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2.3 Soil Forming Processes (Factors Influencing Soil Formation (Time: Past…
2.3 Soil Forming Processes
Soil
A natural body of
mineral
and
organic
constituents differentiated into horizons of variable depths
Soil is only considered mature when
changes are negligible over time
Soil Descriptions
Soil Structure
The way that soil grains are grouped together into larger masses called peds, which are stacked around each other to form the soil's structure
Small Peds: Crumbly, granular structure
Larger Peds: Angular, blocky structure
Organic Soil Horizons
O Horizon
[organic topsoil]
Organic horizon formed from the accumulation of plant and animal organic matter
Consists of
upper Oi horizon
which contains decomposing organic matter like leaves or twigs
The
lower Oa horizon
contains humus which is broken down material
Mineral Soil Horizons
A Horizon
[topsoil]
Uppermost mineral horizon that is rich in organic material, consisting of plant roots and humus translocated from the O horizon and sand grains (minerals)
Darker in colour and thin if the E horizon is well developed
A zone of
eluviation
E Horizon
[transition zone]
Lighter in colour than upper or lower horizons
Lacks organic content and composed of sand grains or coarse silt as clay particles and oxides of aluminium and iron are eluviated from here by downward seeping water
B Horizon
[subsoil]
Receives the clay particles, Fe and Al oxides washed down from the A and E horizons
Dense and tough as natural spaces are filled up by clays and oxides
A zone of
illuviation
C Horizon
[weathered bedrock]
NOT
a part of the soil
Consists of parent minerals of the soil and is the regolith
Below it lies the unweathered parent rocks known as
R Horizon
Soil Texture
Determined by the dominant particle size of the mineral grains, as soil is made up of minerals like
clay (<0.002mm), silts (0.005mm) , sand (1-2mm), irons and aluminium compounds
Determines the ability of soil to retain water;
porosity
and
permeability
Coarse-textured sandy soils have many wide passages that quickly allows water to flow to deeper layers
If the soil contains fine particles, passages and spaces are much smaller so water travels slowly and water retention is higher
Soil Colour
Indicates mineral and organic composition of soil
Due to pedogenic processes or inherited from parent rock
Darker: More humus content (humid)
Lighter: Less humus (arid)
White/Deep Orange Red: Clay
Grey: Silt
Black: Abundance of humus
Soil Formation
R → C → O → A → E → B
Weathering acts on the bedrock but much is left unweathered
R horizon
Mineral matter is produced when weaker materials are weathered and clay content ↑, allowing for the
C horizon
to be formed
Organic matter is decomposed which aids rock weathering and forms top
O horizon
Greater production of organic and mineral matter as decomposition and rock weathering continues, allowing the
A horizon
to be formed
Most of the rock materials are weathered leaving only the resistant ones. Production of organic and mineral matter is the greatest here. Soil forming processes are active as water drains through the profile easily; transporting soluble ions and colloids from A downwards, forming the
B horizon
At this stage, the soil is fully developed with distinctive horizons and is able to support vegetation growth
Factors Influencing Soil Formation
Time: Past Climates
Soil formation is very slow, requiring thousands of years to develop a mature soil horizon
Tropical soils have deep well developed profiles as they have been developing over long time spans in a warm environment of abundant soil moisture, uninterrupted for mil of years as there's no glacial periods
Arid tropics in higher latitudes were too cold from glaciation so soil forming ceased
Parent Material
Refers to rock and mineral materials in which the soils form
Has greatest influence in the
early stages of soil development
or in
dry climates
Different parent materials produces different profile development and soils
Acid igneous rocks (granite, rhyolite) produces sandy soils
Basic igneous rocks (basalt) produces fine-textured clay soils
Topography
: Local
Modifies the development of soil
Steep gradients inhibit the development of soils due to active slope erosion
Most water flows off as surface runoff resulting in the continued removal of surface sediments
Moderate to gentle slopes have better drainage which enhances pedogenic processes like eluviation and illuviation , in turn developing the soil horizons
Distinct horizons are formed in these soil profiles
