The rock cycle
WEATHERING
Physical and Chemical
•Physical weathering- Changes in the degree of consolidation with
little or no chemical and mineralogical changes in rocks and
minerals. Frost wedging, salt weathering in arid climates, thermal
expansion, plant and animal disruption.
•Chemical weathering - chemical changes rocks and mineralogical
composition via dissolution, hydration, hydrolysis, acidolysis,
chelation, and oxidation/reduction.
•Dissolution
•minerals most affected - salts, sulfates, carbonates.
•H2O + CO2  H+ + HCO3-  H2CO3
•CaCO3 is a congruent reaction - entire mineral is weathered and
results in completely soluble products
Hydration and hydrolysis:
Hydration - incorporation of water molecules into minerals which results
in structural and chemical change.
CaSO4 + 2 H2O  CaSO4 . 2H20
anhydrite
gypsum
Gypsum is relatively soluble and can undergo dissolution whereas
anhydrite is less soluble.
Hydrolysis - incorporation of H+ or OH- in to mineral.
KALSi3O8 + H+  HALSi3O8 + K+ (incongruent)
Feldspar
FeOOH + 3H+  Fe3+ + 2H2O (congruent)
goethite
Pathways of water near the land surface.
Acidolysis:
Similar to reaction of hydrolysis where H+ is used to weather
minerals, however, H+ supply not form water but organic and
inorganic acids.
Humic and fulvic acids, carbonic, nitric, sulfuric and low molecular
weight organic acids such as oxalic acid.
Chelation:
Organic acids can also cause weathering by chelation. A chelator is
a ligand capable of forming multiple bonds with metal ions such as
fe, Al, Ca resulting in ring-type structures with the metal
incorporated. Large complex acids in soils can strip metals from
minerals. EDTA (ethylene diamine tetraacetic acid) - common
artificial chelator used in labs.
Oxidation and reduction:
Oxidation and reduction reactions weather minerals by the transfer of
electrons. Minerals containing elements that can have multiple
valence states such as Fe, Mn, S are susceptible.
Fe3+ + 2H2O  FeOOH + 3H+
Goethite
Mg-olivine (Fosterite)
•Mg2SiO4 + 4CO2 + 4H2O  2Mg++ + 4HCO3 + H4SiO4
•
2Mg++ + 4HCO3-  2MgCO3 + 2CO3- + H2O
HCO3- is a good indication of weathering.
Mg precipitates as magnesite, thus 4 moles of CO2 are taken
from air, 2 return when Mg precipitates. So, for every mole of
fosterite--2moles of CO2 get fixed into carbonate.
Minerals in soils are divided into primary and secondary
minerals.
Primary minerals, which occur in igneous rocks, metamorphic ,
and sedimentary rocks, are inherited by soil from the parent
material.
Secondary minerals form in soils and include layer-silicate
clays, amorphous (non-crystalline) minerals, carbonates,
phosphates, sulfide, sulfates, oxides, hydroxides, and
oxyhydroxides.
Primary minerals are typically larger than secondary - high
surface area of secondary minerals make them extremely
reactive in soils.
Basic tetrahedral unit –
Si ion shares charge
equally with four
oxygen ions.
Second most common
building block of
phyllosilicates is the Al
octahedral polyhedron.
Kaolinite
Gibbsite
Feldspar
•Na-Feldspar (Albite)  alkaline solution + kaolinite
2NaAlSi3O8 + 2CO2 + 11H2O  Al2Si2O5(OH)4 + 2Na+ + 2HCO3- +
4H4SiO4
•Ca-Feldspar (Anorthite)  kaolinite, 1 mole Ca, 2 moles HCO3-, but no
H4SiO4
•Ca2+ +2HCO3-  CaCO3 +CO2 +H2O, 1 mole of CO2 returned to
atmosphere
•In addition to carbonic acid, other organic acids -citric acid from plant
roots,
•phenolics (tannins) - decomposition,
•fungi - oxalic acid
•fulvics and humics weathering
•Also chelation (e.g., Fe & Al) combine with fulvic
acids -- can percolate to lower profile.
