Soil Formation Factors and Processes

meaning of soil

a naturally occurring unconsolidated mineral or organic material at the earth’s surface that is capable of supporting plant growth (plus all its other ecosystem services)

how are soils formed

as the pedosphere is exposed to the atmosphere, hydrosphere and biosphere

what is the pedosphere

the lithosphere’s uppermost portion that interacts with the other three spheres

weathering

the break down of the rocks and minerals to initiate the soil formation process

types of weathering

physical and chemical (biogeochemical) weathering

what is the rate of weathering dependent on

the rock (igneous, sedimentary, metamorphic) and the mineral(s) contained in the rock and their resistance to weathering

how are igneous rocks formed and what are they made up of

formed from molten magma. made up of primary minerals - silicates (90%) as magma is dominated by oxygen and silicon

what are primary minerals. give examples

those that are present from the time a rock first formed.

• silicates, aluminum, iron, magnesium, calcium, sodium, potassium

what are the categories of primary minerals

• quartz (most dominant; about 75%)

• light coloured minerals

• dark coloured minerals

how are sedimentary rocks formed

by the accumulation or deposition of mineral or organic particles at earth’s surface. most common type of rock encountered

what is the resistance of a given sedimentary rock to weathering determined by

its dominant minerals and by the cementing agent

what is clastic and how is it formed

a sedimentary rock formed from consolidation of pre-existing particles such as sand, silt or clay

what are the three types of clastic rock

• sand (sandstone)

• silt (siltstone)

• clay sized (shale, mudstone)

how are biochemical sedimentary rocks formed

from shells and bodies of underwater organisms

• ex: limestone, coal

how are chemical sedimentary rocks 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

• ex: halite (rock salt)

how are metamorphic rocks formed

from other rocks by a process of change

explain the steps of metamorphosis for rocks

• plate movements push rocks deep into crust

• rocks exposed to intense heat and pressure

• compression distorts, may partially melt rocks

• mineral bonds break - recrystallization forms larger crystals

• elements recombine - new minerals form

examples of metamorphic rocks

• igneous rock granite → gneiss

• sedimentary rock limestone → marble

• sedimentary rock shale → slate

which type of rock is usually harder and more strongly crystalline than others from which they formed

metamorphic rocks. the particular minerals that dominate a given metamorphic rock influence its resistance to weathering

what is physical weathering

the breakdown of rocks into small fragments which does not alter the original minerals

when is physical weathering the greatest

in cold and dry climates

• drying - cracks the rocks

• cold - freezing expands and thawing contracts: cracking

• ice can cause abrasion

when is physical weathering the least likely to occur

in wet and warm climates

• chemical weathering dominates as water alters the composition of a mineral or cause it to dissolve in solution

• the chemical process is increased by warmer conditions

explain temperature change as part of physical weathering

• expansion and contraction: daytime heating and nighttime cooling causes minerals to expand/contract differently → stresses crack rock apart

• exfoliation (peeling): water freezes in cracks, expands, and forces mineral grains and fragments to break off

explain abrasion as part of physical weathering

• water: sediment-loaded water cuts gorges, valleys and rounds rocks/sand

• ice: glaciers with embedded debris grind and carry away rock

• wind: blown dust and sand wear down rock surfaces

explain plants and animals as part of physical weathering

• plants: roots grow into cracks, prying rocks apart

• animals: burrowing helps break up rock

• overall: minor influence compared to water, ice, wind and temp

what is chemical weathering

process that alters the original minerals

how is chemical weathering enhanced

• by geological agents - presence of water, abundance of oxygen

• by biological agents - presence of carbon dioxide due to microbial and plant root metabolism, which produces weak carbonic acid when combined with water

where is chemical weathering the greatest

in warm and wet climates and least in cold and dry climates

hydration - biogeochemical weathering

intact water molecules bind to a mineral

• hematite + water → ferrihydrite

hydrolysis - biogeochemical weathering

the original mineral becomes altered to a different mineral

• plagioclase feldspar + carbonic acid ←> kaolinite + dissolved calcium ions + dissolved carbonate ions

dissolution - biogeochemical weathering

involves the complete dissolution of a mineral (solute dissolves the solvent to form a solution)

• calcite will dissolve in weak acid to produce dissolved calcium and dissolved bicarbonate ions

• calcite + dissolved hydrogen ions + dissolved bicarbonate ions ←> dissolved calcium ions + dissolved bicarbonate ions

which minerals are more susceptible to oxidation reduction reactions

minerals that contain iron, manganese, or sulfur

reduction

for olivine, in the presence of carbonic acid is converted to dissolved iron, carbonate and silicic acid

• olivine + carbonic acid ←> dissolved iron ions + dissolved carbonate ions + dissolved silicic acid

oxidation

in the presence of oxygen and carbonic acid, the dissolved iron is then converted to the mineral hematite

• dissolved iron ions + dissolved bicarbonate ions + oxygen + water ←> hematite + carbonic acid

what does climate determine for soil

the nature and intensity of the weathering that occurs over large geographic areas

what are the principal climatic variables influencing soil formation

• effective precipitation (water)

