LCC- Soils Final

Features of the Canadian System of Soil Classification (4)

1)Recognizes the concept of soil as a 3-D natural body that develops over time

 

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2)Takes a genetic approach

-it is designed to classify soils on the basis of the processes by which they formed (therefore recognizes the specific pedogenic factors operable) at the site

-therefore the system recognizes real differences and real similarities amongst soils as an effective way of differentiating and delineating specific soil classes

-classification key established on a genetic priority basis

-varies from classification keys established on morphological priority bases

-this concept requires observation and verification of specific diagnostic features, most of which can be clearly read and interpreted in the field

 

3)The system is based on the concept of diagnostic horizons

-these indicate the dominance of one or more soil forming processes due to the action and interaction of specific pedogenic factors

-once the order is identified, the observation and verification of other diagnostic horizons specific to the particular order, great group and subgroup are applied

 

4)Diagnosic horizons are mutually exclusive***

-it will never be both… the conditions are only such that one type can form

Soil Order

Represents the first division within the Canadian System of Soil Classification

-represent MAJOR variations in soil characteristics

-currently there are 10 identified

-based on presence of diagnostic horizons

 

Chernozemic – has Ah greater than 10cm thick

Brunisolic- Btj, Bm (with no Chern Ah horizon) – transition from grassland to forest

Luvisolic– Bt (with no Chern Ah) – alkaline forest

Podzolic– has podzolic B horizon (Bf,Bh,or Bfh) – acidic forest

Solonetzic– Bn, Bnt-

Gleysolic – Aheg, Aeg, Bg – evidence of gleyed solum

Regosolic– No B horizon, C close to surface

Organic – Of, Om, Oh

Cryosolic– z horizons (permafroms within 1m surface)

Vertisolic (vertic v horizon over slickensided “ss” horizon)

Structure of Classification System

Order – 10 categories, specific horizons

Great Group– 2-5 categories

Subgroup – relates to specific sequence of diagnostic horizons

Family– 7 classes, specifies textural class as a broad categories (ie. fine loamy)

Series – thousands of categories, identifies soils with same PM and same geographical area

Phase- identifies additional productivity and management related characteristics

Stepwise method to coplete a soil classification to subgroup level

1)identify al horizons

2)zero-in on specific diagnostic horizon(s).. get order

3)Refer to section of the manual specific to chosen order. Read what soils are and what they are not.

****ALL features must be met!! (unless word “usually” is used)

 

4)Determine Great group, based on book

Determine subgroup

Classification beyond subgroup

Is done to identify:

1)specific management problems or benefits (pros/cons)

2)overall soil productivity

Soil Series use

Soil series is a powerful diagnostic tool used in the interpretation of soil productivity because it implies the characteristics of the soil on the basis of texture and hemistry and relates these to the climate of the area

 

Therefore, soil series directly indicates:

1)TEXTURE

2)CHEMICAL PROPERTIES (fertility, pH, leaching potential, salinity hazard)

 

Soil Zones

Soil zones in W Can are:

1)Brown

2)Dark Brown

3)Black

4)Dark Gray Wooded and Gray Wooded

 

-plus inclusions of organic soils; mostly in poorly drained areas, in northern cool regions

-plus solonetzic soils

 

USE?

1)crops nd crop varieties are recommended by soil zone

2)the optimum yield/productivity potential for zone

-fertilizer recommendations

3)recommended rates of soil-applid herbicide

4)herbicide selection

5)weed distribution

Physical properties affecting soil management include:

1)texture

2)structure and vegetation

3)soil density and porosity

 

 

Also:

-conditions that lead to formation of aggregates

-role of OM in affecting physical properties

-structural management of soils: tillage and tilth

-practical soil management to optimize the physical properties of soils

Non compacted vs compacted soil

NON COMPACTED
-50% volume solids and 50% pore space

-water, nutrients, and air are stored for crop use in soil pore space

 

COMPACTED

-total volume decreases at the expense of pore space

-a loss of pore space means a decrease in the ability to store water and air, and the ability of plant roots to access air, water, and nutrients

Soil Texture

Def: The particle size distribution of the fine earth (mineral) fraction of soils (% sand silt and clay).

