Soil formation factors and processes

objectives

• Describe the meaning of soil
• Explain the influence of rock and mineral characteristics on soil formation
• Describe the weathering processes and their significance in soil formation
• Explain the environmental factors that influence soil formation and development
• Explain the interdependence of the environmental factors
• Describe the relevance of soil profile in soil development
• Explain the basic processes that drive the formation of soil profile

meaning of soil

• Defining soil has become controversial due its complex nature, services it provides, its application in different disciplines (settings).
• Soil was generally thought of as the uppermost geological material
• Over 80 years, pedologists have settled on defining soil as
o A naturally occurring, unconsolidated, mineral or organic material at the earth’s surface that is capable of supporting plant growth. But its more than just a medium for plant growth, think of all the other services it provides. But importantly, soil is soil in its natural environment. When you buy soil from the store, or excavate it from its natural environment and move it elsewhere, it is just a soil sample, even if its alot of soil its still just a sample. Soil will work in different ways, either due to its minerals or organic materials. There is organic soil and mineral soil, different things???
• From soil ecosystem services perspective, is there a limit to the definition of soil? There is more to soil than supporting plant life. We are dealing with more than just plant growth. Different perspectives, think about engineers/architects, they use the soil to support manmade structures. Think about hydrologists, water holds the soil. So many different perspectives. So when asked the meaning of soils, think about all of the other perspectives and services it provides.

Soil is a complex system. The complex nature of the soil is revealed in its formation, development, application, and the services it lends to us. The more we know about it, the more we learn of its complexity.

The spheres impact on soil formation

• Recall:
o Soils are formed as the pedosphere is exposed to
the atmosphere, hydrosphere, and biosphere
§ Pedosphere: The lithosphere’s uppermost
portion that interacts with the other three
spheres to form soils
o Landscape consists of a variety of geographical
features that are characteristics of an area
§ Subject to endogenic, exogenic, and human forces, creates a plethora of different soils with varying landscape
§ Suite of different soils and the ecological
services they provide
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Rocks and minerals

• We need the rocks and the minerals contained in to go through a weathering process
o Weathering is the break down of the rocks and minerals to initiate the soil
formation process. So important! When they break down, that starts the soil formation process

Types of weathering
- Physical weathering
- Chemical (Biogeochemical) weathering, both biological and chemical agents at work

o Rate of weathering is dependent on the rock (Igneous, Sedimentary, Metamorphic) and the mineral(s) contained in the rock and their resistance to weathering

Major rock distribution in Canada

Metamorphic: back into earth and pressurized change

Distribution tells you whats going on in each environment

Igneous rock

• Igneous rocks are formed from molten magma
• They are made up of primary minerals

o Primary minerals are those that are present from the time a rock first formed
§ Mainly silicates (90%) as magma is dominated by oxygen and silicon
§ Other primary minerals include aluminum, iron, magnesium, calcium, sodium, and potassium

o Categories of primary minerals (silicates)
§ Quartz (most dominant; about 75%)
§ Light-colored minerals
§ Dark-colored minerals

These primary minerals lead to the clay formation, to make the active ingredients. Don’t need large amounts of these to affect the soil

Igneous rocks, their mineral composition and rock texture

which ones change/react more?

Sedimentary rocks

• Sedimentary rocks are formed by the accumulation or deposition of mineral or
organic particles at Earth's surface
• Sedimentary rocks are the most common type of rock encountered, covering about 75% of the Earth’s land surface
o Most of what is presently dry land was at some time in the past covered by water

More porous, weather more readily than igneous rock

• The resistance of a given sedimentary rock to weathering is determined by its
dominant minerals and by the cementing agent
o Clastic sedimentary rocks are formed from consolidation of pre-existing particles
such as sand, silt or clay
§ E.g., sand- (sandstone), silt- (siltstone), clay-sized (shale, mudstone)
o Biochemical (biogenic) sedimentary rocks are formed from shells and bodies of
underwater organisms
§ E.g., limestone, coal, have different chemical reactions,
o Chemical sedimentary rocks are formed by chemical precipitation that begins when water traveling through rock dissolves some of the minerals
§ These minerals are carried away from their source and eventually redeposited, or precipitated, when the water evaporates away
§ E.g., halite (rock salt)

