LEC 19-20 SOILS

soil as a material and support

• mineral & organic matter, water, gases

• living/decaying/dead microorganisms

• bacteria, algae, earthworms, insects, mammals

• provides plant growth support

soil formation starts with (1&2):

1. bedrock → parent material of soil → base geologic material

   1. continuous mass of solid rock (crust of the Earth)

2. regolith → sediment from broken rock

example of regolith (sediment)

lunar regolith

3 types of weathering (PCB)

1. physical (mechanical) → physical changes in parent material

   1. freezing, thermal expansion, wind

2. chemical → chemical interactions with parent material

   1. water, gases

3. biological → organisms break down parent material

   1. tree roots, lichens

isostatic rebound

gradual rising of land masses initially compressed by large ice sheets

karst

landform characterized by dissolution of layers of soluble rock

factors that affect soil formation

1. parent material mineral content

2. climate (temperature, precipitation)

3. topography (slope steepness)

4. vegetation (root activity, protective cover)

5. biological activity (microbes, fauna, decomposition of organic matter)

soil profile

• cross-section of soil composed of horizons

  • horizon → layer of soil

  • layers are distinct in colour, texture, nutrient content/chemical composition

leaching

dissolved particles that move down through horizons

soil pH (soil region associations)

• red = acidic (wetter areas due to leaching and loss of base cations)

• blue = alkaline

• yellow = neutral

soil colour

• indicates composition

• black or dark brown = organic matter

• pale grey or white = leaching

• red = iron (i.e., PEI)

soil texture

• proportion of sand, silt, and clay

  • soil particle sizes → gravel > sand > silt > clay

• particle sizes and soil pore sizes influence air and water passage

  • loam = even sand-silt-clay mixture → most ideal for plant growth

soil structure

• how sand, silt, and clay are held together

• organic matter + minerals aggregate into various shapes and sizes

• good soil structure allows plants to establish extensive root systems

humus

• stable, decomposed organic matter

  • improves physical condition

  • decreases erosion losses

  • increases water infiltration

  • improves tilth

  • increases cation exchange capacity (CEC)

tilth

capacity to retain moisture and nutrients, aerate, drain, resist compaction, and allow roots to penetrate

soil nutrients

• C, P, N, S → nutrient cycles move among biological, atmospheric, and mineral phases

cation exchange capacity

• max number of cations that a soil can hold at a given pH

• influences the soil’s ability to hold onto nutrients and prevent from groundwater contamination

• influences the soil’s buffer capacity against acidification

• depends on humus + clay composition → can take years to increase CEC after adding humus

high CEC =

• protection from

  • soil degradation

  • soil acidification

  • groundwater contamination

• nutrient retention

steps in nitrogen fixation

1. N₂ from the atmosphere is inert

2. NITROGEN FIXATION: N₂ → Root nodules containing nitrogen-fixing bacteria → NH₃ (ammonia)

3. NITRIFICATION: NH₃ → Bacteria → NO₂^- (nitrite) → Bacteria → NO₃^- (nitrate fertilizer)

4. DENITRIFICATION: NO₃^- → Fertilizer used by the roots → NO₂^- → Bacteria → N₂ + NO₂^- (returned to atmosphere)

what does soil do in an ecosystem

• medium for roots

• provides water

• contains nutrients for absorption and ion exchange

• nutrients for bacterial processes

degradation of soil via…DECT

• D: depletion of water and nutrients

• E: erosion

• C: compaction (loss of soil structure)

• T: toxicity (heavy metals, pH, salinity)

soil restoration

• measures to counter soil degradation (DECT)

• increase soil porosity for root growth and microbial activity

• provides a source of organic substrate to retain water and nutrients for plant uptake

E: erosion

• transport and deposition of sediments

• natural (aeolian, water, gravity) but accelerated by human activity

• problem occurs when it happens faster than new soil can be formed

• reduces water retention

• loss of plants, seeds, topsoil

• loss of organic matter

• reduction in soil quality, structure stability and texture

C: soil compaction

• structural attack: soil particle compression → increases soil density

• reduces porosity for air and water to enter and penetrate

• loss of soil biota

• tillage can break up compacted soil but also contributes to erosion

D: nutrient depletion

due to

• erosion

• loss of biota, microbiota, vegetation

• leaching

• agriculture

• lack of nitrogen → temperate plant zones

• lack of phosphorus → tropical zone plants

• excess nitrogen → pollution, atmospheric deposition

T: soil toxicity

• salinity

• soil pH

• heavy metal contamination

soil pH affects…

• ability to support plant growth

• bioavailability of nutrients

• toxicity of some compounds (i.e., Al³⁺)

what happens when soils are extremely acidic?

• increased plant uptake of heavy metals

• decreased plant uptake of critical nutrients (N, P, K)

• result sin Ca and Mg deficiencies

salinization

buildup of salts in surface soil layers

causes of salinization

• irrigation → salty groundwater brought to the surface, evaporates, leaving salt deposits

• road salt

• natural processes → mineral weathering

• causes leaf burn → reduced availability of some nutrients

phytoremediation

bioremediation using living organisms to clean up contaminants

phytoextraction

type of phytoremediation where hyperaccumulators (certain plants) are able to draw chemicals out from the soil and store as biomass

human effects on soil and land

• #1 cause of erosion

• BILLIONS ha of cropland suffer from erosion

• HUNDREDS of MILLIONS $ cost of land degradation in Canada, annually

• HALF crop yield in Africa in 40 years

• MILLIONS of cropland lost annually

• BILLIONS of people affected

activities that cause soil degradation (not DECT)

• deforestation

• agriculture

• chemical contamination

__ of the world’s land has been degraded, __ of which is caused by agricultural production.

1/3, 2/3

desertification causes __ of loss of arable land productivity

10%

desertification caused by

• over-cultivation → nutrient depletion

• overgrazing → erosion and loss of topsoil

• toxicity → improper irrigation leading to salinization

cons of irrigation

• depletes groundwater

• waterlogging (overwatered soil)

• salinization inhibits production on 20% of irrigated cropland → costs BILLIONS $

holistic management

stimulating the behaviours of natural herds improves/preserve soil

land productivity is largely dependent on _______

soil biodiversity (bacteria, fungi, microorganims, worms, protozoa, etc.)

land productivity is critical to provision of ___, ___, and ___ regulation

food, water, and climate

crop rotation

• alternating commercial crops grown in specific fields after the harvest season

• using cover crops to protect soil (not to be harvested/not commerical)

contour farming

ploughing furrows (turning topsoil to remove weeds and other residual roots) perpendicular to hillsides to prevent rills and gullies (types of water erosion channels)

terracing

level platforms cut into hillsides to retain water

intercropping

alternating bands/mixed arrangements of crops to increase ground cover

shelterbelts and windbreaks

planting tall perennials along the edges of the field to slow the wind and reduce aeolian erosion

alley cropping

shelterbelts (tall perennial edging) + intercropping (mixed plant varieties)

reduced tillage

furrows are cut from soil to receive seeds and are then closed to form shallow ploughs

afforestation

planting trees where they have not previously existed to prevent erosion and anchor soil

agroforestry

planting trees + crops (not just trees) to reduce soil erosion

tree plantations are ___, NOT ___!

farms, NOT forests