1. Soil forming factors and processes

2. objectives By the end of this section you should be able to:
Distinguish between soil-forming processes and factors
Discuss the parameters influencing weathering
Differentiate between eluviation and illuviation
Describe four composite soil forming processes
Compare processes occurring in soils in humid and dry climates
List Jenny’s five soil forming factors
Give examples of soil formation as affected by parent material, climate, relief, time and biological activity
Explain what a catena is.

3. Processes in soil formation
The processes of soil formation are those which alter the regolith and give it the acquired characteristics that differentiate the soil from its original parent material.
Processes, thus, result in horizon formation that is typical for that particular process.
There are many different types of processes.
There may occur together or separately.
Most different soil types are produced from a combination of processes of weathering, addition of organic matter, transportation of soil constituents and accumulation of soil constituents.

4. weathering
Weathering proceeds first with physical disruption of rocks/particles and this breakdown then facilitates chemical weathering.
Weathering results in reduction in particle size and changes in mineralogy.
Weathering is influenced by many parameter.

5. Parameters of weathering
Mineralogy: some minerals are more stable and resistant to weathering while others break down quickly.
In the fine earth fraction (<2mm) the minerals undergo stages in weathering, which involves a change of chemical composition (Figure 3.1)
The end products in highly weathered soils are inert clays such as kaolinite, gibbsite, hermatite and geothite.
These are the most stable minerals that remain after the other less stable ones are transformed or broken down.

6. Water: the presence of water enhances soil formation, through its effect on mineral weathering, production of organic matter and the movement of soil particles and chemicals within the soil
Water dissolves products and causes hydrolysis, hydration and dissolution reactions.
Temperature: mechanical breakdown of rocks (freezing- thawing, or expansion- contraction changes induced by temperature variations)
Its effect of speeding up chemical reactions
Increase in temperature increases chemical reactions thus promote decomposition of minerals.

7. Oxidation and reduction: weathering is also affected by the amount of oxygen in the zone and the type of minerals present.
Oxidation or reduction reactions are particularly manifest in soils rich in iron as iron is an element that easily undergoes change in oxidation status.
If ferrous iron is (Fe2+) is oxidized to ferric state, this results in a less stable mineral, which is then more prone to weathering

8. In conclusion: weathering produces a soil which consists of a mixture of minerals at various stages of decomposition.
Weathering depends on the original mineral compositions, their resistances to weathering, the amount of water present, the temperature and the oxidation or reducing conditions.

9. Leaching
Leaching is a process by which soluble constituents are removed from the soil. When excess water moves through the soil, it will carry salts with it.
The major effect of leaching is to make soil more acidic and to reduce the amount of bases.
The bases are primarily Ca, Mg, K and Na.
After prolonged leaching, only quartz, kaolinite, iron (III) oxides and aluminium oxides remain in the soil.
Plants may reverse the effects of leaching as they take up water, thereby reducing deep drainage.
Plants also tend to take up nutrients (salts) from the surface and these may then be returned to the surface as plant litter.

10. Eluviation
Eluviation refers to the loss in colloidal suspension, of mineral from the upper soil horizons.
This includes clays, sesquioxides (iron/aluminium oxides) and colloidal humus.
Elluviation is a mechanical loss as opposed to a chemical loss that occurs in leaching (Figure 3.2)
If this occurs to a larger extent, an E horizon (albic or eluvial) might be produced.
An E horizon is typically sandy, pale coloured and very acidic.

11. illuviation
Illuviation is the deposition of eluviated materials, in the B horizon.
The deposited clay may form clay skins around particles/ agrregates
Illuviation may lead to formation of clay rich zones called argillic (Bt) horizons.

12. Organic matter accumulation
When OM falls on to the soil, it can be broken down to form humus (humification) or it can accumulate to form litter layer.
If the organic matter is acidic, it can promote podzolisation.
OM darkens the soil colour, especially of the topsoil (horizon). In conditions of poor drainage, peat formation is encouraged.

