Soil Science Exam 3

exam 7/30/25

Land Degradation

Reduced productive potential & diminished capacity to provide benefits to humanity

• linked to desertification, the spreading of desert conditions caused by overgrazing, felling of rainforest trees, and inappropriate agricultural practices

Erosion

transformation of soil into sediment

Geological Erosion

takes place naturally, natural leveling process

Human-Accelerated Erosion

• occurs when people disturb the soil or natural vegetation by grazing, cutting forest, plowing, or tearing up land for construction

• ~10 to 1000x more destructive than geological erosion

• particularly problematic on sloping lands

• in US, 4 billion Mg soil moved annually by soil erosion (2/3 by water, 1/3 by wind)

On-Site Damages from Erosion

• loss of soil

• remaining soil less fertile

• declining soil productivity or expense to restore it

Off-Site Damages from Erosion

• water quality impairment

  • sediment loading & turbidity (cloudiness)

  • nutrient loading & eutrophication

• increased road and drain maintenance

  • buried roads

  • filled-in drainage ditches

• health hazards from breathing small particles

Mechanics of Erosion

1. detachment

2. transportation

3. deposition

Raindrops

• large ones reach about 30 km//h

• explosive force causes

  • soil detachment

  • destruction of granulation

  • transportation of soil

• impact is more damaging than flowing water

A = R K (LS) C

Universal Soil Loss Equation (USLE)

Annual Soil Loss (Tons/A)

A (USLE)

Rainfall Factor (Intensity & Frequency)

R (USLE)

Soil Erodibility Constant (Texture, Structure, Organic Matter)

K (USLE)

Slope Length

L (USLE)

Steepness

S (USLE)

Crop Management Factor (e.g., cover)

C (USLE)

Erosion Control Practices

P (USLE)

T Value or Soil Loss Tolerance Value

• maximum amount of soil loss in tons/acres/yr that can be tolerated and still permit a high level of crop productivity to be sustained economically & indefinitely

• usually values range between 1-5 tons/acre/yr

Soil Creep

5-25% of total movement by wind

Saltation

50-90% of total movement by wind

soil moisture, wind velocity & turbulence, surface roughness, soil properties, vegetation, soils that are most susceptible

What are the factors affecting wind erosion?

E = I C K L V

Wind Erosion Equation (WEQ)

annual soil loss due to wind erosion (Tons/A)

A (WEQ)

soil erodibility factor (soil characteristics)

I (WEQ)

climatic factor (wind velocity, soil T, precipitation)

C (WEQ)

soil-ridge roughness factor (cloddiness, vegetative cover)

K (WEQ)

width of field factor (width downwind between wind breaks)

L (WEQ)

vegetative cover (mature of cover - living, dead, standing, flat on ground)

V (WEQ)

soil moisture, conversion tillage, wind barriers (shelter belts)

How can we control wind erosion?

Conservation Reserve Program (CRP)

• government programs that pay farmers to shift some land from groups to grasses or forest to reduce erosion

• targets highly erodible land (HEL) and environmentally sensitive areas

Photosynthesis

consumes CO2 and H2O and releases O2 and produces sugars

Respiration

consumes O2 and releases CO2, H2O and energy

Mass Flow

movement due to different total pressure

Diffusion

movement due to difference in partial pressure

Specific Heat

amount of energy to raise T by 1°C per g

• soil water > soil solids » soil air

Thermal Conductivity

movement (or rate) of heat through soil

• influenced by

  • water content

  • compaction

• soil solids > soil water » soil air

Redox

affects the oxidation state of elements, the name of the chemical or biochemical reactions that change oxidation states of elements

Redox Potential

the measure of the tendency of electrons to be transferred

Oxidizing Agent

substance accepts e- easily, will be reduced in the reaction

Reducing Agent

substance donates e- easily, will be oxidized in the reaction

Hydrologic Cycle

the process of cycling water from the earth’s surface to the atmosphere and back again

Evaporation

conversion of liquid water into water vapor

Transpiration

process of water movement in a plant and eventual evaporation from the leaf to the atmosphere

Evapotranspiration

combination of both evaporation and transpiration

Watershed

area of land drained by a single system of streams and bounded by ridges that separate it from adjacent watersheds

P = ET + SS + D

What is the water balance equation?

precipitation (may also represent irrigation)

P (water balance equation)

evapotranspiration

ET (water balance equation)

soil storage

SS (water balance equation)

discharge (drainage or runoff)

D (water balance equation)

Runoff

• liquid loss of water from soil

• waters erode soil

• remove sediment and nutrients

Infiltration

leads to

• percolation

• soil storage

Percolation

formation of water table, drainage water

Potential Evapotranspiration (PET)

the amount of water vapor that would be lost from a densely vegetated soil if soil water content were continuously at an optimal level

Transpiration Efficiency

dry matter yield per unit water lost to transpiration

ET Efficiency

dry matter yield per unit of water lost to evapotranspiration

Transpiration Ratio

kg water transpired to produce 1kg of dry matter

Mulch

any material placed on the soil primarily for reducing evapotranspiration or controlling weeds

Organic Mulches

sawdust, manure, straw, compost, leaves, crop residue

Leaching

movement of inorganic and organic chemicals through the soil with percolating water

handles large volumes of water, low installation cost

What are the pros of surface drainage?

erosion, high maintenance cost, interference with land use flooding

What are the cons of surface drainage?

soil is wetted, low maintenance cost

What are the pros of subsurface drainage?

leaching, high installation cost, flooding

What are the cons of subsurface drainage?