Metamorphic rock

• Metamorphic rocks are formed from other rocks by a process of change- “metamorphism” rock changes to new rock
o As Earth’s continental plates shift, and sometimes collide, forces are generated
that can uplift great mountain ranges or cause huge layers of rock to be pushed deep into the crust
o The movements subject igneous and sedimentary rock masses to tremendous heat and pressure
o The forces may slowly compress and partially remelt and distort the rocks, as
well as break the bonds holding the original minerals together
o Recrystallization during metamorphism may produce new (usually larger) crystals of the same minerals
o Elements from the original minerals may recombine to form new minerals

metamorphic 2

• E.g.
o Igneous rock granite may be metamorphosed to
gneiss
o Sedimentary rock limestone may be metamorphosed
to marble
o Sedimentary rock shale may be metamorphosed to
slate,
• Metamorphic rocks are usually harder and more strongly
crystalline than the sedimentary rocks from which they
formed
o The particular minerals that dominate a given
metamorphic rock influence its resistance to
weathering

igneous strongest, metamorphic in the middle, sedimentary the weakest

Think about the formation of each rock to see which is the hardest, makes sense

Physical weathering

• Physical weathering breaks the rocks into small fragments and does not alter the original minerals
• Intensity of physical weathering

o Greatest in cold and dry climates
Ø Drying- cracks the rocks
Ø Cold- freezing expands and thawing contracts: cracking
Ø Ice can cause abrasion
But rate at which soil forms from this process is so slow

o Least in wet and warm climates
Ø In wet and warm climates, chemical (biogeochemical) weathering dominates as water alters the composition of a mineral or cause it to dissolve in solution
Ø The chemical process is increased by warmer conditions

2 mm or less is soil, rocks must weather to this particle size to be considered a soil, makes sense why it takes over 1000 years to form a soil, weathering takes long to be able to reduce parent rock to such a level. Climate also has such an impact.

rocks break down, more surface is exposed to weather by physical and biochemical processes.

Physical weathering processes temperature change

• Temperature change
o Expansion and contraction: heating (due to sunlight during the day) and cooling (at night) cause alternate expansion and contraction of constituent minerals
Ø Variation in mineral response to heat leads to differential stresses that eventually cause the rock to crack apart
o Exfoliation (peeling away): formation of ice in surface cracks disintegrates and dislodges mineral grains from smaller fragments (when water freezes, it expands with force)

we need the crack to expose more surface area

• Abrasion (grinds away) by Water, Ice, and Wind
o Water loaded with sediments has
tremendous cutting power
§ Gorges, ravines, and valleys, rounding
of riverbed rocks and beach sand
grains
o Moving ice masses embedded within soil
and rock fragments grind down rocks in
their path and carry away large volumes of
material
o Windblown dust and sand wear down rocks
by abrasion
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plants and animals

o Plant roots may enter cracks in rocks and
pry them apart, resulting in some
disintegration
o Burrowing animals may also help
disintegrate rock
• Plant and animal influences are of little
importance in producing parent material when
compared to the drastic physical effects of
water, ice, wind, and temperature change

doesn’t hasten the process like water and temperature do, but they still have an impact

chemical (biogeochemical) weathering

• Chemical weathering processes alter the original minerals, not just physical anymore, chemical changes with minerals, minerals change from the original minerals that used to be there due to chemical reactions.
o Some primary minerals in the rock resist chemical change and may retain their
original character
• Chemical weathering is enhanced by
o Geological agents
§ Presence of water in very important (in the air and on the ground surface), but not just pure water we need a biological agent of acid or something to leach minerals!!!
§ Abundance of oxygen
o Biological agents
§ Presence of carbon dioxide due to microbial and plant-root metabolism,
which produces weak carbonic acid when combined with water
• Biogeochemical weathering is often used to describe the chemical weathering
process
o Intensity: greatest in warm and wet climates and least in cold and dry climates

Living organisms and plants also bring out more biogeochemical process to help leach out calcium potassium and other nutrients. So we need living plant engaging with soil environemnt so microbes can do their job. And soil formation and aggregation continues and helps to bind and build the soil.

