Soil Science Midterm

Soil History

Mesopotamia - agriculture stripped the soil

Egypt - nile renewed soil

China - 9 classifications

Soil functions

purification, habitat, nutrient cycling, engineering medium, plant growth

Dirt v Soil v Humus

Dirt : displaced soil

Soil: unconsolidated mineral or OM subject to ClORPT. It shows history. Represented by the peel of a 1/32 apple slice

Humus: rich OM

Pedosphere

aka soil

Interface of all the spheres (atm, bio, hydro, litho)

Horizons

thickness and boundaries vary, not all horizons are always present (ex: E)

O-humus

A-MOST nutrients, drainage

E-leaching

B-deposited minerals, formation from above and below

C-regolith/substratum, larger pieces starting breakdown

R-hard layer, bedrock

Soil Composition

1. Parent Material/Soil Solids - influences texture and structure

2. Organic Matter - biomass, CO2 sequestration which is binding

3. Water - dissolved in/organic substances

4. Gas - (g) sink CO2 O2

FORMATION: Soil Solids

Parent Material

Weathering

Movement

Profiles

Influences

Parent Material

The chemical and mineralogical composition

Crust is largely composed of O2 and Si

Igneous: cooled molten rock, often quarts

Metamorphic: (s) state recrystallization

Sedimentary - reconsolidated MOST COMMON

Weathering

Physical - Temp, abrasion, sheeting, wedging, organismal "disintegration"

Chemical - hydration, hydrolysis, acid, dissolution, redox

Movement of parent material

Water, gravity, ice, wind

Transformation: modified morphology/constituents x<->y

Translocation: lateral or vertical mvmt, often by water

Addition: outside input, vital for O and A

Losses: removal, leaching, erosion

CORPT

Climate

Organisms

Relief/topography

Parent material

Time

Urban influences

Pediologically - Bioturbation (cities build atop one another

Physically - high compaction, abrupt textures

Chemical - contamination due to human activity

Biological - low organic material, atm impacts life

PHYSICAL PROPERTIES

Color

Texture

Structure

Density& compaction

Pore space

Color

hue: coloration

value: greyness

chroma: brightness

Texture

Size distribution: sand (gritty), silt (floury), clay (smooth)

Determines water flow potential, water holding capacity, fertility potential, construction suitability

Combo = loam

CANNOT CHANGE for the most part without time

-Sand

round-angular

LOW surface area, retention, nutrient content

HIGH strength and pore space

-Silt

need microscope to see, not sticky or malleable

quarts is the dominant mineral

HIGH retention but erosive

loess

-Clay

microscopic booklets/flakes that feels sticky

LARGE SA but TINY pore space

HIGH absorption and retention

Properties: shrink-swell, plasticity, water holding capacity, strength

Structure

Spatial arrangement/aggregation of soil particles

Easily changed, only way to improve BOTH water availability and infiltration (also heat transfer and aeration)

type

size

grade

type: granular, blocky(sub/angular), platy (wedge), single grain(massive),

size: fine, medium, coarse

grade: strong, moderate, weak, structureless

Aggregate formation influences

Flocculation of clays : attraction of clay (-) to organic (+)

Shrink/Swell of clays : platelets move closer together when water rises removed, defining aggregates

Biological processes : organisms, OM, tillage

Density

Particle d: Dp : not counting pore space (OM has low Dp)

Bulk d: Db : mass of oven-dried with pore space

high Bd = high weight = low pore space = high strength

Pore Space

macro --> movement

micro --> storage

1) packing 2) intrepid (structure) 3) bio-pores

sorting and packing affect pore space and subsequent permeability

Soil Classification Taxonomy

- Order (sols)

- Suborder (moisture and temp)

- Great Group

- Subgroup

- Family (size, minerals, temp)

- Series (place)

4 Essential properties of water

1) polarity -- bonds with colloidal (clay) surfaces

2) adhesion and cohesion

3) surface tension -- greater attraction to itself than air

4) capillary action -- ad and co > P of column

a) especially important for irrigation

b) capillary fringe wets the soil above groundwater

Movement of water

HIGH PE --> LOW PE

Flow rate increases as spatial gradient of potential increases

Forces affecting PE

Gravity

Matric (adhesion of molecules to grains/capillarity)

Osmotic (attraction to other solutes)

Sign of Potentials (+/-)

+ = saturated (gravitational) influenced by macropores and preferential flow

- = unsaturated (needs F to moves) (osmotic, matric) influenced by texture

Scalar - only need magnitude not direction ~ P

Measuring Water content

gravimetric

neutron probe

porous block

TDR (time domains reflector)

tensiometer

infiltrometer measures rate

Percolation forms a ___

wetting front, often is uneven

Danger of saline soils

The water is attracted to the salts rather than the roots, which are usually more saline than the soil