•Fe and Al are relatively insoluble, found as crystalline
and hydrous oxides.
•Hydrous oxide
•Crystal oxide
Fe--hematite
Fe-- goethite
Al-- gibbsite
Al--boehmite
•Oxides are common in tropical soils where high
temperatures and decomposition leave little humics to
chelate.
•Can use ratio of Si to Al as indicative of weathering
•kaolinite - 1:1 ratio - more weathered
montmorillonite - 2:1 ratio
•some 2:1 clay minerals hold H2O and NH4 in crystal
lattice. Can represent 10% of total N.
•Bauxite - Residual weathering mineral composed of more
than 50% of Al, Fe, and Ti oxides and hydroxides.
•The primary Al ore occurs in 2 varieties
1. Lateritic-occurs with Al-Si rocks: granite, basalt.
(most widespread)
2. Karst-occurs with carbonate rocks: limestone,
dolomites
Structure of a
1:1 (kaolinite)
and a 2:1
(montmorillonite
) layer-silicate
clay mineral.
•PhosphorusLimited in supply to plants, apatite contains P.
Ca5(PO4)3 OH + 4H2CO3--  5Ca++ + 3HPO4-- + 4CO3- +H2O
•Initially non-occluded then taken up and bound
by Al & Fe oxides-- “occluded”
•Animal P
1. Hydroxyapatite (Bones)
2. Flouroapatite (Teeth)
Cation Exchange Capacity
•Silicate clay minerals in temperate soils have a net
negative charge.
1. Mg++ substitute for Al+++ in montmorilloniteunsatisfied negative charge in crystalline
lattice
2. OH- radicals along edge of clay particles.
Depending on pH, H+ can be more or less
reactive with the radical
Organic matter phenolic (-OH) and
organic acid (-COOH) radicals
Diagram illustration the development of
positively and negatively charged sites on
surfaces of soil constituents, at low and high
pH.
•CEC = Total negative charge (meq/100g soil)
•Assuming equal molar concentrations, cations are held in
sequence and displace one another:
Al+++>H+>Ca++>Mg++>K+>NH+>Na+
Ca2+ forms bases of Ca(OH)2
Si:Al ratio - 2:1 indicates greater exchange capacity
than 1:1 ratio
•Tropical soil has no exchange capacity except via organic
matter.
•Tropical soils have strong CEC - thus, can resist acid rain
•However, acid rain in Northeast U.S. can dissolve gibbsite
Al2O3-leading to Al+++ which is toxic to organisms.
Anion Adsorption Capacity
Soils dominated by oxides and hydrous oxides
of Fe and Al have variable charge depending
upon pH.
a. low pH adsorbs H+ from solution, results in
positive charge - mineral AlOH+
b. high pH adsorption H+ dissociation
AlOH  H+
c. pH>9 - additional H+ dissociates - surface
becomes negative - AlO-  H+
PO43- >SO4-->Cl->NO3There is a low availability of P in soils
Soils with poorly developed crystalline forms of
Fe and Al oxides have greater anion adsorption
capacity (AAC).
Tropical soils all controlled by organic matter.
SOILS
SOIL LAYERS
A.Forest Floor
L-Layer or Oi--undecomposed litter
F-Layer or Oe--fungi and bacteria
(Fermentative)
H-Layer or Oa--humus amorphous
organic matter,
increase in % mineral
In tropics decomposition occurs rapidly so there is
little time for development of soil structure (L-Layer,
F-Layer, H-Layer are all the same)
O-Horizon -- all organic; A Horizon--alluvial
processes (removal). Organic matter and minerals chelation with organic acids - Fe and Al percolate
down from forest floor - referred to as podzolization,
not common in tropics because the decomposition is
too complete.
