Midterm Study Guide

Soil profile

vertical section of soils that forms distinct layers

horizon

portion of the soil that can be differed from other layers

parent material

fragmented bedrock or superficial deposit that soil was originally composed of

A horizon

top of the horizons near the surface with highest amount of OM, or removal of material (eluviation)

B horizon

development of soil structure, color change, and deposition of material (illuviation)

c horizon

negligible change form original parent material, close to regolith with no structures developed.

h suffixes

enrichment with organic matter (humus)

m suffixes

change in color, structure, or both compared to c horizon

k suffixes

presence of carbonate material

differentiated factors of soil horizons

texture, structure, color

soil formation processes

illuviation, eluviation, enrichment

enrichment

addition of materials to a soil body

eluviation

removal via downward transport of tiny soil particles (colloids)

4 affecters of soil formation

1. Parent material
2. Climate
3. Organisms
4. Topography
5. groundwater

glacial till deposition agent

glacial ice

glacio-fluvial deposition agent

moving melt-water

glacio-lacustrine deposition agent

stagnant melt-water

eolian deposition agent

wind

glacial-till

* contain coarse fragments
* wave-like topography
* loamy textured
* variable S, Si, C

glacio-fluvial

* coarse fragments, numerous and round
* lower slopes, winding
* level or flat topography
* good drainage and aeration
* sandy, silt, content may be high with some clay

glacio-lacustrine

* few to no stones
* level or flat topography
* high in clay content
* poor drainage/aeration

eolian

* no rock fragments
* sand dunes or hills
* sandy, silt, very fine sand
* good drainage/aeration

texture by pm

* main soil property affected by PM
* affects porosity, water retention, drainage, leaching, compaction, erosion, fertility

pH by PM

* natural presence of finely ground limestone (free-lime)
* main source is calcium carbonate

fertility by PM

* soil high in free-lime do not require Ca, Mg, fertilizers near as much

2 factors of climate affect on soils

1. temperature
2. moisture

Climate: Temperature

* causes biological process increase
* soils in warmer climates develop deeper profiles (with adequate precipitation)

Climate: Moisture

* semi-arid conditions result in slower rate of formation
* soil profiles are deeper in humid regions
* affects amount and kind of vegetation
* affects leaching and eluviation
* major reason for horizon differentiation

Western Canada soils

* precipitation increase and temp decrease increases Ah SOM content. horizon from brown to black soil zones
* brown, dark brown, black and gray soil zones

soil formation affected by organisms

* biotic activity is necessary
* SOM acc. grassland= more SOM underground with thick Ah
* SOM forest= more SOM aboveground

Ah horizon

* granular structure
* high porosity
* organic matter enriched

Grassland soils

chernozemic, higher fertility,

forested soils

luvisolic type

soil formation and topography

* causes differential erosion and moisture content because of difference in elevation and slope
* modifies the effects of macro climates on a site specific basis to create a microclimate due to aspect
* wind exposure
* energy distribution
* erosion: negative and positive

slope

angle which any part of the earths surface makes with a horizontal surface

aspect

direction of slope relative to the points of a compass, effecting temperature and rate of evapotranspiration which regulates moisture content and vegetation type.

toposequence

sequence of different soils down a hill slope

groundwater affects on soil formation

* transports sodium and soluble salts
* groundwater rich in Na+/soluble salts is brought back to soil to form saline soils

time affects on soil formation

* based on degree of weathering, some are young, some are very old
* carbonate presence indicates young soil

Texture does not include:

* gravel, stones, and boulders
* SOM

sand

* round, irregular, medium, coarse, very coarse
* platelike
* large pores
* good aeration and drainage
* low water holding capacity
* low erosion risk
* feels gritty

silt

* irregular
* fragmented
* medium aeration, pores, drainage, water-holding, water-erosion
* feels smooth and slippery> not gritty, not sticky

clay

* platelike
* layered or book-like
* many fine pores
* poor aeration, slow drainage, high water-holding, low erosion risk if aggregated
* feels sticky and easily molded

specific surface area

area per weight in contact with solution, clay has a large area

colloidal behaviour

ions are shelf on surface area of charged particles (high amount of negative charges)

adsorption

* process which ions are loosely held to the surface of charged particles

Na+ effect

* breaks apart aggregates and clay particles
* big molecule that stifles the calcium’s hold

Ca+

* brings together aggregates and clay particles
* Mg does it a bit too

soil structure

* arrangement of soil solids
* includes OM
* S, Si, and C into aggregates via microbial by-products
* peds and clods

peds

naturally formed large aggregates

clods

management formed large aggregates

3 aggregate forming processes

1. biological
2. chemical
3. physical

biological aggregate formation

* microbial activity (influenced by amnt and type of OM in soils)
* vegetation

chemical aggregate formation

* influence by amount and type of clay
* Ca++ bind OM and clay components

physical aggregate formation

* freeze/thaw
* wetting/drying
* root pressure

5 factors affecting soil structure

1. texture
2. freeze/thaw, wet/dry
3. tillage
4. dominant cations in structure
5. SOM content

texture and structure

* aggregate formation requires at least 15% clay

freeze/thaw, wet/dry and structure

* frequent cycles can destroy aggregates

tillage and structure

* puddling formation>soil loses structure due to tillage when wet, leads to crusting when the puddled soil dries

dominant cation and structure

* Ca, Mg, and Al bind bridge clay particles together
* Na+ disperses them and causes columnars

