Topic 3: Soil Formation Processes

Definition of soil

Soil is a loose surface material that has distinctive chemical, physical and biological

qualities allowing it to be used to support life and plant growth. It is a complex mixture

of mineral matter, organic matter and living organisms.

Man’s perception of meaning of soil differs with regard to the use of soil.

i. Soil to a mining engineer/miner: it is an overburden that has to be removed in

order to access valuable minerals.

ii. Soil to a road construction engineer: it is a loose surface material forming the

road surface surface/bed.

iii. Soil to a farmer/forester: it is a material supporting plant growth.

iv. Soil to a geologist: it is a loose material at a stage before the formation of the

rock unit. As such soil must die, buried, cemented, and compacted to form

rocks.

v. Soil to a pedologist: It is a natural body and must be studied in terms of its

origin, property types and utilization.

vi. Soil to a landscaper: it is a resource for beautifying parks and gardens/loans.

Components of soil

All four parts of soil are essential to plant development, and each is necessary for

plants to survive. The composition of the components inside soil varies to create

different soil types.

a. Minerals (free phase): naturally occurring chemical elements or compounds that

possess a crystalline structure and are the constituents of rocks e.g. silicon,
 aluminium, iron, calcium, sodium, potassium, and magnesium.

b. Organic matter (free phase): is the material that forms from living matter.

Decayed plants and animals provide the organic materials found in soil. Through

decomposition, organic materials are broken down and turned into nutrients that

plants can use. Mineralization also occurs through decomposition, and through

this process organic materials become inorganic.

c. Water (liquid phase): water fills much of the spaces between minerals particles.

Water exists in solution form and is important for most chemical reactions in the

soil.

d. Air (gaseous phase): air also fills spaces among mineral particles, organic matter

and water.

Stages of soil development

Stage 1: Parent material

This is the stage at which the loose surface material is not capable of supporting plant

growth. It is a stage where the weathered products are not yet translated into soil.

These are called recolith, geolith or saprorite.

Examples are beach sand, volcanic deposits, landslide deposits.

In the soil profile, these materials present the C-horizon and are just above the bed rock.

Stage 2: Young soil

This is the top part of the recolith called regosol which has been converted to soils and

can now support simple plant growth. It has organic matter.

Regosols form the A-horizon and is transformation stage of C to A horizons.

The soil is called solum.

Stage 3: Mature stage

All horizons are present in this stage. A-horizon is rich in organic matter. B-horizon is

rich in clay particles. The accumulation of clay in C-horizon is because clay is very small

and can easily be moved in suspension during to the lower levels.

At stage 3, the soil is fully fertile and can support full plant growth.

Stage 4: Old soil (Planosol, paeosol or geosol)

This is a well-developed soil profile with several horizons. From this stage, soil can die

by burying itself and by building infrastructure above it.

A Hypothetical soil profile

The soil profile is defined as a vertical section of the soil from the ground surface

downwards to where the soil meets the underlying rock. It can be exposed by a soil pit.

A00: has leaves and organic debris.

A0: contains partly decomposed leaves

A1: is dark in color due to decomposition of organic matter.

A2: is light in color as it is zone of leaching of colloids/clay particles. It is called zone

of eluviation (leaching)

A3: is a transitional zone where soil is changing from A to B but is more like A

B1: is a transitional zone from A to B but is more like B.

B2: is a deep colored zone because of accumulation of colloids/clay particles. It is

called zone of illuviation (deposition of leached materials from A2).

B3: is a transitional zone to C but is more like B.

C1: is a transition zone to C but is more like C.

C2: is saprolite which is weathered parent material.

D: is the unweathered parent material/bedrock.

Main features of the HSP

a. It assumes that precipitation is adequate.

b. It assumes that nutrients are moved down the profile.

c. It assumes that the soil is deep enough to allow for horizontal differentiation.

For class discussion

What could be the criticisms of the HSP? – Refer to the main features above…

Soil formation (Pedogenesis)

Two main processes are involved for a soil to be formed:

a. Rock weathering process

b. Pedogenic process

Weathering

The process of erosion which results in breakdown of the rocks to smaller particles and

finally its constituent minerals resulting in the formation of mineral soil.

Types and agents of weathering

There are basically two types of rock weathering (physical and chemical weathering),

each with its own agents. We can as well have biological weathering.

1. Physical Rock Weathering Processes

This is the physical disintegration of rocks into smaller particles without considering

changing the chemical changes in the rocks minerals. These are divided into two:

thermal and mechanical.

a. Thermal Rock Weathering

 This takes place due to contraction and expansion of rock minerals or rocks

themselves or the enclosed gases.

i. Differential expansion and contraction of minerals in rocks

Rocks contain different rock minerals which expand and contact differently.