How fast can OM decay? After harvest and the chaff is left over. But then winter comes. Then spring comes and its still too cold and too wet. Microbes are using nitrogen from the OM to live and they are respiring and releasing CO2 and making your soil acidic. So yes OM is good but in what quanitities and kinds? Should we be using OM that is maybe already degraded enough to a simple form where soil can access the nutrients.

Grinding and abrasion are great for change, but chemistry provides real change. Need chemistry to change it from sandy soil to silty soil to CLAY.

mineral in soil and resistance levels

There was a time we used gypsum to determine soil moisture content. But it would just start dissolving and you wouldn’t get accurate results. Be mindful of climate and time of year you put it on.

Biogeochemical weathering processes

Don’t need to know equations

Processes
• Hydration
o Intact water molecules bind to a mineral
o 5Fe 2 O 3 + 9H2 O Fe 10 O 15 • 9H2 O
o (hematite + water ferrihydrite)
• Hydrolysis (breaks into components)
o The original mineral becomes altered to a different mineral
§ For example, feldspar mineral is altered to form clay minerals plus some ions in solution
Ø CaAl 2 Si 2 O 8 + H2 CO 3 + ½O 2 ↔ Al 2 Si 2 O 5 (OH)4 + Ca2+ + CO 32−
Ø (plagioclase feldspar + carbonic acid ↔ kaolinite + dissolved calcium
ions + dissolved carbonate ions)

Feldspar drops caclium in this equation, then clay can get calcium. We like caclium present. SOmething about carbonates

Clay too acidic? Aluminum released and attacks soil structure.

Dissolution

o Involves the complete dissolution of a mineral (solute dissolves in solvent to form
a solution)
o Calcite, for example, will dissolve in weak acid to produce dissolved calcium and dissolved bicarbonate ions
§ CaCO 3 + H+ + HCO 3− ↔ Ca2+ + 2HCO 3−
§ (calcite + dissolved hydrogen ions + dissolved bicarbonate ions ↔ dissolved
calcium ions + dissolved bicarbonate ions)

Oxidation reduction

o Minerals that contain iron, manganese, or sulfur are especially susceptible to
oxidation–reduction reactions
§ Reduction: For olivine, in the presence of carbonic acid is converted to
dissolved iron, carbonate, and silicic acid
Ø Fe 2 SiO 4 + 4H2 CO 3 ↔ 2Fe 2+ + 4HCO 3− + H4 SiO 4
Ø (olivine + carbonic acid ↔ dissolved iron ions + dissolved carbonate ions
+ dissolved silicic acid)
§ Oxidation: But in the presence of oxygen and carbonic acid, the dissolved iron
is then converted to the mineral hematite
Ø 2Fe 2+ + 4HCO 3− + ½ O 2 + 2H2 O ↔ Fe 2 O 3 + 4H2 CO 3
Ø (dissolved iron ions + dissolved bicarbonate ions + oxygen + water ↔
hematite + carbonic acid)

Need carbonates present to take the place on attatchments so soil isn’t so acidic

gain or loose oxygen

Certain reactions occur in the presence of oxygen

these changes occur because of the chemistry, change from primary to secondary mineral

Factors that influence soil formation and development

• Suite of different soils and the ecological services they provide in a landscape
• Individual soils have distinctive properties influenced by environmental factors

all three rocks go thru changes and give foundation for soil to form but we still need all the other factors in the photo

More develops at bottom than top of slope

TIME: certain soils are older than others, think about equators vs poles maybe?

Parent material

will form the texture it will have, also what if it forms in place? or what if its transported?

Soils on residual parent material (stayed in place) will form faster. Takes longer when transported away.

• Geological processes have brought to the Earth’s surface numerous parent materials
from which soils form
• The nature of the parent material influences soil characteristics
• E.g.,
o Soil texture (sand, silt, clay)
§ Quartz-rich parent material, such as granite or sandstone, generate sandy soils
§ Quartz-poor material, such as shale or basalt, generates soils with little sand
o Chemical and mineralogical composition of soil
§ Felspar and micas minerals engage in biogeochemical weathering to form
clay minerals
§ Limestone (calcite mineral) slows the development of acidity that typically
occurs in humid climates
§ Soil nutrients useful for plant needs (Ca, Fe, Mg, K,...)

