Soil Geography

• Soil geographers focus on the relationships between soils and landscapes.– How and when were soils formed in a given area?

– How are the physical properties of soils related to

topography, climate, vegetation and fauna?

– How do soils contribute to ecosystem function/health?

• Pedologists are more concerned with the specific chemical and biological properties of soils, though some spatial analysis is still done.

Functions of SoilsSupports growth of higher plantsa. mediumb. nutrient elements

Hydrological regulation a. supplyb. purification

Nature’s recycling system a. role in life cycleb. global climate

Habitat for living organisms mammals, reptiles, insects, bacteria

Engineering medium a. building materialb. foundation

 

Volume composition of a loam surface soil

Soil as a Medium for Plant GrowthPhysical supportanchor root system

VentilationCO2 &O2 for root respiration

Water high water-holding capacity promotes cooling, nutrient transport, turgor & photosynthesis processes)

Temperature Moderation amplitude of temperature wave decreases with depth

Protection from Toxins gas ventilation & decomposition or adsorption of organic toxins

Nutrient Element Supply Dissolved ions: metallic – K, Ca, Fe & Cu; non-metallic – N, S, P & B; Plants acquire nutrients directly – animals indirectly through plants

Regulation of Water Supplies Nearly all water in lakes, rivers and aquifers passes through or over soilsConsider the impact of soil removal on pathway and timing of water delivered to a stream in a mountainous catchment.

Storage in soils, usage by vegetation, seepage to groundwaterGroundwater may take months or years to reach a water body as baseflow.

Water is purified and cleansed while passing through soils.

Contrast with destructive flash flood of muddy water with shallow soil of low permeability

Recycler of Raw Materials Nutrients must be reused to maintain productivity

Environments with poor recycling end up with deep organic layerThe most productive environments have soils that recycle rapidly (tropical rainforest) Organic waste is converted to useful, nutrient-rich humus

Mineral nutrients re-converted to forms useful to plants

Carbon returned to atmosphere as CO2, the required gas for photosynthesis, and an important greenhouse gas

Habitat for soil organisms A handful of soil may contain billions of organisms belonging to thousands of species

How is this possible? Range of niches and habitats (anoxic vs. aerated pores, temperature variation, pH variation etc.)

(a) (b)

Organic matter and plant roots (a) promote the growth of microbesand higher plants.

Soils low in organic matter generally are associated with lowerproductivity and biodiversity.

High organic matter content Low organic matter content

Engineering Medium 

Designs for roadbeds or buildings need to account for soil properties

Poor soil management and population pressure are often citedas reasons for the downfall of greatcivilizations

Is same happening today on a larger scale?

http://www.physicalgeography.net

Silicate clays, ironoxides, aluminium oxides,and calcium carbonatesaccumulate (little organic matter)

Regolith (above bedrock)May be transported (ie., can be distinct from parent material)

Least weatheredpart of the soil profile

Good mix of mineraland organic particles (mainly mineral)

Partially decomposed organic material dominates

SOIL HORIZONS

ELUVIATION

ILLUVIATION

The exposed wall ofa soil pit or road cut is called the soil profile

E Horizonmay be present

(a)

(b)

It is not always easy to differentiate between distinct soil horizons

Taking samples from each levelidentified can help

Topsoil•The organically-enriched A horizon at the soil surface in a cultivated soil•Most nutrient-rich portion of cultivated soils•Contains the majority of plant roots

Subsoil•The soils that underlie the topsoil•Lower in most nutrients•Drainage properties important in determiningsusceptibility to waterlogging and soil moisture stress

Notice the concentrationof roots in the more nutrient-rich, aerated,looser organic layers near the surface

No crop residues orfertilizers

Fertilizers and cropresidues received

Mineral constituents of soils

*

* The smallestclays (<0.001 mm)display colloidal properties, as doesvery fine organic matter

Soil Texture

Particle Size Distribution

Surface area: 24 x 106 m2

Particle Size Differences• Different properties based on the size of the

particles, even if same mineral.

• Function of surface area.2

mm

2 mm

2 mm

4 mm2*6 = 24 mm2

2 um

2 um

2 um

4 um2*6 = 24 m2

Potential surface area within sand grain volume: 24 x 109 m2

LARGESANDCLAST

LARGECLAYCLAST

Note: Most clasts are not square and would not fit together, leaving pore space.