**no referece to SOM

 

Effects on Soil Productivity

-more clay means water retention, compaction potential, and slower infiltration/hydraulic conductivity

-also decreased aeration, increased cohesion b/w particles, and tendency for swelling and sinking soils

-also increased fertility (clay prevents leaching, clay richer in nutrients)

 

Ranges of sizes for mineral solid particles in soils

SOLID MINERAL CLASS

-Boulders: over 250mm

-Cobbles: 80-250mm

-Gravel: 2-80mm

 

FINE EARTH FRACTION

-Sand: 2-0.05mm

-Silt: 0.05-0.002mm

Clay: less than 0.002mm

 

Colloidal clay is any mineral particle less than 0.0001 mm diameter

Soil TExture Triangle

-used to identify textural class

These include: S, LS, SL, L, SiL, SiCL, CL, SCL, SiC, SC, C, HC

-just find where %S and C intersect

Significance of Loam

1) Loam is not a soil particle or a soil particle size

2)it is a textural classification that is a mix of S,Si,C that no one particle size dominates

3)not an even mix by mass, rather it emphasizes the most positive properties of each particle without the negative implications associated with each

4)OM content has no effect on placing a soil in the loam textural class

Good vs Bad texture

Is site-specific, depends on:

1)climate

2)topography

3)geographic location

4)specific vegetation/crop in area

 

GNERALLY:

-fine textured soils are most productive in the driest areas (Brown and Dark Brown zones)

-coarse textures soils are more suited to the moist, higher rainfall areas

-medium are a compromise

 

*soil texture is the single property that has the widest range of implications affecting soil performance including physical, chemical (ie. fertility) and biological characteristics

**texture canot be changed on a practical basis on a field scale…. impractical

Bulk Density

-is the ratio of the mass of oven dry soil to total volume of soil (measure of compactness)

 

Db=oven dry mass/total volume (cm3)

 

-usually expressed to 3 sig digs

-normal range is 1.0-1.7

-generally Db greater than 1.80g/cm3 are considered threshold problematic

 

FACTORS AFFECTING Db

1)Compaction**

-can be caused by cultivation or scarification (forms a plow pan below soil surface)

-a plow pan can usually be eliminated by subsoiling

-compaction is single greatest effect

 

2)Loss of Soil OM

-causes direct and indirect (granular soil structure) increase

 

3)Loss of Soil Structure

-puddling is the complete loss of structure

-likely to occur when clay content is high (over 30%) and OM is low (less than 4%

-when puddles soils dry, a crust forms

 

4)coarse texture

-as sand content increases, Db increases (providing everything else remains the same)

-in the NON COMPACTED state, coarse textured soils have higher Db values than textured soils

(therefore “light” and “heavy” soils are poor terms)

 

5)Solonetzic Bnt Horizons

-these have extremely high Db when dry due to deflocculation

 

Ideal Db is b/w 1.25 and 1.50

Compaction at depth is fixed by subsoiling (80-100cm)

-relatively expensive

-avoid this by keeping OM!!

Particle Density

Dp is the mass of soil divided by the volume of soil solids

-it is the mean density of soil particles

 

Dp=oven dry mass/volume of soil solids

-expressed in g/cm3 or Mg/m3

-normal range is 2.60-2.75

-always greater than Db

-when porosity = 0, Db=Dp

 

Affected by OM content (inversely proportional)

-affected by differences in the densities of mineral particles (therefore main variable is OM content)

Soil Porosity

-ratio of the volume of the pore space compared to the total soil volume, usually expressed as a percentage of the total porosity

 

Porosity= volume of pore space /total soil volume * 100

-difficult to get vol of pore space… so…

 