E-Horizon-some minerals remain, podzolization is less
intense; B-Horizon--illuvial (deposition); C-Horizon -lowest soil layer least weathered and most similar to
parent rock material
Grassland soils (i.e., tallgrass prairie at base of Rocky
Mountains).
•Classified as Mollisols - high organic content and
base saturation
•In forests precipitation exceeds
evapotransportation, however, not typically in
grasslands. Thus, less H2O, slower decomposition,
high pH and Ca - same as in East African
grasslands.
•Clays complexed with organic acids
•In great plains- leaching is so limited CaCO3
precipitates and accumulates in calcic horizons.
Spodosols - intense podzolization (common in
temperate northeast U.S soils)
Alfisols - low podzolization
Utisols - southeast yellowish, deep-reddish color
in B horizon
Soils in Tropics are many meters thick and have
endured millions of years without disturbance
Deserts
•Entisols - recent with little profile
development
•Chemical weathering proceeds slowly
•Soils in southwest U.S. deposited by alluvial
transport from adjacent Mountain Ranges
CaCO3 - horizons called caliche
NaCl
can contain illuvial clays - eolian dust
Typical soil
horizon
sequence for a
Spodosol
developed under
a coniferous
forest (left) and
a Mollisol
developed under
grasses and
herbaceous
plants.
Soil Organic Matter and Humic Substances
The term soil organic matter (SOM) is generally used
to represent the organic constituents in the soil,
including undecomposed plant and animal tissues, their
partial decomposition products, and the soil biomass.
Thus, this term includes:
1.identifiable, high-molecular-weight organic
materials such as polysaccharides and proteins,
2.simpler substances such as sugars, amino acids,
and other small molecules,
3.humic substances.
Soil is a complex, multicomponent system of interacting
materials, and the properties of soil result from the net
effect of all these interactions.
One of the major problems in the field of humic substances
is the lack of precise definitions for unambiguously
specifying the various fractions.
The term humus is used by some soil scientists
synonymously with soil organic matter, that is to denote all
organic material in the soil, including humic substances.
SOM consists of humic and nonhumic substances.
Nonhumic substances are all those materials that can
be placed in one of the categories of discrete
compounds such as sugars, amino acids, fats, etc.
Humic substances are the other, unidentifiable
components. Even this apparently simple distinction,
however, is not as clear cut as it might appear.
Humic acids - the fraction of humic substances that is not
soluble in water under acidic conditions(pH < 2) but is
soluble at higher pH values. Humic acids are the major
extractable component of soil humic substances. They are
dark brown to black in color.
Fulvic acids - the fraction of humic substances that is
soluble in water under all pH conditions. They
remains in solution after removal of humic acid by
acidification. Fulvic acids are light yellow to yellow-brown
in color.
Humin - the fraction of humic substances that is not
soluble in water at any pH value. Humins are black in
color.
“Classical
view” of the
formation of
kerogens.
Turnover of litter and soil organic fractions in a
grassland soil.
Humic acid
Linking of structural units according to
Kleinhempel.
Pathway 1 – Formation of Humic Substances
Pathway 1 – Lignin Theory - Wakman’s Theory
Lignin utilized by microbes, and residuum material
becomes part of soil humus. Modification of lignin
includes loss of methoxyl (OCH3) groups with
generation of o-hydroxyphenols and oxidation of
aliphatic side chains to form –COOH groups.
Pathways 2 and 3 – Formation of Humic Substances
Pathways 2 and 3:
Pathway 3 - Phenolic aldehydes and acids are
release from lignin during microbial attack and
undergo enzymatic conversion to quinones. These
quinones polymerize in the presence or absence of
amino acids to form humic-like compounds.
Pathway 2- same except that polyphenols are
synthesized by microorganisms from non-lignin
sources (i.e., cellulose). Quinone-lignin theory now
currently accepted by most.