Soil moisture

Determined by climate and replenishing ability

More moisture is retained with higher OM

Transpiration

Water potential decreases as water moves up through a plant

ET water often comes from subsoil

As soil drys ... potential increases ... potential decreases

Matric - gravimetric

Measuring Soil Wetness

Max Retentive Capacity - fully saturated pore space (gravitational)

Field Capacity - degree where soil will hold the max usable water by plants with aeration (capillary)

Permanent Wilting Percent - water adheres to clay, no access by plant (hygroscopic)

Available water holding capacity =

Field Cap - PWP

What soil texture generally has the highest field capacity

Silt loam

Water cycle, where does it go

Interception

Surface Runoff

Infiltration

a) impacted by timing, vegetation, management, soil properties

PET (potential evapotranspiration)

Amount of water lost through evaporation and transpiration in need of replacement

Influenced by temp, humidity, sun, wind, etc.

Controlling water loss practically

mulch, herbicide, weed control, fallow field, conservation tillage

Irrigation considerations

Depth and distribution of roots

Water retention and potential at root depth

Rate of water use by plants

Available water

Irrigation Methods

Surface Water (getting less efficient)

Sprinkler

Micro-irrigation (highest cost and efficiency, lowest labor)

Irrigation Optimization Methods

Reduce runoff

(terracing, mulching)

Reduce evaporation

(early planting)

Increase water supply

(control weeds, water collection, deeper rooted crops)

Accumulation of soil salts

occurs with little drainage but high evaporation

Drainage benefits and issues

Benefits - earlier planting, removing salts, less (g) accumulation, reduced frost heaving, toxicity, and fungus

Issues - loss of wildlife,(drained wetlands), loss of soil OM, increased downstream flooding and contam

Types of drainage

Surface - ditches, land smoothing, grading

Subsurface - tile lines, mole drains

Fauna sizes

macrofauna >2mm

meiofauna 0.2-2mm (mites, water bears)

microfauna <0.2mm

suspended animation

tardigrades

some nematodes

means they basically hibernate. Both are highly tolerant

Roles of Microorganism

N fixation

Decomp

Growth regulators

Ion and colloid stabilizers and mobilizers

Foundation of food chain

1 meter of soil can have ___

10^9 bacteria - 10^2 nematodes

30-300 earthworms

Conditions affecting microbial growth

moisture, oxygen, temp

C availability

Texture, acidity, salinity

Source of carbon

heterotrophs: breakdown of other materials

autotrophs: CO2, generally are less of them

Source of energy

Chemohetero/autotrophs : biochemical oxidation of C in organic compounds

Photohetero/autotrophs : solar radiation

Ecosystem engineers

make major changes that impact habitat of other organisms

Abundance of organisms

impacted by

- food availability

- physical factors

-biotic factors

-chemical factors

forests have more diverse fauna but LESS abundance than grasslands

Cultivated fields have low richness and biomass

Biomass

related to OM

usually 1-5% SOM dry weight

Hotspots of microorganisms in soil

plant roots

detritus

OM lining earthworm burrows

feces

etc.

Soil metabolism

80% due to microflora

earthworms and termites do heavy lifting

Earthworms

Egg-laying hermaphrodites, eat detritus, change chem composition, burrows are pathways for infiltration and root growth, casts mix and stabilize soil aggregates and increase nutrients

Grouped based on burrows

- Epigeic: surface/litter dwellers

- Endogeic: upped 10-30cm of soil, shallow horizontal burrows

- Anecic: large, make relatively permanent vertical burrows meters down

Termites

Mix soils because they work above and below the ground

Nematodes

roundworms that hang out in water "cryptobiotic resting state"

Increase soil N

1) Predatory 2) Parasitic

Control: resistant crops, fumigation, hardwood bark, marigolds

Soil protozoa

active around plant roots and around the surface

like well-drained soils but swim

can also have resting stages

Soil fungi

measured in biomass and by hyphae

decomposers, nutrient cycling, stabilizing soil, trap nematodes, competition

Mycorrhizal fungi

plant symbiosis - attain C (almost 12-20% more) from plant in exchange for nutrients and water, help form macroaggregates, help with defense

Arbuscular (bust through the cell)

Ectomycorrhizal (penetrates roots but not the cortex cell walls

Soil Prokaryotes (bacteria, archaea)

billion-trillion per gram of soil

Cyanobacteria - photosynthesize

Actinomycetes - final stages of composting

Conditions affecting microorganisms

OM - competition is high

O2, moisture, temp - sensitive

cations and pH - helps establish niches

Benefits for plants

OM decomp

Toxic compound breakdown

Inorganic transformation

N fixation

Rhizobacteria - dense pops near roots