Porosity = (1- Db/Dp)*100

 

-high Db indicates a decrease in mean pore diameter and quantity

 

Significance:

-infiltration rates decrease by a logarithmic multiplier of the mean pore diameter

-under compaction, the rates of hydraulic conductivity (water movement through soil) and infiltration (water movement into a soil) are decreased, y as much as 2 to 3 orders of magnitude or more

-decrease in infiltration can cause excessive runoff, ponding and poor aeration in low areas as well as an increased potential for water erosion

Aggregation and Soil Structure

Structure describes the arrangement of the primary soil particles (S,Si,C,humus) into individual aggregates

 

ped= naturally occurring aggregate in the soil

 

Structure is defined on the basis of 3 criteria:

 

1)Type of structure

-refers to the specific arrangement of primary particles into aggregates

-include: granular (high humus), platy (Ae-ish), subangular blocky (common to B horizons /w-out clay accumulation), angular blocky (Bt horizons /w clay), prismatic/columnar (longer in vertical..Bm to Bnt(large))

-there are also structureless forms of soil aggregation (single grain, amorphous)

 

2)Size of the aggregates (peds)

-fine medium or coarse

 

3)Grade or strength

-weak moderate or strong

-assessed by effort required to crush.. subjective

 

*descriptions of soil structure must indicate all three evaluations to be complete

-exceptions are:

-granular (always weak)

-single grain

-amorphous

Factors affecting Soil Aggregation and Soil Structure (5)

1) Texture (at least 15%C to have structure…. more clay usually means larger and stronger

 

2)OM content… responsible for granular structure

 

3)Dominant cations within the soil (Ca and Mg lead to well aggregated.. beneficial)

-Na in high concentrations causes problems (strong, coarse columnar/prismatic

 

4)Wetting/Drying

-creates lines of weakness in the soil mass and the formation of structure

 

5)Freeze/thaw

-imparts lines of weakness to form soil structure

-may also destroy structure of aggregates left unprotected at the surface over winter (low OM soils, chinook conditions: icnrease the hazard of wind erosion)

 

BENEFITS

-aggregates greater than 0.84mm prevent wind erosion

-also reduces water erosion

-aggregates can decrease soil Db

-aggregates increase rate of hydraulic conductivity

The value of granular soil structure

-soil humus is require for granular structure to form

-humus also provides for water stable aggregates (aggregates can readily reak down under the pounding effect of rain drops (and irrigation); and also the freeze/thaw effect over winter

-sugars and resins in humus preserve aggregates when soil is wet

-humus is the ONLY factor that does this

-effect is noticeable at 2-3%

Soil Consistence

Consistence is the field term that relates to the resistance to deformation exhibited by the soil mass. Also referred to as soil plasticity when wet, toughness when moist, and hardness when dry

 

POSSIBLE DESIGNATIONS

dry:loose, soft, slightly hard, hard, very hard, extremely hard

moist:loose, very friable, friable, firm, very firm

wet:nonsticky, slightly sticky, sticky, very sticky, OR nonplastic, slightly plastic, plastic, very plastic

 

FACTORS AFFECTING CONSISTENCE

-clay content

-dominant types of clay mineral present

-OM content

Tilth

refers to the overall condition of the physical properties of soils as these relate to soil productivity

 

Product of:

1)NATURAL

-soil texture

-OM content

-chemical composition (Ca and Mg vs Na)

-Ca and Mg promote good tilth, sodium causes problematic soil structure and poor tilth

 

2)HUMAN

-frequency and nature of tillage/scarification

-timing of tillage (don’t work when too wet or dry)

-management that promotes soil conservation and the preservation of soil OM

;

3)ENVIRONMENTAL FACTORS
-climatic/weather conditions (frequency and intensity of rainfall around seeding time)

-frequency of freeze/thaw cycles (chinooks)

;