13. Gleying
The presence of water for long periods in the soil, brings about anaerobic conditions (hydromophy)
Under these conditions of low oxygen content, bacteria in the soil reduce various compounds including iron.
The iron is changed from the iron (III) Fe3+ (responsible for formation of red coloured compounds) to iron (II) Fe2+.
The iron is much more soluble in this form and may be removed by draining water, leaving the soil grey coloured or gleyed
If there is a fluctuating water table, the iron (II) compounds may be re-oxidised when the water table drops, forming mottles of rust coloured iron (III) compounds.
Gleyed soils are very common in vlei soils and dambos
If the gley soil has a prolonged exposure to air (dries out) then the grey layer with mottles hardens irreversibly into a layer called laterite (indurated ironstone)

14. Ferralization
This process is characteristic of hot, tropical climates and only occurs after extreme weathering and leaching have occurred in soils.
This process in the past was referred to as laterization or kaolinization.
It involves the relative accumulation of iron and aluminium oxides and hydroxides (sesquioxides) with the loss of silica. The soils produced are acidic with a characteristic red colour due their high sesquioxide content.

15. Podzolisation
Podzolisation is a combination of soil processes that occur in cool humid parts of the world such as Russia and Canada, often where siliceous parent material (lots of silica/quartz) is covered by coniferous forests.
The conifers produce a very acidic leaf litter.
As rain falls on the litter and then moves into the soil, it facilitates breakdown of compounds.
OM and iron are mobilized and removed from the surface soils.
The removal of iron is enhanced by the formation of soluble organo- metal complexes called chelates

16. After some time, mainly silica (quartz) remains, giving a charateristic bleached, acidic E (albic) horizon, below the surface.
The organic substances and iron, may be deposited lower in the illuvial horizon (Bh) as water movement slows down.
The soil is strongly acidic and there is little earthworm or soil fauna activity. As a result the horizons are not mixed and stay very distinctly separate.
This profile that develops from these processes, is called a podzol, which consists of a humic layer overlying an albic horizon which further overlies a Bs horizon

17. salinization
This is a combination of processes where there is accumulation of salts in soils usually due to weathering, low leaching, and high evaporation rates.
High groundwaters can also give rise to saline soils.
Saline soils often have white crusts or white concretions in the soil due to the presence of these salts.

18. Overview of factors affecting soil formation
In 1941 H. Jenny, in the USA, produced a book which hypothesized that soils are formed as a result of the interaction of many factors, mainly climate (cl), parent material (p), time (t), relief or topography (r), and organisms (o).
Soil (s) is therefore a function of the above factors or: s= f (cl, p, t, r, o) also referred to as Jenney’s five soil forming factors.
The factors can be studies by holding 4 factors constant and varying only one. E.g. if climate alone is varied, then the range of soils formed is a climosequence.
Similarly, one can define a toposequence (vary r) or chronosequence (vary t).

19. climate
Climate is one of the many influential factors in soil formation because it determines the many of the processes in soils.
The key components of climate that affect soil formation are moisture and temperature (white 1989)
These components influence on the type of vegetation at any given site and this in turn influences the organic status of the soil.

20. Effect of moisture
The actual amount entering and remaining within the soil depends on:
The form and intensity of precipitation (snow, rain,hail)
The seasonal variability of precipitation
The evaporation rate from the soil and plants
The amount of runoff
The permeability of the soil/ parent material
In arid regions, any water that enters the soil is quickly lost by evaporation.
Some weathering may occur but the soil is too dry much of the time for weathering to take place.

21. The limited water movement in the soil also prohibits the removal of soluble products from the soil, through the process of leaching
The result is that soils may tend to be shallow due to limited weathering
They may also contain large amounts of salts and are therefore saline.
Soils of humid regions may have enough water present all the time for weathering and leaching to take place.
In general, an increase in rainfall is associated with an increase in weathering and leaching.

22. In hot, humid regions, soils are often very deep due to intensive weathering and leaching.
As weathering progresses, the primary minerals break down and clays (secondary minerals) are synthesised from the breakdown products.
Moisture status also has influence on vegetation.
Higher rainfall encourages greater plant growth and favours accumulation of OM in soils
In contrast, lower rainfall areas are likely to have less OM due to poor vegetation growth.
Temperature also affects the amount of OM in the soil. Higher temperatures tend to favour more production of vegetation which adds OM to the soil.