Eg. Clays hold water more tightly than do sands

Later, we’ll learnwhy loamy soilswith a high organicfraction provide themost ‘available’water

Soil texture isof great significance to plant growth

Hand Texturing (see Box 4.2)

• Used to determine the relative contributions of the fine fraction.

• Very useful in the field to determine soil texture.

• Based on physical properties and “feel”.• Sand feels gritty as you can feel the

individual particles. Silts are smooth, and clays are sticky.

Start by Making a Ball1. Falls apart? SAND (or not enough water)

Does not fall apart? Continue by making a ribbon.

2. Will not form ribbon? LOAMY SAND

3. Ribbon breaks <2.5cmSANDY LOAM, SILTY LOAM or LOAM

4. Ribbon moderately sticky, firm, 2.5 – 5.0 cmSANDY CLAY LOAM, SILTY CLAY LOAM or CLAY LOAM

5. Ribbon sticky and firm, >5.0 cmSANDY CLAY, SILTY CLAY or CLAY

Ribbon Test

SANDY LOAM SILT LOAM

CLAY

SANDLowest surface area (weak particle attraction).Won’t hold together unless saturatedLoses water easily

SILT Particles are small enough to hold water well (0.05 – 0.002 mmToo large to feel sticky, just smooth

CLAYClay particles are the smallest (<0.002 mm)Cohesive particles are so small, that they feel sticky.

Why Hand Texturing Works:

Soil Texture

• Different relative amounts of sand, silt, and clay (see soil texture triangle).

• Coarse fraction not considered in texture assessment.– Not important for soil texture.– Important for soil structure.

• Fine fraction describes the soils ability to hold moisture and store nutrients.

Soil Structure

•Particles sometimes remain independent•May also form aggregates

- roundish granules- cube-like blocks- flat plates

•Both texture and structure affect waterand air movement within soils•Important for plant growth

Soil Organic Matter

What is organic matter?•remains of plants, animals and microorganisms•soil biomass (living organisms)•Organic compounds produced by floral and faunal metabolism

Relevance to carbon balance•atmospheric CO2 sequestered by plants andstored in soils•CO2 is also lost to atmosphere via microbialdecomposition

Organic matter as a ‘glue’•plant roots and soil organisms produce glue-like substances•mineral particles are bound by this ‘glue,’resulting in a granular soil structure •causes productive, loose, easily managed soil

Organic matter as a ‘sponge’•Increases volume of water that can be held•Increases proportion of water a plant can use(difference between wilting point and field capacity)

Organic matter as a ‘fertilizer’•primary source of N, P and S •nutrients released as soluble ions as organic matter decays •food and energy source for soil organisms

What is humus?•stable, colloidal fraction of organic matter•acts as contact bridge between larger particles•surface charges hold soluble nutrients•water held tightly when pores small •stimulates plant growth more effectively than colloidal fraction of clays

What is humus?

•stable, colloidal fraction of organic matter

•acts as contact bridge between larger particles

•surface charges hold soluble nutrients

•water held tightly when pores small, especially when soil is dry (see figure)

•stimulates plant growth more effectively than colloidal fraction of clays

SUCTION

Figure 1.21

The Soil Solution•Contains soluble, inorganic compounds that supply elements for plant growth•Organic and inorganic colloidal particles releasethese elements to the soil solution

Acidity vs. Alkalinity•H+ and OH- ions in soil solution•Affects solubility and availability of soil nutrients•pH is the negative logarithm of H+ ion activity(pH=6 has 100 times more H+ ions than pH=8)

(roots grow)

(soil water flows)Nutrientstaken up through hydrophilicchannels(binding sites on protein carrier molecules)

Soil Air•Pores filled either with air or water•High [CO2]; Low [O2]•Effects exacerbated if pore size is smallor if soil moisture is high

Soil Formation

FACTORS AFFECTING SOIL FORMATION

1. Parent Materials (resistance, composition)2. Climate (precipitation, temperature)3. Biota (vegetation, microbes, soil fauna)4. Topography (slope, aspect, hillslope position)5. Time (period since parent material exposed)

Review of Minerals• Basic building blocks of rocks.• All started as igneous rocks (even metamorphic

and sedimentary rocks), but most have

been altered and redistributed at surface.• Chemical composition is a reflection of

environmental conditions & parent material.• Different levels of stability.

– Quartz (SiO2) more stable than Olivine (Mg2SiO4).