**ideal tilth conditions change according to time of year

Example of Optimal tilth under varying conditions

FALL

-want size of clods to be 10 cm across min… (30cm not too large)

-prevens erosion

-will trap snow

-too big means fall herbicides will get too deep

;

SPRING

-reduced size and number of clods

-ensures a good soil-to-seed contact so that soil water moves to contact with the seed coat by capillary action

-clods should be 2-5cm in size or less

-at no time should clods allow for wind or water erosion (be too small)

Forces Affecting Retention and Supply of Soil Water

Unique molecular structure

-105 degree bond angle… water is a dipole

hydrogen bonding is a separate but equally significant pheomenon

-displays both cohesive and adhesive forces (tension)

adhesive forces are the attraction b/w dissimilar molecules (water and clay)

cohesive is attraction b/w similar ones

matric tension– sum total of cohesive and adhesive forces in porous media

;

Dipolar structure gives rise to 2 practical phenomena:

1)Capillarity (water moves from wet to dry trough capillary pores due to higher matric tension in dry soils.

2)Matric Tension (water is held in the soil due to forces b/w water molecules and the surface area of soil solids

-this is the major mechanism by which water is retained in the soil

;

;

MEASURING MATRIC TENSION

-when soil pores are totally filled with water, the soil is saturated and matric tension is zero

-matric tension is weaker than the force of gravity when water drains (generally in the middle)

gravitational water: water that drains under the influence of gravity

-as soil dries, water molecules are removed and the tension of the ater remaining in the soil increases. Therefore, as soil dries, the water remianing in the pores becomes increasingly difficult to remove from the soil because the matric tenstion increases as the soil dries

;

***see diagram

Methods Applied For Expressing teh Moisture Content Retained in a Soil

1) % water on a mass basis (Pw)

Pw = Moist mass – over dry mass/oven dry * 100

-most common method

-not practical or meaningful on a field scale.. good in lab

-very accurate with simple equipment

;

2) % water on a volume basis (Pv)

-Pv=Pw*Bsg(same as Db)

-not entirely practical in the field… but is easily converted to depth equivalent

;

3) % water expressed as depth equivalent (mm water/depth soil)… depth of root zone

-most relevance/meaning

-Depth Equiv = Pv* depth soil

-depth is root zone….

-for annual species, 1000mm (1m)

-for perennial forage species, 1.5m (1500mm)

-actual depth is dependent on plant species and stage of development

;

*Only a portion of the total water retained in a soil is actually available to plants

Definitions of Saturation, Field Capacity, Wilting Point, and Max Avail. Water

Saturation= The max quantity of water a soil can hold when the pores are completely filled

;

Field Capacity= The Maximum quantity of water a soil can retain after drainage by gravity occurs (assuming no barrier to drainage)

-this more accurately represents max water content than saturation

;

Wilting Point= The moisture content at which the soil has dried to the point that plants can no longer extract oil water and therefore wilt and die

-this is the lower limit

;

Max Available Water= [email protected][email protected]

-then convert to Pv

-then to Depth equivalent (assume 1000mm)

*check out examples in notes

Factors that Control Max Avail Water in Soils

TEXTURE !! (most significant)

-more clay means more matric tension

-water at 1/3 bar tension (field capacity) increases)

-water at 15 bar tension (wilting point) increases

;

However..

% water at field capacity increases more rapidly than that at wilting point

-at soil textures finer than SiL, SiCL, or CL, the percent water content at wilting point increases by a slightly greater degree than the % water content at Field Capacity

-Si, SiCL, and CL classes have the highest degree of available moisture retention while the C and HC actually show a minor decrease in quantity of available water since %water at FC has increased slightly bu t % at WP has increased to a greater degree.