23. However, increased temperatures also increase rate of decomposition, which breaks down the OM more quickly.
On balance though, many humid tropical soils have greater OM contents than humid temperate soils, as the annual production of OM is so great in the tropics.
Interestingly, OM content of tundra soils are often high.
Under tundra conditions, precipitation and temperatures are low with meagre biomass production.
However, decomposition is so slow that OM accumulates over a long time in these soils.

24. Parent material
PM is the building block on which the other factors act.
PM may consist of consolidated material (rocks) or unconsolidated material such as alluvium or fresh volcanic ash.
Usually physical weathering takes place before chemical weathering occurs
Weathering can occur in situ which means that the weathering and soil formation occurs in and on top of the PM.
However soils can also be formed from transported material, meaning that they are weathered at one site then transported and deposited on another site.
They, therefore overlay PM that is not necessarily the PM from which they were originally derived
PM ranges from igneous, metamorphic, and sedimentary rocks to unconsollidated deposits formed by wind, water, glaciers or gravity.

25. Types of PM
Igneous rocks: these are formed by solidification of magma in the earth’s crust.
They are divided broadly into light coloured, acidic, rocks (with relatively high quartz contents e.g. granite) and darker coloured, basic, rocks (which are lower I quartz but rich in ferromagnesian minerals e.g basalt).
Granit rocks which are rich in quartz tend to weather to produce soils which are sandy and high in quartz, as quartz is resistant to weathering.
Granite soils cover most of the central and south-western Zim

26. In contrast, basic rocks such as basalt, which contain more calcium and magnesium, produce clayey soils with very low sand contents.
These can typically be seen in south-eastern Zim (Chisumbanje) where basalts have produced black clay soils.
Ultrabasic rocks such as serpentines, which contain large quantities of ferromagnesium minerals, may weather to produce magnesium-rich, clay soils such as those found along Great Dyke in Zim

27. Sedimentary rocks: these are composed of the weathering products of igneous and metamorphic rocks and are formed after deposition from wind and water.
Usually the sediments are deposited in layers and are the subjected to consolidation and hardening. Which leads to the formation of the sedimentary rocks.
These rocks can also be subjected to folding, faulting and tilting.
The size of fragments making up sedimentary rocks decrease in the order from conglomerates (very coarse), sandstones (coarse), siltstones (medium) to mudstone (fine).

28. These rocks give rise to different soils depending on their mineralogy and fragment sizes. Sandstones generally give rise to sandy soils while mudstone usually produce clay soils.
Other sedimentary rocks are formed from precipitation of calcium compounds which give rise to limestone and chalks. These weather to produce calcium-rich (calcareous) soils.
Extensive areas of sedimentary rocks occur in the Zambezi Valley and Escarpment area in the nortwest of the country ranging from grits, sandstones and siltstones to mudstones.

29. Metamorphic rocks: when igneous or sedimentary rocks are subjected to intense heat and great pressure, they are transformed into metamorphic rocks.
Examples are gneiss, slate and schist. The minerals usually undergo transformation during the heating and pressurization and generally change into stable compounds that may be resistant to weathering.
The type of minerals and their size in the rock again influences the soils developed from the rock. So gneiss weathers to produce sandy soils while slates weather to produce clay soils.

30. Water deposited material: transport of particles by water followed by deposition, produces alluvial soils.
Example of these soils can be seen on flood plains of rivers.
During transport, the particles may be abraded and sorted by density.
Typically, alluvial soils smooth rounded particles.
These soils frequently have distinct layers where particles have been sorted due to speed of water flow or deposited during successive floods.

31. Glacially deposited material: deposits of materials are formed as glaciers move and melt. These deposits are common in Asia, North America and Europe and are called morraines of tills.
Wind deposited material: wind moves particles by rolling, saltation and suspension processes. These wind blown particles may then be deposited when the wind velocity is slowed and they form aeolian deposits. Eg Kalahari sands in the west of the country.
Gravity deposited material: loose rocks and stones formed by weathering of exposed rocks will move downslope under the influence of gravity. These are called colluvial deposits.

