Soil ecology Flashcards

soil structure
Terms Definitions
microboial colonies
Volcanic Ash
Relatively unweathered
Related to Inceptisols
Highly weathered
Low fertility
Acidic, humid tropics
Oxic (Bo) horizon
Residual Fe and Al
Not found in continental US
Agrillic (Bt) horizon
Base saturation over 35%
Typical forest soil of midwest
Grassland soils (farm-belt)
High in OM
Fertile, bas saturation above 50%
Mollic horizon, typically cultivated
more develope than entisols but still young
Finer textured than Entisols with a cambic horizon
Dead organic material undergoing decomposition.
Oi and Oe horizons.
Principles governing exchange reactions
2.Charge equivalence
3.Ratio law
4.Mass action
5.Cation selectivity
Lime as an acid neutralizer
Sources of H+
1.carbonic acid-from rain
2.metabolism-from organisms matter-produces H when it decays
4.uptake-plants often release H+ to balance their charge when they uptake cations.
5.Aluminum (especially below pH of 6)-dissacociates from the clay minerals.
dung beetles
detritovores. enhance nutrient cycling. bring surface nutrients downwards.
Macro detritivore. engineer burrows which naturally till soil. Casts enhance nutrient cycling.
Water-stable protein excreted by a guild of rhizosphere fungi. Coats soils particles, acting as glue to bind aggregates.
Bulk Density
Weight (solids only)/volume (solids and pores)
1.33 is an ideal soil bulk density. Above 1.6 starts to make root growth difficult.
Compaction increases bulk density, influences movement of water and gases.
Factors mediating CEC
1.exchanges are reversible
2.exchanges are charge-for-charge
3.ions in equilibrium with solution (most important)
4.mass action sites are cation selective
pH Buffering
OM increases buffering
finer textured soils are better buffered
Physical and chemical alteration of materials from which soils form. Process for soil formation.
Benefits of Lime
Increases base cations/base saturation
Increases nutrient availability
Improves soil structure
Promotes microbial growth
Minimizes acidifying effects of fertilizers
Reduces metal toxicity to plants
CEC by % clay and O.M.
Soil Classification
1. Organizes knowledge about soils
2.Groups soils with similar properties
3.Provides meaningful basis for soil mapping and management
Diagnostic horizons are key to soil taxonomy
A horizon; Northwest USA
Like mollic but lower in carbonates
Forest soils with thick A horizon
Bw horizon
Found on young, coarse textured soils
Weakly developed, always above C horizon
herbivores. shredders- reduce size of plant matter.
Soil Structure
Arrangement of soil particles. Grouped spatially into aggregates
Aggregates form through physical-chemical processes and biological activity
Aggregates change soil properties
Influences water movement, fertility, pore space, and biodiversity
A horizon; Easter US forest soils
Pale color, lower OM
Low base saturation
Thinner A than mollic and umbric
Hard and massive structure when dried
O horizon
Protects soil from changes in temperature, moisture, and disturbance. Creates cooler, moister, and higher fertility soils.
Tetrahedral sheets
silicon surrounded by four oxygens. One of two fundamental units of silicate clays.
Soil Pore Space
Function of texture and structure. Macropores a function of structure and micropores a function of texture. Small pores generally good for water and nutrient movement. Larger pores good for gas exchange and root growth.
Aluminum hydrolysis
Al(OH)^2+ + H2O <--> Al(OH)2^+ + H+
Al(OH)n^+ + H2O <--> Al(OH)3 + H^+
Method of acid buffering
feed on fungi, bacteria, algae ro are predatory. Release nitrogen from eaten bacteria and control insect pop.s . some parasitic to plants
Five factors of soil formation
Parent Material
Climate (water and heat)
Topography (microclimate)
Organisms (add organic matter, organic acids)
2:1 silicate clay
when each clay layer has one octahedral sheet sandwiched between two tetrahedral sheets.
Smectite and vermiculite (expanding/ Due to water filling in micropores)
illite (mica) and chlorite (nonexpanding)
Iron and Aluminum Oxides
Iron or Aluminum hold cation position in octahedral sheet. Isopmorphous substitution is rare, not much charge.