Groundwater

saturated zone that lies above a layer of impermeable rock or clay

Water Table

upper surface of groundwater

Vadose Zone

unsaturated zone above or below groundwater

Solution

• < 1 nm

• usually clear/colored

• homogenous

• transparent

• no settling

• no separation by filtration

Colloidal Suspension

• 1-100 nm

• cloudy

• homogenous or heterogenous

• translucent, opaque, or transparent

• no settling

• no separation by filtration

Suspension

• > 100 nm

• cloudy

• heterogenous

• often opaque, but can be translucent

• settles

• separate particles by filtration

small size, high surface area per unit of mass, negative surface charge, absorbs ions & water

What are the properties of soil colloids?

layer silicate clays, Fe/Al oxides, amorphous silicates (from volcanic ash), humus

What are the types of colloids?

Layer Silicate Clays

smectite, vermiculite, mica, chlorite, kaolinite

Fe/Al Oxides

gibbsite, geothite

Amorphous Silicates (from volcanic ash)

allophane

Phyllosilicates (Layer Silicate Clays)

dominant inorganic colloids in most soils

• layer-like crystalline structure

• negative charges

• alumino silicate minerals

Fe & Al Oxides

• common in highly weathered Ultisols & Oxisols

• significant quantities in some Alfisols & Inceptisols

Allophane & Imogolite

• non-crystalline silicate minerals

• shape: hollow spheres or tubes

• known for phosphate adsorbing capacities

Humus

• highly charged molecule, surrounded by cations

• structure is NOT crystalline

• made up of chains, rings, branches of C bound to H, O, and N

Silica Tetrahedral

Si surrounded by 4 O

Octahedra

cation (Al, Mg most common) surrounded by 4 O and 2 OH

Tetrahedral Sheets

• all tetrahedral point in the same direction

• basal oxygen are shared

1:1 Type Minerals

• 1 Si sheet

• 1 Al (or Mg) sheet

• adjacent layers held together through H bonds

• effective surface limited to external surface

• tends to be less plastic & less sticky than other clays

• includes Kaolinite

2:1 Type Minerals

• 2 Si sheets

• 1 Al (or Mg) sheet

• layers held together through different mechanisms

  • cation bridging

  • hydrated cation bridging, water bridging

• some are expanded and some are not

• minerals

  • smectite

  • vermiculite

  • mica

  • chlorite

Smectites

• high interlayer expansion

• hydrated cations in interlayer

• shrink when dry & swell when wet

• particle size is very small

• montmorillonite

Vermiculites

• some interlayer expansion

• hydrated cations in interlayer

• shrink/swell less pronounced than smectite

• particle size is larger than smectite, but smaller than kaolinite

• Mg is common in interlayer

Micas

• non-expanding, plate-like structure

• interlayer occupied by tightly bound K

• no shrink-swell

• higher charge than smectite & vermiculite

• high charge keeps interlayer ions bound tightly

• particle size is larger than smectite & can range from sand/silt fraction

• muscovite & biotite

Chlorites

• Mg-oxide interlayer

• sometimes classified as 2:1:1

  • 2 Si sheets

  • 1 Mg (or Al) sheet

  • 1 Mg oxide sheet in interlayer

• low Al content, basically Fe & Mg

• particle size is similar to fine-grained micas

Permanent Charge

constant charge, created by imbalance due to imperfections in crystalline structure

pH-Dependent Charge

variable charge, created due to broken edges or functional groups that are protonated depending on pH; all minerals & humus have pH-dependent charges

2:1 type clays

What has permanent charge?

humus, 1:1 type clays, Fe- & Al- oxides, allophane

What has pH dependent charge?

Isomorphic Substitution

during mineral formation, sometimes a cation is substituted for another similar size, but not necessarily the same charge

Cation Exchange Capacity

ability of a soil to retain exchangeability (i.e., weakly held) cations

Anion Exchange Capacity

ability of a soil to retain exchangeable anions

Ion Exchange

exchange of cations from surface to solution

• rapid

• reversible

• charged based

Percent Base Saturation

the % of the CEC due to basic cations

Outer-Sphere Ion Adsorption

• held relatively loosely

• exchangeable cations

• usually ions are hydrated

Inner-Sphere Ion Adsorption

• direct reaction with surface forming a complex with the surface

• non-exchangeable

• ions are not completely hydrated

Active Acidity

• H activity in the soil solution

• what is measurable with a pH meter

• very, very small pool of H

• determines the solubility of many substances

Exchangeable Acidity / Salt-Replaceable Acidity

• associated with exchangeable Al and H ions

• typically found in highly acid soils

• edge of mineral

• released by unbuffered salt solutions, e.g., KCl or CaCl

• acidity caused by release & hydrolysis of Al to form additional H

• approximately 100x greater than active acidity in moderately acid soils

• weakly held