Formation of brown-colored substances by
reactions involving quinones is not rare – well
documented in melanine formation in fruits.
quinone - any member of a class of cyclic
organic compounds containing two
carbonyl groups, > C O, either adjacent or
separated by a vinylene group, -CH CH-, in
a six-membered unsaturated ring. In a few
quinones, the carbonyl groups are located
in different rings. The term quinone also
denotes the specific compound para- (p)benzoquinone (C6H4O2). The quinone
structure
Pathway 4 – Humic Substance Formation
Pathway 4 – Sugar-Amine Formation
Reducing sugars and amino acids, formed as byproducts of microbial meatbolism and then undergo
non-enzymatic polymerization to form brown
nitrogenous polymers.
Sugar- amine condensation involves addition of amine
group of the sugar to form the n-substituted
glycosamine.
Major objection to this theory is that the reaction
rates is very slow in typical soil temperatures.
Attractive feature is that the reactants are produced in
abundance in soils.
Biopolymer degradation and abiotic
condensation for humic substance formation.
Carbohydrates in Soils
Carbohydrates constitute 5 to 25% of the
organic matter in most soils.
Plant remains contribute carbohydrates
in the form of simple sugars,
hemicellulose, and cellulose, but these are
more or less decomposed by bacteria, and
fungi, which in turn synthesize
polysaccharides and other carbohydrates
of their own.
The significance of carbohydrates in soil arises
largely from the ability of complex polysaccharides
to bind inorganic soil particles into stable
aggregates. Carbohydrates also form complexes
with metal ions, and they serve as building blocks
for humus synthesis. Some sugars may stimulate
seed germination and root elongation. Other soil
properties affected by polysaccharides include
cation exchange capacity (attributed to COOH
groups of uronic acids), anion retention
(occurrence of NH2 groups), and biological activity
(energy source for microorganisms).
The major groups of carbohydrates
They can be divided into 3 subclasses:
•1. Monosaccharides, which are aldehyde and
ketone derivatives.
•2.Oligosaccharides, a large group of
polymeric carbohydrates consisting of a
relatively few monosaccharide units.
Monosaccharides
Oligosaccharide
3. Polysaccharides- contain many
monomeric units (8 or more)
•The carbohydrates material in soil
occurs as:
1. free sugars in the soil solution
2. complex polysaccharides
3. polymeric molecules of various sizes
and shapes which are strongly attached to
clay and/or humic colloids.
Polysaccharide
Soil lipids
The class of organic compounds designated as
lipids represents a convenient analytical group
rather than a specific type of compound.
They represent a diverse group of materials ranging
from relatively simple compounds such as fatty
acids to more complex substances such as the
sterols, terpenes, polynuclear hydrocarbons,
chlorophylls, fats, waxes, and resins. The bulk of
the soil lipids occurs as the so-called fats, waxes,
resins.
In normal aerobic soil the lipids probably exist
largely as remnants of plant and microbial tissues.
From 2 to 6% of soil humus occurs as fats, waxes,
resins.
Lipids are physiologically active. Some compounds
have a negative effect on plant growth whereas others
act as growth hormones. Waxes and similar materials
may be responsible for the water repellent condition of
certain sands.
Lipid
Amino acids
Amino acids exist in soil in the following
different forms:
1. As amino acids, peptides or proteins bound to
clay minerals
• on external surfaces
• on internal surfaces
2. As amino acids, peptides or proteins bound to
humic colloids
• H-bonding and van der Waals' forces
• In covalent linkage as quinone-amino acid
complexes
3. As mucoproteins
4. As a muramic acid
Amino acids, being readily decomposed by
microorganisms, have only an ephemeral
existence in soil. Thus, the amounts present in the soil
solution at any one time represent a balance between
synthesis and destruction by microorganisms.
The free amino acids content of the soil is strongly
influenced by weather conditions, moisture status of
the soil, type of plant and stage of growth, additions of
organic residues, and cultural conditions.