Rebecca S. Franklin
SWES 541 Final
1. Ferrolysis is the destruction of clays by disintegration and by solution in water by both the
oxidation and reduction of iron, occurring alternately. Soils undergoing this process become
clay poor, acidic and sandy. The cation exchange capacity of the soil is lowered as its clay
minerals are destroyed.
This process occurs in soil horizons that are alternatively dry and saturated, like in a monsoon
climate. For example, this can happen in a slowly permeable B horizon that has a perched water
table, or aquiclude, in the wet season. During this wet season, or anaerobic stage, the Fe3+ is
reduced to Fe2+ by biota in the soil. This occurs rapidly because of the ready supply of organic
matter that comes from the near surface A horizon. The Fe2+ ions can displace other ions such
as Al3+ and cations from cation exchange sites. These removed ions can then be removed via
solution and precipitation. The saturated conditions and possible perched water table facilitates
this removal.
At the same time as this process is occurring, the organic matter used by the soil organisms is
oxidizing, which forms OH- but more oxidation occurs in the dry or aerobic stage. The Fe 2+
ions that were reduced during the wet season are now oxidized to Fe3+. These ions can combine
with water and O2 to form minerals that give oxidized soil its characteristic red color such as
ferric hydroxide. The net result is the leaching of base cations, the increase of H+ cations and
the destruction of clay minerals.
Al is released into soil solution and the destruction of
phyllosilicate clays and release of silicic acid lead to sandy soils. The resulting soil is acid and
sandy over the aquiclude.
The soil of a profile undergoing ferrolysis would have an A horizon with organic matter,
underneath that you will see an alternately dry and saturate Eg horizon. Right below the E
1
Rebecca S. Franklin
SWES 541 Final
horizon there is an aquiclude which would be a B horizon, the aquiclude higher in the Btg/Bt
region of the B horizon in the wet season and down lower in the BC horizon in the dry season.
2. See attached diagram
3.
Strain, a volume change from parent material to the horizon of interest where positive strain
represents an increase in volume and negative strain a decrease, is effected by organic matter
accumulation in the A horizon. An accumulation of OM thickens the A horizon but pedogenic
equilibrium, the lower part of the A horizon transforming into an E or B horizon, causes the A
horizon to stay at about 20 – 50 cm. (Wang and Folmer 1998). This phasing occurs because the
loss of OM material like leaf decomposition will outpace the addition of any new OM. Soluable
materials such as carbonates are also removed. Lateral and vertical removals also occur through
processes such as leaching and translocation.
The accumulation of OM at the surface also effects primary mineral weathering by increasing the
rate at which the weathering occurs. Release of oxalic acid can “etch” minerals, breaking them
down and increasing the weatherable surface area (Robinson 1994).
Organic matter also
increases the incident of fauna in the surface horizons. The flora and fauna added as OM to
these layers, through bioturbation, by generating chelating compounds, by changing pH level
through CO2 production and increasing the surface area of minerals accelerate the rate of
primary mineral weathering.
Calcium carbonate redistribution is an important factor in horizon differentiation. The increased
weathering of primary minerals allows for more Ca to be taken up to be formed into calcium
2
Rebecca S. Franklin
SWES 541 Final
carbonate concretions. The Ca taken out of solution cannot be as readily translocated up or
down through the soil profile.
4. Dahlgren and Ugolini (1989) wanted to find if in situ formation of immogolite in the Bs
horizon is possible, if immogolite formation was favorable in the upper horizons of the spodosols
in their study site and if this formation of immogolite was under present or past conditions.
Dahlgren and Ugolini found that constant pulses of ash were common the spodosol morphology.
These soils were dominated by fulvic acid weathering and low pH brought on by fulvic acid
disassociation. Chelation was also another podzolization process and caused the end products,
Al and Fe to remain in solution in the form of organo-metal complexes. The organo-metal
complexes precipitate as SRO Al oxyhydroxides as Bhs and Bs horizons. There is then a shift to
scidity being sominated by carbonic acid as the soil atmosphere in these horizons is now about
20 – 30 times greater than that of the free atmosphere.
The H2CO3 that initiated the weathering process in the tephra layer came from the CO2 from the
buried forest floor while the HCO-which mitigated cations released from the weathering came
from carbonic acid weathering. The weathering rates, then, were about five times greater in the
tephra treatment.
5. Sulfur cycling, the cycling between organic and inorganic forms of sulfur added to the soil
system from atmospheric deposition, organic material decomposition, and mineral weathering,
may lead to acidification, in conditions such as the reducing environments of tidal marshes.
Here, sulfur is reading available to the ecosystem from sea water. Sulfuric acid is released as the
sulfur bearing minerals are exposed to oxidizing conditions. The acids create very low pH
3
Rebecca S. Franklin
SWES 541 Final
values and weather the soil minerals.
The jarosite that was formed under the oxidizing
conditions will also weather, leading to more sulfuric acid production.
Alkanization due to sulfur cycling can occur under conditions where there is a heavy base cation
level in the soils brought about by the large transport of nutrients from a productive upland with
a high precipitation regime.
.
REFERENCES
Chadwick, O., Brimhall, G. H, and Hendricks, D., 1990, From a black to a gray box- A mass
balance interpretation of pedogenesis: Geomorphology, v. 3, p. 369-390.
Dahlgren, R.A. and Ugolini, F.C. 1989. Effects of Tephra Addition on Soil Processes in
Spodosols in the Cascade Range, Washington, U.S.A. Geoderma, 45: 331-355
Hong Wang, H and L. R. Follmer, L.R., 1998. A polygenetic model for pedostratigraphic units in
the Chinese Loess Plateau region Quaternary International 51/52: 52.
Robinson D.A., and Williams R.B.G., 1994. Rock Weathering and Landform Evolution. John
Wiley & Sons, Inc., New York.
4