Because of this minerals will acquire different volumes. This will create stress in

the rock hence it will disintegrate.

ii. Differential expansion and contraction of a large rock

Most of the rocks are bad conductors of heat. When temperature rises, only the

outer layers expand therefore they peel off, a process known as exfoliation.

iii. Expansion of trapped gases in rocks

Most rocks contain spaces in which gases are trapped. Contraction and

expansion of gases of the interstitial gases (trapped gases) will create stress in

the rock and cause it to disintegrate. When temperatures are very high, the rocks

can explode.

iv. Freeze thaw

v. Colloidal swelling

vi. Pressure release/dilation/exfoliation or unloading.

b. Mechanical Rock Weathering

This takes place due to the influence of external factors which makes the rock to

break into parental material.

Agents of Mechanical Weathering

i. Hoofed animals e.g. cattle, buffaloes. These will crush the surface of the

rock through trampling.

ii. Human activities through cultivation using heavy machinery, road works

and quarrying.

iii. Mineral or crystal growth: Some rocks can have cracks containing water

with dissolved nutrients/salts. If water is evaporated. Minerals will still

form and this will cause rocks to fragment.

iv. The growth of ice in cracks of rocks can as well disintegrate rocks.

v. Root penetration into pre-existing cracks of rocks.

2. Chemical Rock Weathering Processes

The original rock is changed through the reactions of minerals and other substances to

produce loose material. Chemical weathering takes place through the following

processes:

a) Hydration

Involves the additional of water to minerals to produce hydrous compounds which are
weak or less resistant to weathering hence the rock will disintegrate. For example, when
haematite (iron oxide) acquires water, it changes to geotite (hydrate).

b) Hydrolysis

Involves the reaction of minerals with hydrogen and hydroxyl ions to produce new
compounds which are less resistant to weathering. For example, when mixed with water,
olivine produces silica acid.

It is this acid which weakens the rock.

c) Oxidation

Involves the reaction of minerals with dissolved oxygen. Mineral rich in iron
sulphide undergo oxidation.

d) Reduction

Involves the removal of oxygen or addition of hydrogen from or to a mineral.

e) Carbonation

Involves the reaction of dissolved carbon dioxide with minerals.

f) Dissolution or solution dissolved

Involves the dissolving of some minerals when they are in contact with water.

3. Biological weathering
This has similar effects to those of physical actions and chemical processes.
a. Tree roots growing in cracks or joints of rocks and exert outward pressure. This
is same as in freeze thaw.
b. Organic acids: biological activities enhance chemical weathering.
  Plants and animals live and die and decay on the surface relief
environment and create their own chemical reactions.
 Plants roots extract nutrients from soil and replace them with hydrogen
ions that increase acidity and basicity.

Factors influencing the rate of weathering

These factors may be human, physical or temporal. However, these do not operate in
isolation but hand in hand in influencing rate of weathering.
a. Climate
b. Geology
c. Relief
d. Vegetation cover
e. Human activities
f. Time

For class discussion

Discuss the complementarity of factors influencing rate of weathering.

Products of Weathering
1. Solid soil material:

 Rock fragments (regolith, scree, gravel, sand, silt, clay)

 New minerals from chemical weathering.

2. Chemical precipitates

3. Biological materials (shell, bones, organics).

4. Dissolved material

 Gases from atmosphere and chemical reactions

 Dissolved solids - inorganic and organic

For class discussion

 Explain the impact of weathering on human activities.
Think of:
a. Agriculture and farming
b. Scenic value for tourism.
c. Extraction of raw materials
d. Quarry for construction
e. Causes damage to built environment and ground transport network.

Soil Pedogenesis (Pedogenic processes)

Pedogenesis can be defined as the process of soil development where parent materials

are changed into soil.

Types of pedogenic processes

a. Physical pedogenic process

i. Addition: when organic matter is added to the loose surface materials which

eventually mix to form soil. Most of these will come from dead organic matter

which rests on the weathered material for adequate time.

ii. Leaching/depletion/translocation: is the removal of nutrients and clay

particles (e.g. colloids, bicarbonate ions, nitrates, ammonia, iron etc.) from

the upper to the lower horizons due to excessive rainfall precipitation.

Because of this downward movement, the upper layers are said to be reduced

or depleted of these nutrients, and enriching the lower horizons.