;

FACTORS THAT AFFECT THE QUANTITY OF AVAILABLE WATER IN SOIL

-texture

-OM content

-structure (granular preferred)

-porosity (no compaction)

-salinity (imparts osmotic effect.. salinity increases tension with which water is retained)

Parent Material

-represents the starting material from which soil isformed (also referred to as IC)

can be either mineral or organic in origin

;

-Mineral PM refers to the geological material from which the soil has been generated (C horizon closely esembles the PM of mineral soils)

;

Mineral PM vs Organic PM

-PM is referred to as mineral if the OM content is less than 30%

-PM is referred to as organic if the OM content is greater than or equal to 30%

-organic PMis found on imperfectly ot poorly frained sites in central to northern locations in Can

-organic is the most common PM in West Can

-climate in these areas does not allow for agricultural production

Pedogenesis

Soils form due to the action and interaction of 5 distinct soil-forming factors**

1)Type of PM

2)Climate

3)Organisms

4)Topography

5)Time

Influence of PM in pedogenesis

PM is a soil forming factor which influences many properties of the soil that ultimately forms at a particular site

-when the type of PM varies, the end result is a wide variety of very different soil types, characteristics, and inherent (natural) productivity

***the PM ultimately determines the physical and chemical properties of the soil

;

PHYSICAL AFFECTED BY PM

texture

-in turn, texture affects:

1)water retention

2)erosion (wind severe with high S/Si, water severe when high Si or C)

3)rate of infiltration of watr

4)compaction hazard

;

CHEMICAL PROPERTIES AFFECTED BY PM

1)soil salinity (high salt in PM)

2)Reaction (pH)

3)Fertility (high C means more fertility)

4)susceptibility to leaching (coarse leached of ions

Effects of PM on soil development

1) Texture is affected by type of PM

-Till tends to give medium texture

-fluvial tends to give coarse

-lacustrine fine

-aeoliaan tends to give medium (very fine though- holds moisture well)

;

;

2)Topography

-till gives waves (hummocky)

-fluvial lower in elevation and meanders through landscape

-lacustrine usually fills in topo lows b/w tops of hills… generally flat

**although lacustrine and other types of PM may be relatively thick, in SAB glacial till is usually found somewhere below the unconsolidated material over bedrock

-aeolian PM may be seen as inactive or dormant sand dune

;

3)In southern AB and SK, there are two main general effects of siol PM with respect to the chemical proeprties of soils:

a)PM tends to be high in free lime

b)soils may be saline

Climate as a Pedogenic Factor

Two major components: Temperature and Moisture

;

Effects of Temperature

1)increase in the mean annual temperature causes the rate of soil formation increases

-10C increase, rate of soil formation increases by approx 2X

-warmer usually means deeper soil (when ppt is there)

;

Moisture

-semi-arid to arid conditions result in slower rate of osil formation

-dry regions usually not as deep profiles

;

-therefore very dry and cold means slow rate of formation (Arctic soils take thousands of years)

;

Soil Moisture:

a)indirectly affects soil formation by determining and modifying vegetative cover and other organisms.. (sep. ped. factor)

;

b)directly affects soil formation by causing translocations of ions of compounds in solution as well as extremely small (colloidal) particals of clay and humus downward in the soil profile (eluviation)

;

c)causes differential movemen of water through the soil profile

-cited as major reason for formation of different horizons within profile since these zones experiencedifferent degree of water percolation

Organisms as a Pedogenic Factor

Soils cannot form in the absence of biotic activity. Furthermore, the species distribution and productivity affect the very nature of soils formed.

;

General categories of organisms that have a direct effect on soil development include:

-vascular plants*

-mesofauna

-microorganisms**

-vertebrates/burrowing animals

-humans*

Vascular plants as a pedogenic factor

The major effects of vegetation include:

1)control of the quantity of the aerial portions and plant roots that regulates the effective additions of OM when OM stabilizes as humus

-directly controlled by the quantity and type of veg

;

2)the actual species composition and nature of vegetation is important

-grassland soils favor the accumulation of humus in the topsoil… forest vet results in a much less desirable soil humus content and type

;

-low quantities of OM incorporated into forested soils results in a thin Ah or non-existant Ah