• Classification based on the mode of placement in their current location

Lacustrine (lake aggasiz)

fluvial (red river floodplain on stream bed, deposited and carried by stream on sides)

marine (parent material exposed from where ocean used to be)

alll them can still be transported by wind

Parent material distribution in the Prairie provinces of Canada
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Parent material distribution in the Prairie provinces of Canada

Type of soil you get is largely determined by the parent material

older soil, thicker A horizon which has a lot of nutrients

Climate

Water distribution and availaiablity so important

Need water to dissolve CO2 to make carbonic acid

• Climate determines the nature and intensity of the weathering that occurs over large geographic areas
o The principal climatic variables influencing soil formation are
§ Effective precipitation (Water)
§ Temperature
o They affect the rates of chemical, physical, and biological processes

Climate - Water

• Water from rain and melting snow is a primary requisite for parent material weathering and soil development
o To fully promote soil development
§ Water must enter the profile and participate in weathering
§ Water must percolate through the profile and translocate soluble weathering
products
Ø Percolating water stimulates weathering and helps differentiate soil horizons
§ Too much water (e.g., in rainforests) can lead to the leaching of important
chemical components (e.g., carbonate minerals - calcite) and hence lead to acidic soils
§ Too little water (e.g., in deserts and semi-arid), results in very limited downward chemical transportation and the accumulation of salts and carbonate minerals (e.g., calcite)

Water 2 

• Seasonal distribution of precipitation, temperature and evaporation, topography,
and permeability of the parent material influence how much water percolates
through the soil and promote soil development
o Seasonal distribution:
§ Let’s consider a site that receives an average of 600 mm of rainfall per year
§ Case 1: The 600 mm of rainfall distributed evenly throughout the year, with
about 50 mm each month
§ Case 2: The 600 mm of rainfall distributed only during the six-month rainy
season with about 100 mm each month, ditribution is not uniform, it is important to preserve the soil we have, MB unpredictable rain dist 
§ Conclusion:
§ Case 1 is likely to cause less soil leaching or erosion and floods

water 3

• Temperature and evaporation
o Evaporation from soils and vegetation is much higher in hot than in a cool climate
o In hot climate, much less of the 600 mm of precipitation will be available for
percolation and leaching
§ Most or all will evaporate soon after it falls on the land
o In a cool climate most of the 600 mm of precipitation may cause more leaching
and profile development

espeically in planting season, impeded by the snowmelt present, evapotranspiration rate very low, a lot of moisture in atmosphere, all this extra water will either run off and cause erosion or sit on top of the soil and cause floods because the soil is already so saturated and the water can’t percolate down.

Tile drains? Before you unpiut the drain, you must first understand the whole soil profile and its characteristics?

water - topography

Water falling on a steep slope will run downhill so rapidly that only a small portion will enter the soil where it falls
o Level or concave sites experience more percolation and leaching than steeply
sloping sites
o Concave site will receive the greatest effective rainfall because, in addition to
direct rainfall, it will collect the runoff from the adjacent sloping site

Is it developing on top of a hill, on the slope, or at the depression?
In depression, you get lots of minerals from run off, but also a lot of standing water. Not enough oxygen going through the system, grey soil.

Water - permeability 

water must be able to filtrate and percolate through it moving through at a good speed 

o Considering similar seasonal distribution of rainfall, temperature and
evaporation, and topography,
§ More rainwater and snowmelt will infiltrate and leach through a coarse,
sandy profile than a tight, clayey one
§ The sandy profile will experience a greater effective precipitation, and
more rapid soil development
§ In humid and poorly drained regions, swampy conditions may prevail,
producing soil that is dominated by organic matter

climate - temperature

• For every 10 °C rise in temperature, the rates of biochemical reactions more than
double
o Soil forms most readily under warm climate and where precipitation amounts
are moderate (not dry, not too wet)
§ The processes of weathering, leaching, biological activities, plant growth are maximized
§ Temperature and moisture both influence the organic matter content of soil through their effects on the balance between plant growth and microbial decomposition

Frozen water vs liquid water

Rise in temp: increased rate of biological activity, ex. respiration increases CO2 level, water interactions increase carbonic acid content to release and breakdown minerals. Microbial activity super important and impactful but pauses in the winter

living organisms

• Activities of living organisms enhance
o Biogeochemical weathering, organic matter accumulation, production of organic acids, profile mixing, nutrient cycling, aggregate stability, reduction in soil erosion, soil protection and development (by humans)
Prarie soils can stay intact in high wind conditions due to plant roots holding them together. Native grasses have very deep roots that keep things together. But when we cultivate the soils it removed the root structure holding things together. We have to consider this to protect our fertile soils. We have to find a way to perserve the soil after crops are harvested becasue the wind will take its toll.