Climate
:globe_with_meridians:
Directly
affects soil formation by supplying water and heat to react with the parent and associated material
Indirectly
determines the flora and fauna activities which produce a source of organic matter to soils
Weathering rates ↑ with higher temperatures
When ppt > evpt, soil moisture levels would be high which promotes the weathering of bedrock and laterization
Only determines broad characteristics, unlike meso or local which determines specific characteristics
Regolith Characteristics
Affects soil composition where CW = more clay
Thickness of regolith encourages infiltration
Developed in Af and Aw due to hot and wet conditions for rock weathering and soil development over long periods of time
Esp Af: soils are rich in many minerals esp Fe & Al oxides
Sub-Processes
Translocation
Process where materials are moved up or down the soil profile with water movement due to a difference in water gradient
↑ when evapotranspiration > rainfall
↓ when evapotranspiration < rainfall
Consists of 2 processes that occur simultaneously
Eluviation
🡳
Downward transport of fine particles like clay and dissolved substances from upper horizons when water drains through the soil
Illuviation
🡱
When evpt > rainfall, capillary action draws water up the soil , along with minerals and dissolved substances causing an accumulation of these materials in the B and C horizons
Key Processes
Salinisation
:
EVPT > PPT :arrow_up: :hot_pepper:
Process that involves the upward movement of saline groundwater resulting in deposition of salts at or near the topsoil surface
Common in drier
BSh & BWh
climates with little soil moisture and high evpt due to high temps (does not occur in Aw as evpt cannot > ppt even during dry seasons)
Evpt is always greater than ppt
High evaporation rates where capillary action draws a continual flow of groundwater ↑ through illuviation, along with minerals and dissolved soluble salts like compounds of Na & K within the soil profile.
Salts precipitate and accumulate/deposit as a
distinctive salic (salt rich) horizon
, forms salt crusts and highly alkaline soils due to high concentration of Na salts
Calcification
: :arrow_up: :arrow_down: :hot_pepper:
Process where Ca ions are precipitated as white or pale coloured grains of CaCo3 in the B horizon
Soils contain a substantial proportion of calcium carbonate from disintegration of limestone during carbonation processes and weathering
Common in low moisture
Aw, Bsh and Bwh
climates
In Aw climates:
Wet
seasons: rainfall dissolves CaCo3 in upper layers and translocated down to B horizon where water penetration reaches its limits and Ca ions are precipitated
Dry
seasons: these dissolved salts are drawn upwards from groundwater by capillary action through illuviation as evpt rates > ppt and precipitated at B horizon to form a hard layer of pale coloured grains called
caliche
Laterization
:
PPT > EVPT :arrow_down: :evergreen_tree:
Forms
laterites / lateritic soils
where soils are deficient of soluble salts but enriched with insoluble Fe and Al2O3
Rocks are intensively weathered due to high temp & rainfall thus large presence of soluble minerals like Si,CaCo3, insol Fe & Al oxides
Movement of large amounts of water into the soil causes the soluble ions to move down and out of the soils, where some are deposited at B horizon due to ineffective leaching
Insoluble salts left behind to form laterite in A horizon(does not eluviate/get carried ↓ by water)
Common in
humid
areas of high soil moisture
:thunder_cloud_and_rain:
In Af
: heavy leaching where almost all of the pdts of weathering are leached out of the soil and lost, leading to development of horizons that are depleted in base cations but enriched in Si and oxides of Al/Fe, forming
soft laterite
that is less close to surface than in Aw
:partly_sunny_rain:
In Aw
: Occurs during the wetter seasons where mineral salts in the topsoil (A horizon) dissolves in rainwater (
transformation
) and percolates down to subsoil (
translocation
), where some are deposited at B horizon due to ineffective leaching. When exposed to alternate wetting and drying due to rainfall seasonality, oxides accumulated at the top recrystallize & harden as an impermeable layer of
hard laterite
(pans/duricrusts)
that limits infiltration, due to a thinner soil layer in Aw
Laterite gives soil a
red colour
for both climates due to strong presence of Fe and Al oxides that remain in the soil profile
involves sub processes
include diagrams to show movement of water/salts across horizons