SOM and structure

more SOM = more aggregation

provide water aggregate stability (only factor that does this)

Granular structure

* peds are spherical
* porous to very porous
* aids water infiltration and seed germination
* Ah horizon

platy structure

* thin flat horizontal plates
* can be inherited from PM
* can result from clayey soil compaction
* naturally formed in eluviated horizons
* Ae horizon

angular blocky structure

* illuviated clay
* Bt horizon

subangular blocky structure

* promotes drainage, aeration, and root penetration
* horizon B, may occur in A

prismatic structure

* vertical axis longer than horizontal and spikelike top
* Bm horizon of arid/semi-arid regions

columnar structure

* vertical axis longer than horizontal axis with a flat or rounded top
* Bn horizons high in sodium

Structureless

* no visible aggregations or structural units
* Sand

Massive structure

* solid mass without aggregates
* C or Ck horizons

particle density

\-ratio of soil mass divided by soil solid volume

\-influenced by mineral density in soils

\-compaction, loss of structure, and tillage have no effect

\-assumed to be 2.65g/cm

bulk density

\-ratio of soil mass divided by total soil volume

\-reflects amount of pore space in soil

ideal bulk density values

\-1.2 g/cm^3> for clay

\-1.5 g/cm^3> for loamy sand

root restricting bulk density

\-more than 1.45 g/cm> for clay

\-more than 1.85 g/cm> for loamy sand

bulk density and SOM

* High SOM= Low Pb
* Low SOM=High Pb
* SOM creates granular structures
* ideal pore space is 50% of soil volume

saturation state

all pores are filled with water, has limited usage to plants and reduces soil aeration.

field capacity (FC)

maximum amount of water that can be stored against the force of gravity. water is stored in micro to medium sized pores. ideal conditions for plant growth, aeration, and microbial activity.

permanent wilting point

water is held so strongly in the soil that plants can’t use it

* low risk of subsoil compaction

available water

portion of soil water that can be used by plant roots

readily available water

portion of available water that can be used by plant roots without stressing the plant.

less available water

at this moisture level crops start to experience drought stress

unavailable water

soil water held so firmly to soil particles that it cannot be used by plants

hygroscopic water

corresponds to air-dry moisture

traditional tillage

* multiple passes with a moldboard plow, followed by a disc plow or a disc harrow
* associated with summer-fallow, pest management, and see prep

no-till advantages

* economical gains> less time and fuel
* soil health> improves OM and aggregation
* decreases subsoil compaction, topsoil drying, and soil erosion
* improves infiltration and drainage

no-till disadvantages

* delayed planting may occur with wetter conditions
* reduced seedling emergence
* high herbicide usage
* nitrogen immobilization
* long-term fertilizer required

influence of compaction

reduces pore space and increases bulk density

soil strength

Hardness or resistance to root growth

crusting compaction

* caused by aggressive tillage which reduces soil residue and cover can break apart more easily
* can happen via large amounts of water added via irrigation
* surface seals when dry and forms a _____
* restricts water infiltration and seed emergence

crusting compaction prevention

* keep soil covered with stubble, mulch, or cover crops
* correction via: no-till or reduce tillage, light tillage before spring seeding

subsoil compaction

* happens with wheel traffic vis heavy machinery
* more likely to occur on 100% FC soil or where machine axle load is greater than 10 tons
* produces ruts and compaction

subsoil compaction prevention

* no working fields at 100% soil capacity
* restrict traffic to specific lanes
* reduce machine axle load
* use chisel plow or spring-tooth harrow

penetrometer

* measure soil strength in psi
* measures depth and thickness of compaction
* anything over 300 psi is severe limitation

deep-ripping (subsoiling)

goes 35-50cm deep to loosen compaction in subsoil. not to be completed as an annual process

100 % FC

* use penetrometer
* soil is at high risk of compaction

50-75% FC

* use tillage

40% FC

* perform deep-ripping

Composition of SOM

* 60-90% stable OM (Humus)
* 10-20% active fraction
* 10% fresh residue
* 5% living organisms

climate and SOM

* low temp = high acc, low decomposition
* high temp = low acc, high decomposition

SOM Decomposition

* performed by soil microbes
* nutrients go from organic to ionic or mineral form
* produce organic glue which promotes aggregate formation and stability

SOM nitrification

* performed by soil microbes
* bacteria can oxide elements to nitrate
* ammonium>nitrate