sooner than we did
Definitions
• Geologist’s concept
– Regolith or weathered surface of the Earth
– The junk that hides the good stuff
• Agronomist's view—stuff to grow plants in
• Soil Scientist’s View (a la Mueller + additions)
– Soils are natural bodies which support plant life, with
characteristics resulting from the integrating and
synthesizing effects of living matter and climate acting upon
parent material, as conditioned by relief, over periods of
time, and modified by man.
Factors in Soil Formation
• Climate
– Most important single factor
– Controls both temperature and rainfall
• Those control both mechanical and chemical weathering
• Influence both type and abundance of vegetation
Factors in Soil Formation
• Living Organisms
– Plant and animal matter
– Organic accumulation
– Nutrient cycling
• Plants maintain the type of soil they like
– Plants take up the nutrients they need
– Other stuff is removed
– Needed nutrients are returned when plant dies
• Locally, plants love acid soils
– Acid-making nutrients are held in the biomass
– Alkali elements are leached away
– Soil remains acid
Factors in Soil Formation
• Living organisms (continued)
– Profile mixing
• Ants bring yellow subsoil to the surface
• In Iowa, the upper meter of soil is completely overturned by earthworms every
75-100 years
– Nitrogen fixation
– Roots impart structural stability to soil
Factors in Soil Formation
• Parent Material
– Material that breaks down to form soil
• Often, but not always bedrock
• Locally it is sand that was transported
– Controls initial chemical and mineral composition
• Carbonate valley soils are basic
• Local soils are acidic
• Soils formed on shales are clay-rich, never sandy
– Influences rate and course of the weathering cycle
– Floodplain soils are rich because new parent material and nutrients are
added regularly
Factors in Soil Formation
• Topography
– Controls rate of erosion—always greater on steeper slopes
– Controls moisture accumulation which influences rate of chemical
weathering and vegetation
• Soils thickest in valley bottoms where it is wettest
• Thinner on hilltops
• Thinnest on slopes where there is erosion
– Can produce microclimates—low areas are coolest
– Slope aspect
• North-facing slopes are wetter and cooler and have more vegetation
• South-facing slopes experience faster weathering due to more sunlight
Factors in Soil Formation
• Time
– Older soils have better developed profiles
• At what developmental stage is this soil?
• Has this soil been subjected to a complex history?
– As soils age they become more similar—Parent material becomes less
and less important
• Man modifies soils
– Cultural activity
– Argiculture
Types of Changes
• Translocations
– Leaching—removal of material in solution
– Precipitation—Deposition of material from solution Eluviation's—
removal of material in suspension
– Illuviation—addition of material in suspension
• Transformations
–
–
–
–
Organic matter changing to humus
Weathering processes
Oxidation of carbon
Work of microbes and other organisms
Soil Development
• Surface exchanges
–
–
–
–
Material additions/removals
Energy
Biocycling
Water
• Intrasolum translocations and transformations
• Lateral gains and losses
• Lower solum exchanges
– Weathering adding material from below
– Removal from below
Soil Horizons
• O horizon
– Surface horizon
– Recognizable organic matter
• A horizon
– First mineral horizon
– Humidified organic matter
• E horizon
– Dominated by removals
– Loss of silicate clay, iron
– Resistant minerals appear to be concentrated
– Lighter in color
Soil Hozizons
• B horizons
– Illuvial concentration of silicate clay, iron, aluminum, carbonates, gypsum, and/or
humus
– Residual concentration of oxides or silicate clays
– Often brightly colored
– Bt—silicate clays
– Bk—carbonate
• C—Weathered parent material
• R—Bedrock
Weathering and Soil Formation
Soil Classification
• “New Classification”
– Quantitative—Measure properties such as pH, chemistry, etc.
– Useful for soil science, but not engineering
– Hierarchical
•
•
•
•
•
•
12 World Soil Orders
60 Suborders
303 Great Groups
Subgroups—(~1500 in U.S.)
Families—(~10,000 in U.S.)
Series—(~17,000 in U.S.)
SOILS TABLES
Element
SiO2
Ca
Mg
K
Fe
Al
Organic
Matter
Arid Climate
Cool/Wet Climate
Stays put; not affected
Not removed; appears to
be concentrated
Accumulate as they are Most leached; severely
removed from rocks
depleted
but quickly deposited
as water evaporates
Not weathered out of
Removed from E, but
parent material
precipitated in B; gives B
its color
Not much due to lack
Acid compounds enhance
of water
removal of Fe and Al, but
not quartz
Hot/Humid Climate
Under severe weathering
conditions will be removed
Completely gone
Form iron and aluminum
oxides and hydroxides
Neutral soil; Quartz is most
soluble at
pH = 7, quartz is removed
Soil Order
Formative Element
Description
Entisol
Inceptisol
Recent
Inception
Soils without pedogenic horizons
Soils having weakly differentiated horizons showing
alteration of parent material
Soils of arid or semi-arid climates; dry soils with pedogenic
horizons; low O.M.; accumulation of bases (Ca++, Mg++,
Na+, K+
Soils with nearly black, organic-rich surface; high base
supply
Soils of cold regions; high organic matter; typically
saturated; underlain by permafrost
Solis that have accumulation of amorphous material (al,
O.M., and/or Fe) in subsurface horizon; cold, wet climate
Gray-brown surface horzon; medium-high base supply;
silicate clay accumulation in B
Low base supply; accumulation of silicate clays; typical of
high rainfall and high temperature
Highly weathered; inactive clays (mainly kaolin and
hydrated oxides of Fe and Al); no silicate clay
accumulation
Soils with high content of organic matter; peat or muck
Soils developed on volcanic rocks; dark color from ironrich volcanics, not O.M.
Soils with a high content of shrinking and swelling clays;
no horizon development
Aridisol
Mollisol
Gelisol
Spodosol
Arid
Mollify (mollis,
soft)
Gelid (very cold)
Alfisol
Podzol (spodos,
wood ash)
Pedalfer
Ultisol
Ultimate
Oxisol
Oxide
Histosol
Andisol
Histology (tissue)
Ando (black soil)
Vertisol
Invert