1. PARENT MATERIAL

Time for a quick review of Geography 1010/2030 – the rock cycle…

MineralA natural, inorganic compound with a specific chemical formula and a crystalline structure

Examplessilicates (quartz, feldspar, clay minerals), oxides (eg., hematite) carbonates (eg., calcite)

A rock is an assemblage of minerals bound together

• Igneous (solidify and crystallize from molten magma)

• Sedimentary (settling)

• Metamorphic (altered under pressure)

Existing rock is digested by weathering, picked up by erosion, moved by transportation, and deposited at river, beach and ocean sites.

Lithification follows (cementation, compaction and hardening)

Laid down in horizontally-layered beds

Conglomerate largest clastsSandstone sand cemented togetherSiltstone derived from siltShale mud/clay compacted into

rock

Limestone calcium carbonate, bones and shells cemented or

precipitated in ocean watersCoal ancient plant remains

compacted into rock

Any type of rock is transformed, under pressure and increased temperature

• Often harder and more resistant to weathering

• Compressional forces: (i) collision of plates, (ii) rock thrust under crust, (iii) weight of sediment above

Shale Slate

Granite Gneiss

Basalt Schist

Limestone, dolomite Marble

Sandstone Quartzite

Mineral composition affects resistance to weathering

OxygenSiliconAluminiumIronCalciumMagnesiumSodiumPotassium

Percentage by Weight

Most Common Elements

Fe

AlSi

K, Al

K, Al

Ca

Mg

Relativesusceptibilityto weathering

Mineral Residual Products Material in Solution

Quartz quartz grains silica

Feldspar clay minerals silica, K+, Na+, Ca2+

Amphibole (hornblende)clay minerals, limonite,

hematite silica, Mg2+, Ca2+

Olivine limonite, hematite silica, Mg2+

Sample minerals and their products

(SiO2)

PHYSICAL WEATHERING

Rocks broken down into smaller rocks, sand, silt and clay

(i) Temperature (cracking, exfoliation, freeze-thaw)Expansion and contractionDifferential stresses since mineral composition variesCracking or exfoliation may occurFreeze-thaw weathering in temperate and arctic regions

(ii) Abrasion (water, ice and wind)Sediment carried by water, ice and wind abrades

(iii)Plants and animalsRoots enter cracks and pry apart rockBurrowing animals

Frost Wedging

•Adequate moisture•Cracks in rocks•Freeze/thaw cycles

•Adequate moisture•Cracks in rocks•Freeze/thaw cycles

Glacier National Park,USA – formed due to freeze-thaw weathering)

Abrasion bysediments carriedby wind

Freeze-thawweathering

SLATERESISTANT SILICATECLAY MINERALS

MARBLELESS RESISTANT CALCITE

Biological Wedging

• Biological wedging – plant roots penetrate into cracks causing cracks to widen.

• Must have:– Climate hospitable for

plants.

– Adequate moisture and temperature.

Trees (Pinus flexilis and Pinus contorta) growing on very little soil

Roots grow into cracks, prying them apart

Lakeview Ridge, Waterton Lakes National Park

Exfoliation Dome

Removal of pressure of deep burial.

Unloading

Abrasion and Plucking

Glacial ice is not clean…loaded with sediment that abrades the surface.

Transport by Ice

Wind Erosion

Particles of sand and dust wear away relatively soft rock.

More resistant Less resistant

BIOGEOCHEMICAL WEATHERING

(i) HydrationH2O molecules bind to a mineral through HYDRATIONOxides of Fe and Al are common

(ii) HydrolysisWater molecules split into hydrogen and hydroxyl componentsH often replaces a cation in the mineralReleases nutrients (eg. K+) and forms secondary minerals

(iii) DissolutionCations and anions hydrated until they dissociate

(iv) CarbonationAcids such as carbonic, nitric and sulphuric acid acceleratedissolution

(v) Oxidation-reductionFe, Mn and S can be oxidized (loses and electron) in the presence of air and water during soil formationCauses destabilizing adjustments in crystal structureMay be visible as a change in colour

                        

                           

Iron-rich rock weathered by oxidation: Trout River, NL

Photo source: http://www.stmarys.ca/conted/webcourses/GEO/GEO99/pubweather/chemcombined.html

Crustal warping(eg. due to compressional forces) followed byweathering and erosion near surface

Leads to abruptchanges in parent material(complexity),soil quality andeven vegetationcomposition

Parent material sediment can be classified by itsmethod of deposition

Alluvial/fluvial sediments deposited in a floodplain

Alluvial Fans

                                                  

Glacial Deposits

1 – till2 – glaciolacustrine deposits3 – loessial blanket (aeolian)4 – unglaciated (loess)

*nearly all of Canadawas glaciated!