The disadvantage is that nutrients are lost to underground drainage where

they cannot be reached by plant roots.

iii. Eluviation: is the leaching of soil nutrients from upper layers of soil to lower

levels by downward precipitation of water across soil horizons. It occurs

when precipitation exceeds evapotranspiration. When evapotranspiration

exceeds precipitation, nutrients are moved from lower layers to upper layers
 through capillary rise.

iv. Illuviation: is the accumulation/deposition of nutrients (illuvial deposit) in

lower levels.

b. Chemical pedogenic process

These are the same as those involved in chemical weathering processes such as

reduction, oxidation, carbonation, hydration, hydrolysis, solution.

c. Biological pedogenic process

These are also called biochemical pedogenic processes/ they involve microbial

activities with the main interest on bacteria and fungi. These microorganisms

influence chemical processes taking place in the soil.

i. Humification: involves the transformation of organic matter/litter into humus.

ii. Mineralization: involves the transformation of organic matter into inorganic

compounds plus water and gases.

iii. Ammonification: involves the production of ammonium salts from organic

nitrogen.
 iv. Nitrification: involves the transformation of ammonium salts into nitrates.

Factors of soil formation/development

These are conditions responsible for the development of a particular soil type in a
particular area. The kinds of soils that develop in a particular area are largely
determined by five interrelated factors: climate; living organisms; parent
material; topography; and time.

Climate
The two most important climatic variables influencing soil formation are temperature
and moisture through precipitation. Humidity and wind also are climatic factors.

 High temperatures increase rates of bedrock weathering.

 High temperatures increase the activity of soil microorganisms, the frequency
and magnitude of soil chemical reactions, and the rate of plant growth. (i.e.
chemical reactions)
 High moisture availability in a soil promotes the weathering of bedrock and
sediments, chemical reactions, and plant growth.
 Moisture also influences soil pH and the decomposition of organic matter.
 Humidity determines the direction to which eluviation and illuviation will occur
hence the extent of leaching in a particular soil.
 Wind, especially in deserts, help in introducing parent material by transporting
sand and other debris material from one place to another.

Living Organisms

Microorganisms (bacteria/fungi), macroorganisms (humans, mammals, and large

animals), mesoorganisms (worms/termites) have a role in a number of processes
involved in pedogenesis including organic matter accumulation, profile mixing,
and biogeochemical nutrient cycling.
  Microorganisms aid in decomposition of dead organic matter and turning it into
soil.
 Macroorganisms provide organic matter which when decomposed become part
of the soil.
 Mesoorganisms aid soil formation by improving soil structure through tunneling,
mixing and addition of nutrients from their excreta and dead matter.
 Through litterfall and the process of decomposition, organisms add humus and
nutrients to the soil which influences soil structure and fertility.
 Surface vegetation also protects the upper layers of a soil from erosion by way
of binding the soils surface and reducing the speed of moving wind and water
across the ground surface.

Parent Material

Refers to the rock and mineral materials from which the soils develop. It provides the
framework of the soil profile. Parent material determines:

 The mineral composition of the soil in an area. For example, granite rock which
contains quartz will form soil rich in quartz.
 The color of the soil. For example, parent material rich in iron (Fe) will produce
red soils.
 Soil porosity – the easiness of water infiltration.
 Water holding capacity.

Topography

Generally, topography modifies the development of soil on a local or regional scale

through slope and altitude.

 Slope regulates rate of infiltration of water into the soil and amount of radiation
received from the sun. For instance:

i. Steep slope will have small infiltration and enhances erosion leading to
 thin/shallow soil profile.
ii. Gentle slopes will have will have high infiltration leading to deep soils.
iii. South facing slopes tend to be warmer and drier than north facing slopes.
These results in difference in the soils in terms of depth, texture, biological
activity, and soil profile development.

 Topography influences the amount of rainfall to be received in an area. Good

drainage enhances illuviation and eluviation that are responsible for the
development of soil horizons.

i. High altitude areas will have more rainfall and therefore high chances of
developing deep soil profiles.
ii. Low altitude areas will have low rainfall and therefore high chances of
developing shallow soil profiles.

Time

All the processes required to produce a particular soil will require a passage of time.
Time influences the temporal consequences of all of the factors described above.

 Many soil processes become steady state overtime when a soil reaches maturity.
 Pedogenic processes in young soils are usually under active modification.

Soil formation equation

Russian geologist Vasily Dokuchaev (1889) commonly regarded as the father of

pedology determined that soil formation occurs over time under the influence of climate,
vegetation, topography, and parent material. He demonstrated this using the soil
forming equation:

soil = f(cl, o, p) tr

(where cl or c = climate, o = organisms, p = biological processes) tr = relative time

(young, mature, old).