Base Cations
Acid Cations
Al, H
Soil Formation Processes
Transformation (weathering)
Translocation (Illuviation and Elluviation)
Addition (organic matter or deposition)
Losses (Leaching or erosion)
Weathered Alfisol
Argillic horizon with <25% base saturation
Strongly acidic
Southeastern US forest soils (acid rain may be turning Alfisols of SE Ohio into Ultisols)
Inverted soils
Swelling clays
Crack and open when dry (plowing itself)
Fairly uncommon
Unit of Charge
cmol=atomic weight/charge
Interface between lithosphere, atmosphere, biosphere, and hydrosphere where soil lies.
Recently formed
No pedogenic horizons
Alluvial beech sand; coarse texture
Bo horizon; tropical regions
Highly weathered soils, low cation-exchange
Accumulation of Fe and Al
Poor for agriculture
Feels sticky
Minerals: products of weathered minerals
High water holding capacity
Available water low
Soil Texture
Sand: 2-.05mm
Silt: .05-.002mm
Clay: <.002mm
What is Soil?
A matrix of....
living organisms
dead organic matter
single celled predators. control bacteria populations.
herbivores and detritovores. less beneficial to soil productivity. bring subsoil upwards.
Base Saturation
Total base cations/CEC
Basis for knowing soil fertility. 50% and below is generally bad.
Bt horizon; common throughout SE Ohio
High percent of clay
Required long time to form (geomorphic stability)
Typical of forests (undisturbed)
Octahedral sheets
Central Aluminum or magnesium surrounded by 6 oxygens. One of two fundamental units of silicate clays.
Total soil water potential
gravitational potential+matric potential+hydrostatic potential+osmotic potential
Carbonate Hydrolysis
CaCO3 + H2CO3 <-->Ca(HCO3)2
Ca(HCO3)2 + H2) <--> Ca^2+ +2OH^- +H2CO3
Mechanism of base buffering
Five Master Soil Horizons
O-Plant matter
E-zone of elluviation
B-zone of accumulation
C-zone of least weathered parent material
Material displaced across a soil profile by water
O horizon
Muck and peat
Saturated with water
Thick (> 20 cm)
Very high OM (>12%)
E horizon
Elluvial horizon below A horizon
Light in color, no coating of Fe (all leached away leaving just sand and quartz)
Also forms over long periods of time
If a Bt horizon is present, an E horizon must also be present. (indicative of old-growth)
"A zone of accumulation necessitates a zone of leaching"
Arid and dry
High base saturation
Salt accumulation at depth (Caliche layer like concrete)
Physical Weathering
Reduction in particle size
Freezing and thawing (frost wedging)
Heating and cooling
Water and wind (most prevalent, especially for sedimentary rocks)
Feels gritty
minerals: quarts or other primary silicates such as feldspar
Low water holding capacity
Available water very high
Athens Soil
Fine-loamy, mixed, active, mesic Typic Hapludalf
The stacking of clay platelets with cations such as calcium and sodium. Adding lime will improve soil structure, makes it more granular.
Chemical Weathering
Minerals are decomposed by chemical reactions. Primary minerals into decondary minerals.
Hydration (binding of water to mineral ion; rust)
Hydrolysis (mineral breakdown by water)
Redox (oxidation and reduction)
Complexation (decomposition by organic acids)
Dissolution (water dissolving minerals)
Soil Water
Supports life. Primary mechanism in nutrient transport to plant roots.
Acid leaching soild; Northern US and Florida
Spodic horizon (Bs)
Has an E horizon
Acidic, sandy soils
Blood-red coloring
Acidic, coarse textured forest soil
A or B horizon
Illuvial horizon formed under cultivation (plowing)
Directly below an Ap horizon
Creates unique floral and faunal community
Hydrated ion radius
electrostatic attraction of water to an ion. Depends on ions density and charge. Greater water attraction for smaller ions. Larger radii indicate weaker attractions.
Soil Organic Matter
Generall 5% is important. Improves soil fertility, improves water-holding capacity, and buffers soils against change.
Aluminum acidity
Al^3+ + 3H20 ---> Al(OH)3 + 3H^+
Soil Density
Volume of soil solids to volume of soil. Ideal soil is 50% solids and 50% pore space.
Cation exchange capacity
sum total of the exchangeable cations that a soil can absorb. Expressed as the number of moles of positive charge adsorbed per unit mass.
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