Certain forested areas have acidic soil due to leaf dropping and leaf litter.

Topography

• Topography relates to the configuration of the land surface and is described in terms of differences in elevation, slope, and landscape position—in other words, the lay of the land
• The topographical setting may either hasten or retard the work of climatic forces
o Steep slopes generally encourage rapid soil loss by erosion and allow less rainfall to enter the soil before running off
o In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation
o Soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites

Plowing along slopes, see rills start to form and water being transported down, erosion is occuring

• In depressions that collect runoff water, the parent material is usually more deeply weathered, and soil profile development is more advanced
o However, in the lowest landscape positions, water may saturate the soil to such a degree that drainage and aeration are restricted
§ The weathering of some minerals and the decomposition of organic matter are
retarded
§ In such low-lying topography, special

profile characteristics of wetland soils may develop

B horizon from upper level area can erode and fall down onto basin lower area so your topsoil is actually b horizon soil. 

time

• Time interacts with the other factors of soil formation
o Residual parent materials have generally been subjected to soil-forming processes longer than transported parent materials
§ E.g., soils forming on glacial materials have generally had far less time to develop than those soils that escaped disturbance by the glaciers
Ø Western portion of Yukon vs. rest of Canada
ü Never glaciated or was covered by only the earliest glaciations more than 2 million years ago, older soil

Soil formation and development continuous

All the environmental factors that influence soil formation and development are
interdependent
• Soil development is a continuous interactive process
• The fragmented rocks and the further development of the resulting loose debris
take over several many thousands of years to form a more or less stable internally
ordered, functioning soil body
o Ultimately, there will be a formation of a characteristic soil profile

As technology improves we apply new stressors to the soil we must consider the affects

from top to parent material you must look at the whole profile and soil character

Clay soil restricts verticle movemnets. Cant install tial drains and expext water to move vertically when you have clay that sends water moving horizontally. Ditches allow for hoizontal movement.

Soil profile

• The vertical section of the soil running from the soil surface through
to the parent material below
• Reveals the character of the soil as a whole
• Consists of succession of more or less distinct strata
o The strata may result from the pattern of deposition or
sedimentation
o The strata (horizons) may form by internal soil-forming
processes
• The character of a profile depends primarily on the climate and
secondarily on parent material, vegetation, topography, and time
• Matured soils have been subjected to these factors for a sufficient
length of time and full profile development has taken place

from top to parent material you must look at the whole profile and soil character

Clay soil restricts verticle movemnets. Cant install tial drains and expext water to move vertically when you have clay that sends water moving horizontally. Ditches allow for hoizontal movement.

• A: Soil is minimally weathered with an organic matter overlying parent material
• B: Soil has an organic matter, thin greyish bleached layer, reddened layer, transitional
yellow-red layer, and unaltered parent material
• C: The soil has an organic matter, thick white-gray bleached layer overlying a dark,
iron and organic matter enriched layer that grades to parent material
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Soil profile eg

Soil profile

A: Forms as a result of loosened materials from C are taken
over by living organisms. Generally, has aggregated
structure, stabilized to some degree by organic matter
cementation. Typically, darker in color than the underlying
soil. Major biological activity zone

E: Bleached zone below A horizon as soluble weathering
products and clay minerals are moved by water and
organic acids percolating down from the liter layer
B: Forms due to continuous disintegration and downward
migration of finer particles and transportable materials
(such as soluble salts) from A. Generally thicker than A
horizon. Porosity is reduced due to clay accumulation and
pressure of A horizon. Gets very dense.
C: Soil's parent material resulting from disintegration of
exposed rock formation or continuous disintegration of
deposited minerals

Four basic processes that drive the formation of soil profile

• Transformation from
weathering processes
• Translocation of fine particles
and salts across layers and horizons, can even go from bottom layer up by groundwater
• Additions of organic matter,
salts from groundwater, dust
• Losses through leaching,
erosion