Glaciated, U-shaped Valley

Deposition from Outwash Plain

Aeolian Deposits

Organic Deposits

Stages in peatland formationN.B. Many wetland ecologists now believe that forested peat is not necessarily the final stage!

Mer Bleue Bog,Ontario

Climate• Most influential of the five soil forming

factors over large areas.• Determines the nature and intensity of

weathering.• Greater precipitation = greater degrees of

weathering.• Water percolates through the profile

transporting soluble ions and suspended materials (clays).

Climate

• Water deficiencies can cause problems.– Soluble salts are not carried away.– Over time, these salts can cause salinity

problems.

• What are the dominant climatic characteristics of Lethbridge?

• How do these conditions affect soil development?

http://sis.agr.gc.ca/cansis/taxa/landscape/slc_prairie.html

From de Blij & Miller, 1996, Physical Geography of the Global Environment. Adaptation by M.J. Pidwirny,Okanagan University College

Credit: Government of Alberta, 2002

Different Dimensions

Soil Zones of Western CanadaSoil Zones of Western Canada

Alberta

Saskatchewan

Manitoba

Black

BrownDark Brown

GreyDark Grey

Temperature & Moisture

• For every 10° rise in temperature, biochemical reactions more than double.

• Temperature and moisture influence the amount of organic matter.

• If you have moisture and temperature present at the same time, weathering and leaching are maximized.

• Is this the case in our environment?

Biota

• Biological activity is the primary contributor to the organic constituent of the soil.

• Organisms play a strong role in profile mixing and nutrient cycling. Which ones?

• Grassland soils have large accumulations of organic matter.– Beneficial for moisture retention, nutrient

storage, and defense against fire.

Biota

• Forested soils.– Generally lower in soil organic matter.– Not really necessary as the environment has

plenty of moisture.– Leaves on forest floor are the principal source

of OM.• Very acidic, inhibits the action of soil organisms

used to decompose.

• Most trees can withstand low pH.

Same parent material.

Different environment.

Crotovinas

Topography

• Three essential factors– Elevation, slope, landscape position.

• Can change in response to climate factors.– More gentle slopes in warm, moist climates.

• Causes change in local microclimate.– Different slope aspects.– Lateral changes in soil moisture conditions.

Soil Catena

Poorly developed B

Development of B

Deeply weathered B

• Depressions also have greater depths of weathering.

• Can get the development of very different soils along a slope from top to base.

• Same parent material…just different topographic position / characteristics.

• Milne (1935) recognized this property and called it a catena (chain).

• Steeper slopes have larger amounts of soil loss due to erosion.

• Less complete vegetation cover.• Shallower soil development.• Depressions tend to accumulate runoff of

moisture and sediment.– Not generally connected to external drainage

networks.

Time

• Takes time to form soils.• Difficult property to gauge.• Over what sort of time scales are soil

forming processes significant enough to develop a soil.

• Complex system.• Easier to solve if we can control the time

factor…known disturbance.

Soil Formation in Loess Over Time.

Time: Buried Horizons

Soil Forming Processes

• So we have the five factors…what are the processes that create a soil.

• Also known as pedogenic processes.• All processes are in action, but the

relative importance is variable.• Transformations, translocations,

additions, losses.

SYNERGISTIC INTERACTIONS OF MULTIPLE VARIABLESOVER TIME

Transformations

• Soil constituents are chemically or physically modified.

• Primary minerals are converted into secondary products.

• Decomposition of organic material into organic matter.

• Change of particle sizes.

Translocations

• Movement of inorganic and organic materials laterally within a horizon or vertically from one horizon to another.– Percolation down (vertically and laterally due

to gravity and slope).– Capillary action drawing materials to the

surface.

• Incorporation of surface organic material into A and B horizons.

Losses

• Loss of material due to groundwater flow, and erosion of surface materials.– Erosion affects clays and silts more than sands

Net effect: Leaves a more sandy profile– Agricultural activities can lead to the removal

of large amounts of OM.

The Master Horizons