• temperature

they affect the rates of chemical, physical and biological processes

what are the four basic processes that drive the formation of soil profile

• transformation from weathering processes

• translocation of fine particles and salts

• additions of organic matter, salts from groundwater, dust

• losses through leaching, erosion

explain the A soil horizon

forms as a result of loosened materials from C are taken over by living organisms. generally has aggregated structure, stabilized to some degree by OM cementation. typically darker in colour than underlying soil. major biological activity zone

explain the E soil horizon

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

explain the B horizon

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

explain the C horizon

soil’s parent material resulting from disintegration of exposed rock formation or continuous disintegration of deposited minerals

what is parent material in soil science

the geological material from which soils form; it influences soil properties

how does quartz rich parent material (like granite or sandstone) affect soil texture

produces sandy soils

how does quartz poor parent material (like shale or basalt) affect soil texture

produces soils with little sand

which minerals in parent material weather to form clay minerals

feldspars and micas

how does limestone as parent material affect soil chemistry

its calcite slows soil acidification in humid climates

what key soil nutrients can come from parent materials

calcium, iron, magnesium, potassium

what is a primary requisite for parent material weathering and soil development

water from rain and melting snow

what two roles must water play to fully promote soil development

enter the soil profile for weathering and percolate through it to move soluble products

how does percolating water affect soil horizons

it stimulates weathering and helps differentiate horizons

what can too much water (e.g. in rainforests) do to soils

leach important minerals like calcite, leading to acidic soils

what happens with too little water (e.g. deserts)

limited chemical movement and accumulation of salts and carbonates

what factors influence how much water percolates through soil

seasonal precipitation, temperature, evaporation, topography and parent material permeability

in two sites with 600mm of annual rainfall, which causes less soil leaching: evenly distributed rainfall or rainfall concentrated in a wet season

evenly distributed rainfall

how does temperature affect soil water balance

hot climates → high evaporation, less water for leaching

cool climates → more water available for leaching and profile development

how does a hot climate reduce effective precipitation

most or all rainfall evaporates before it can percolate into the soil

how does topography influence soil water entry

steep slopes → rapid runoff, little infiltration

level/concave sites → more percolation and leaching

which site gets the greatest effective rainfall; steep slope, level site or concave site

concave site (collects both rainfall and runoff)

how does soil permeability affect infiltration

sandy soils → more infiltration and rapid development

clayey soils → less infiltration

what kind of soil develops in humid, poorly drained regions

organic matter rich soils (swampy conditions)

how does temperature affect biochemical reaction rates in soil

every 10 degree rise more than doubles reaction rates

in what climate does soil form most readily

warm climate with moderate precipitation

what soil processes are maximized in warm, moderately wet climates

weathering, leaching, biological activity and plant growth

how do temperature and moisture influence soil organic matter

by affecting the balance between plant growth and microbial decomposition

how do living organisms contribute to soil development

through weathering, organic matter input, acid production, profile mixing, nutrient cycling and erosion reduction

how do humans act as living organisms in soil formation

by protecting and developing soil through management practices

topography

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

how does topography influence climatic forces on soil

it may hasten or retard their effects depending on slope and landscape position

what happens on steep slopes regarding soil

• encourage rapid soil loss by erosion

• allow less rainfall to infiltrate (more runoff)

• result in less vegetative cover → less plant contribution to soil formation

• produce shallow, poorly developed soil profiles compared to level sites

why do steep slopes in semiarid regions have less vegetative cover

lower effective rainfall reduces plant growth, leaving soils more exposed

how does topography affect soils in depressions that collect runoff

parent material becomes more deeply weathered, leading to more advanced soil development

what occurs in the lowest landscape positions where soils are saturated

• drainage and aeration are restricted

• weathering of some minerals is slowed

• organic matter decomposition is retarded

• wetland soil profiles with special characteristics may develop

how does time influence soil formation

time interacts with other soil forming factors, determining how long processes have acted on the parent material

why do residual parent materials show more soil development than transported ones

because they have generally been exposed to soil forming processes for a longer time

how do soils forming on glacial materials differ from those not glaciated

they usually have had far less time to develop since glacial activity disturbed them

what’s an example of regional soil development differences in canada due to glaciation

• western yukon: never glaciated or only glaciated over 2 million years ago → soils are older and more developed

• much of the rest of canada: glaciated more recently → soils are younger and less developed

what distinguishes matured soils

they have been subjected to soil forming factors long enough to reach full profile development

what primarily controls the character of a soil profile

climate (primary), with influences from parent material, vegetation, topography and time (secondary)

what does a soil profile consist of

a succession of more or less distinct strata (soil horizons)

what does a soil profile reveal

the character of the soil as a whole

what is a soil profile

a vertical section of soil from the surface down to the parent material

what is the end result of long term soil development

the formation of a characteristic, ordered, functioning soil profile

how long does it take for fragmented rocks and debris to develop into a stable soil body

several thousands of years

are soil formation and development isolated or connected processes

they are continuous and interactive, with all environmental factors being interdependent