Hans Jenny (1941) refined the ideas of Dokuchaev and came up with “Clorpt” as a
mnemonic for the factors influencing soil formation. He treating time (t) as a factor,
adding topographic relief (r), and left the ellipsis "open" for more factors (local variables)
to be added as our understanding becomes more refined.

S = f(cl, o, r, p, t, …)

 S soil formation

 cl (sometimes c) climate

 o organisms (soil microbiology, soil mesofauna, soil biology)

 r relief

 p parent material

 t time duration of the soil-formation process.

When water is included as an independent factor, the equation becomes
S = f(cl, o, r, p, t, w…) where “w” is water.
For class discussion
To what extent can we consider soil formation processes as open systems?

Principal Pedogenic Processes

These refer to processes that operate as a team to produce a particular soil type. There
are five main principal pedogenic processes and these are laterization, podsolization,
calcification, salinization, and gleization.

Laterization/ferallization

This is a pedogenic process common to soils found in tropical and subtropical

environments (i.e. wet, warm tropical region, and equatorial and monsoon land – warm
humid climate).
 High temperatures (not below 200C) lead to the growth of broad leafed
vegetation, and provides suitable temperature for bacteria and fungi existence.
  Broad leafed vegetation provides adequate organic matter.
 The bacteria and fungi decompose organic litter into humus and nutrients.
 High temperatures increase the rate of chemical reactions for example
hydrolysis, dissolution, and hydration.
 Heavy precipitation (750mm per annum) results in the rapid weathering of rocks
and minerals.
 Movements of large amounts of water through the soil
cause eluviation and leaching to occur.
 Almost all of the byproducts of weathering, very simple small compounds or
nutrient ions, are translocated out of the soil profile by leaching if not taken up by
plants for nutrition.
 The two exceptions to this process are iron and aluminum compounds. Iron
oxides give tropical soils their unique reddish coloring. Heavy leaching also
causes these soils to have an acidic pH because of the net loss of base cations.

Podzolization
This is associated with humid (temperate) cold mid-latitude climates
and coniferous vegetation. Decomposition of coniferous litter and heavy summer
precipitation create a soil solution that is strongly acidic.
 This acidic soil solution enhances the processes of eluviation and leaching
causing the removal of soluble base cations and aluminum and iron compounds
from the A horizon. This process creates a sub-layer in the A horizon that is
white to gray in color and composed of silica sand.
 Cool temperatures inhibit growth of microorganisms.
 Adequate precipitation will favor growth of coniferous trees.
 There is accumulation of organic litter due to reduced rate of decomposition
because of less number of microorganisms.

Calcification
This is a pedogenic process of the semi-arid and arid regions and middle latitudes (i.e.
steppe climate and tropical steppe climate) where evapotranspiration is greater than
precipitation.
 Calcification is common in the prairie grasslands.
 High evapotranspiration causes the upward movement of nutrients (dissolved
alkaline salts) from the lower B-horizon to the upper A-horizon through capillary
action.
 These deposits can form a hard layer in A-horizon called caliche or calcrete.
When it is soft, it is called mollic.
 Nodules, slabs and hard stones form in B-horizon.
 Pedicals and chernozems are the resulting soils.
 The most common substance involved in this process is calcium carbonate
(CaCO3) which accumulates in B-horizon from C-horizon.

Salinization

This is a process that functions in the similar way to calcification in arid and semi-arid
regions (hot deserts areas) where evapotranspiration is greater than precipitation. They
are most located to the western side of continental land masses.

It differs from calcification in that the salt deposits occur at or very near the soil surface.
 Salinization involves mobilization and redistribution of soluble salts e.g. sodium
chloride (NaCl), sodium sulphate (NaSO4), and sodium nitrate (NaNO3).
 During rainy season, salts are mobilized from A-horizon to B-horizon.
 During dry season, salts are mobilized from B-horizon to A-horizon.
 This balances up removed and accumulated salts.
Salinization produces
i. Saline soils known as solonchaks which shows total accumulation of salts
ii. Solonetz soils which are partly saline because some free salts are leached.
iii. Solodized soil where most of the salts are leached from the profile.

Gleization

This is a pedogenic process associated with poor drainage in low lying areas where
slope is almost horizontal for example Lake Chilwa plain.
  The underlying rock is almost impervious. Coupled with horizontal slopes, water
logged conditions develop.
 Water logged conditions make oxygen absent (no oxidation) in the soil thereby
hindering biological actions.
 Ferric oxide reduction therefore occurs producing sulphides and ammonia.
 Instead of having organic vegetative matter decomposing, vegetative debris
accumulates forming peat soils with blue-grayed stains.