-much greater quantity of the C and H in forest is lost as CO2 (more volatile)

-results in less than ideal physical properties and fertility

-results in low base saturation (this is related to soil acidity and poor fertility)

Microorganisms as a pedogenic factor

;

Main functions:

-decomposition and resynthesis of soil OM (formation of humus)

-the release of available nutrients and nutrient cycling in general… total vs available nutrient… microbes can make available

;

NUTRIENT CYCLING

-specific microbes are an essential vector in “fixing” N2 gas from the atmosphere into soil as NH4

-other microbial species convert ammonium in soils to nitrate, and others are also responsible for incorporating ammonium and nitrate into organic… blah blah… NITROGEN CYCLE

;

heterotrophic microbes: obtain body carbon and energy by oxidizing carbon from organic sources

(also called decomposers)

;

autotrophic microbes: obtain energy by inortanic (mineral) oxidation and reduction reactions and obtain body carbon from CO2 (inorganic)

;

1)and extremely important combination of autotrophic microbes are responsible for converting ammonium to nitrate in N-cycle

2)other autotrophic microbes are responsible for oxidation of elemental sulfur to sulfate which is a major factor in acid rain and soil aidification as well as the reduction of iron and other metal ions under anaerobic/anoxic conditions (ie. the; gleying process under saturated soil conditions)

;

The main families of soil microorganisms include bacteria and actinomycetes (unique to soils)

;

-some soil organisms cause disease in plants and vegetation

;

*nematodes and earthworms are classified as microfauna rather than microflora

*bacteria and actinomycetes have high live biomass in soil than algae, nematodes, and earthworms
-fungi is the highest

Effects of Topography on Soil Formation

The two components are aspect and eleveation

;

Elevation

1)When the difference in elevation is significant, soils at higher elevations resemble those developd under cooler, moisture conditions

2)however, even when changes in elevation are relatively small, the effects on soil formation can still be significant

-the main effect is due to differential erosion and also changes in microclimate

;

*The variation in soil characteristics due to variations in elevation in a conined area is referred to as toposequence

-generally Ah gets deeper the lower you go

-same with OM, however, if you’re going below tree line OM may decrease

 

 

Aspect

-refers to the orientation of the slope and soil surface relative to the points of a compass

-In AB S+W facing slopes are generally drier, grassland, thick Ah, neutral pH, high fertility, better tilth

-In AB N+E aspect are high in effective moisture, forested, Ah degraded, low OM, solum pH is acidic, lower nutrient content, poor tilth

Time as a factor in Soil Formation

Time is often referred to as the passive or inactive pedogenic factor.

-Under drier and cooler climativ conditions, soil formation required more time to reachequilibrium

-the rate of soil building has been estimatd as 0.75mm/yr in central prairies

 

PRACTICAL APPLICATIONS IN INTERPRETING TIME AS A PEDOGENIC FACTOR

-Soil profile characteristics can also indicate the relatie period of time the PM at a particular site has been subjected to pedogenic factors. Over time, soil profiles will reach a terminal point (equilibrium) that can be used as reference point for the “typical” soil profile of the area.

 

Variations from the typcal soil profile, particularly in depth, may be indicative of soil disturbance and/or soil erosion. Following are a number of applied views on using time vs soil profile development in asessing history of soil development

 

1)free lime within the boundaries of the solum indicate insufficient time to have cleared and translocated carbonates to C

-indicates a young, developing soil profile yet to reach equilibrium.

 

CAUTION:

-carbonates in solum may also indicate slow or incomplete drainage of the soil due to an impermeable layer or a perched water table and saturated subsoil conditions that prevent translocation of carbonates in percolating water

 

2)”poorly developed” horizons compared to typical soil profiles in the area

-therefore the horizon characteristics are not as well expressed as compared to soils that hve reached equilibrium with pedgenic factors

 

3)a profile lacking a B horizon… as this requires time to form