32. Time
The length of time during which materials have been subjected to weathering plays a significant role in soil formation.
If the PM is hard and resistant (e.g. Granite), then the soil may take many thousand of years to develop.
Young soils retain many of the features of the PM. As they become older, they acquire other features such as OM and increasing development and distinctness of horizons.
a chronosequence of soil development is shown in Figure 3.4
In general, soils take many thousand of years to develop. The work of Owens and Watson (1979) on rates of soil formation in Zimbabwe, indicated that 11.0mm and 4.1 mm of soil were produced per thousand years, under moderate temperature and rainfall conditions

33. Topography or relief
Topography modifies soil profile development in three ways :
By influencing rainfall absorbed, and therefore affecting the amount of moisture in the soil.
By influencing the rate of soil erosion
By influencing the amount of subsurface water movement which will transport soil materials from one place to another.
The main influence is that on the movement and distribution of water within and over the soils
As steepness of slope increases, there is greater runoff and erosion and less water enters the soil to be available for chemical and biological activity, or weathering.

34. Therefore soils on steep slopes tend to be shallow and poorly formed.
The role of topography is closely linked with catena concept.
The concept of the catena was first proposed by Milne (1935) to describe a toposequence in East Africa.
Catena is defined as a sequence of soils derived from the same PM but differing in properties because they occupy different topographic positions.
The difference in soil properties arise because of:
The differences in soil moisture regimes that exists between soils in the sequence, and
The degree and nature of lateral water movement at each position.

35. A simple catena, therefore, shows a repetitive soil pattern linked to changes in topography (Figure 3.5)
Taking an example of UZ (Harare) catena. The catena developed on basic (mafic) PM (figure 3.6)
In the upland position, just below the crest, sufficient rainfall penetrates the soil to allow for weathering and development of deep profile.
This site is well drained (above water table) and water moves internally and carries bases down slope.
The soil is mostly composed of stable iron oxides and hence has a red clour.

36. Lower down the slope, the water table is closer to the surface and influences the soil morphology.
The soil experiences seasonal waterlogging which induces formation of mottles in the subsoil.
The soil is brown colour due to the iron oxides being reduced.
Still further down the slope, the soils become permanently waterlogged and under these reducing conditions all iron is reduced to the ferous state.
There is little stream flow and poor drainage encourages accumulation of bass, OM and presence of active swelling clays.

37. The clays combine with OM to produce black soil above the waterline.
However, the soils below the waterline which are permanently wet, exhibit gley colours.
Other examples on catena is the Great Dyke (Nyamapfene, 1991; 1992)

38. organisms
Biological activity plays an important role in soil development.
Biological agents (organisms) include vegetation, soil fauna, soil micro-organisms and man.
Plants affect soil genesis through addition of OM, cycling of nutrients and water, and through root activity
Plants such as pines and spruces produce acidic leaf litter which may lead to creation of a specific soil type called podzol.
Organisms also affect soil genesis as consumers and decomposers of OM and through their earth moving activities.

39. Of particular importance in Zimbabwe is the action of termites and ants.
Many different types of termites mounds (termitaria) and anthills are found here.
The soils in these termataria/ anthills have distinctly different properties compared to the surrounding soils, including more bases and higher clay contents
Termites may bring fine soil up to the surface leaving stones deeper down.
Earthworms have a major effect on soil properties in temperate regions, promoting well aerated and aggregated soils.
Man influences soil formation through his management ( or often mismanagement) of vegetation, through farming and keeping of domestic animals, and by urban and industrial development.
Many of these changes induced by man lead to degradation/ pollution of soils.

40. summary
Factors are external conditions which cause various processes to occur in soils.
Weathering is most affected by temperature and precipitation.
Soil forming processes such as leaching, eluviation, illuviation cause movement and redistribution of salts, clay, OM and sesquioxides in soils
There are five soil forming factors (climate, PM, time, topography and organisms) which give rise to different types of soils.

41. PM include consolidated igneous, sedimentary and metamorphic rocks and unconsolidated material such as alluvium, colluvium, aeolian and glacial deposits.
Topography affects soil formation primarily through its effect on water movement and distribution down the slope. Catenas are a sequence of soils developed at different positions on the slope.