Soil characteristics

1. Soil texture: The relative proportions of sand, silt and clay. It affects the soils’

ability to hold moisture and a nutrient thereby affecting the soil’s potential to

support life.

2. Soil structure: this describes how individual grains of soil are bound together. It

determines the stability of the soil and its ability to support plant growth by giving

plant roots ready access to nutrients, water air.

3. Soil chemistry: This describes the type and amount of chemical elements in the

soil. It determines soil fertility and soil acidity.

4. Soil colour: This indicates the processes operating in the profile. Soil can be

described as black, brown, red, orange, grey and white. The colour of soil is

controlled by the distribution of organic matter through the profile.

(Albert) Mansell chart: This is a colour chart code that is used internationally to

describe specific soil colour.

Soil classification (guidelines in soil classification)

1. The principle of graded likeliness and finite differences

This states that no two soils are totally similar.

 2. The principle of areal transition

This is states that a soil type turns into another soil gradually from place to place.

This is to say soil boundaries are unreal (arbitrary).

3. The principle of continuous alteration of soil form over time

This states that soil map will change over time. This is because the processes of

soil formation are continuous and the changes in climate are possible.

4. The principle of the tendency to develop towards a steady state

This assumes that within a given environment, soil will develop towards a steady

state of equilibrium.

Ways of soil classification

There are four ways: empirical, morphological, genetic and integrative.

1. Empirical method

This is based on observation and experiments where texture and fertility characteristics

are observed. For example;

a. Sandy soil should have 85% sand or more, less than 5% clay and about 10% silt

and gravel.

b. Loamy sand soils should have 75% sand, 10-20% clay, and 5-15% silt and gravel.

c. Loam soils should have 40-50% sand while clay and silt are in the same range.

d. Clay loam soil should have over 30% clay.

e. Clay soil should have over 40% clay

f. Silt loam soil should have over 60% silt and about 40% clay.

2. Morphological method
 This method is based on the soil formation processes and conditions under

which soils are formed. G.W. Robinson (1994) used leaching and soils drainage

system to classify the world soils. He came up with two classes”

a. Freely drained soils: the completely leached soils were called Peldafers while

the partly leached soils were called peldacols.

Peldafers were subdivided into:

i. Soils without humus called latosols (ferrallites)

ii. Soils with humus called podzols (red soils or brown soils).

Peldocols show some accumulation of humus. When humus is absent, brown

desert soils are produced.

b. Soils of impeded drainage (imperfect drainage)

This type of soil shows accumulation of soluble salts sucha s chloride (NaCl).

these are divided into three:

i. Soils formed under temperate conditions e.g. greyed soil, peat, grey

podzol.

ii. Soils formed in sub-arctic region e.g. Tundra soils.

iii. Sols formed in tropical and sub-tropical conditions e.g. alkaline soils of

the area around Lake Chilwa.

3. Genetic method

This type was adopted from botany and zoology. It groups soils in order and sub-

orders. Using this order, the world soils can be group into three: zonal soils,

azonal soils and intra-azonal soils.

i. Zonal soils order

 These develop in areas with drainage system especially uplands with suitable

biotic conditions.

Suborders of zonal soils include

 Light coloured podzolized soil of the forest regions

 Lateritic soils of the warm moist tropical and sub-tropical and equatorial

regions e.g. latosols.

 Soils of the forest grassland areas e.g. chernozems.

 Dark coloured soils of semi-arid and sub-humid grassland e.g. prairie soils.

ii. Azonal soils

These are soils without a well-developed profile because excessive soil erosion.

They are young soils.

Suborders of azonal soils include:

 Legosol (juvenile young soils) formed from alluvial or colluvial depositions.

 Lithosols: stony thin soils on steep slopes.

iii. Intra-azonal soils

These are soils with impeded drainage and the areas are always in water logged

conditions. The slope is horizontal and the parental material is impervious.

Suborders of intra-zonal include:

 Hydromorphic soils of marshy areas. In temperate regions these are

called meadows; in Malawi they are called dambos.

 Holomorphic soils of coastal regions which are rich in sodium chloride.

 Calcimorphic soils rich in calcium.

4. Integrative method
 This method classifies soils based on the three methods described above. Soils

are classified by looking at the stages of development and parent material, and

the mineral accumulation.

As such, soil can be classified as young if they are

a. dominated by parent material of vertisols

b. rich in magnesium or ferric soil

c. rich in iron.

For class discussion

Using the soil classification methods discussed in this topic, classify the soils of

Malawi.