Soils Exam 2

How does the hydrometer method determine soil texture?

sedimentation

How do bulk and particle densities differ?

- Bulk - mass per unit of dry soil (includes pore space) ALWAYS LESS THAN PARTICLE bc of larger denominator including pore volume, increase with soil depth

- Particle - mass per unit volume of dry soil solids (doesn't include volume between particles) - 2.65 constant, increased by iron and heavy metals, decreased by organic matter

How does bulk density affect root growth?

- Root growth is favored by lower bulk densities

- Higher for sands

- High bulk density impedes root growth

What are the effects of soil compaction on particle density, bulk density, and percent pore space?

- Doesn't affect particle density

- Bulk density increases with compaction

- % pore space decreases with compaction

How is percent pore space calculated from bulk and particle densities?

- 100-(bulk density/particle densityx100)

- Soil porosity inversely related to bulk density

How do macro- and micropores compare for soil air and water movement?

- Micropores have low conductance for air and water movement and can limit subsoil aeration

Why is tillage more detrimental to aggregate stability for Mollisols than for Oxisols?

- Mollisol aggregates are formed with the aid of organic matter and are stabilized, tillage aerates and makes microbes more active and they deplete organic matter and deplete stability

- Oxisols have iron and aluminum that build aggregates and aren't affected by tillage

What is soil organic matter and what are its functions?

- The organic component of soil consisting of living organisms (biomass), organism remains (residues), and decay products (humus)

- Energy and carbon source for heterotrophic microbes

- Supplies N, P, and S to plants and microbes

- Increases nutrient holding capacity

- Binds chelates/micronutrients

- Contributes buffer capacity

- Improves soil structure, aggregation, and microporosity

- Enhances water infiltration and aeration

- Increases water holding capacity for sandy soils

- Reduces erosion and compaction

- Promotes root growth

- Decreases evaporation

- Reduces crusting

- Essential to microbes, earthworms, and nematodes

- Major carbon reservoir

How does residue decomposition vary with C/N ratio?

- Microbes need nitrogen to burn enzymes

- Decomposition with low c:n goes fast

- High c:n and microbes are limited by that deficiency

How do soil orders vary in organic matter content?

- Highest organic matter in histisols (peats and mucks)

- Then mollisol and andisols

- Then alfisols

- Then aridisols which are very low because plants cant grow

- Spodisols under pine forests

- Histisols are highest in organic matter (it's an organic soil), mollisols and andisols are next, alfisols, spodisols least

What effect has synthetic N fertilization had on soil organic matter levels in the Morrow Plots?

- Synthetic n fertilizers promote organic matter depletion

- Increased residue cardon inputs but not SOC storage

- SYNTHETIC N DOES NOT REPLACE CROP ROTATION

How do polarity and H bonding affect the properties of water?

- Hydrogen bonding

o Reason water polymerizes and high in boiling point, specific heat, and viscosity

- Greater electron affinity by O than H

- Polarity

o Reason water is universal solvent

What causes capillarity?

- Adhesion - to surfaces

- Cohesion - together

What affects the height and rate of capillary rise in soils?

- Height - pore size/diameter

- Fine texture means higher rise

- Rate - macro

- clays height of rise, sand for rate

How does water flow in relation to potential?

- High energy to low energy

- Matric potential lowers free energy of water and is the most important

What are the four potentials that determine the energetics of soil water? Which one is most important?

- Solute (uptake of water into plant roots), hydrostatic (below water table), matric, gravitational

- Osmotic potential - amount of work needed to remove water from a solution with given solute concentration

- Gravitational potential - removes water from surface

- Hydrostatic potential - below water table

How is soil water classified with respect to saturation, field capacity, the wilting coefficient, the hygroscopic coefficient, and oven-dryness?

- Macro and micro pores filled at saturation and water potential is 0 kpa

- Field capacity - macros have drained, micros still full, matric -10—30

- Wilting co - -1500kpa, plants are struggling to take in water

- Hydroscopic is air dried soil

- Oven dried has only chemically bonded water still present

How is gravimetric water content calculated?

Theta m = wet weight - dry weight / dry weight x 100

What two factors cause infiltration, and how do infiltration rates compare for fine- versus coarse-textured soils?

- Matric potential differs

- Gravity

- Matric effect/potential, attraction of water to soil solids

What distinguishes saturated from unsaturated flow?

- Saturated through macros - matter of liquid water flowing in response to gravitational from higher to lower - gravity is driving force

- Unsaturated is still liquid water moving from thick films to thin films, no longer just a flow, its films of dif thicknesses, have to be physically connected - matric potential is driving force

Why and how is water flow restricted in stratified soils?

- Sand lens

- How does that effect water flow? Water moving down above sand lens, combination of matric and gravity

- When reaches sand, no longer attracted to soil solids and instead piles up above it and only then will water drain from saturated flow

How does water move by vapor equalization?

- Hydroscopic coefficient

- All just water vapor moving from high potential to low potential from warm to cool

How do texture and organic matter content affect soil water availability?

- Texture

o Water holding capacity (lowest for sands, highest for clays)

- Organic matter

o Increasing water-holding capacity

Stabilizes soil structure to increase infiltration and macroporosity

How has water management changed over time, and what is the major use today?

- Used to settle near water, harvest rain, live as nomads

- Now adapt to environment, irrigate arid land, build cities in desert, deplete aquifers

How is water balance estimated for a watershed?

Precipitation=Soil storage + evapotranspiration + discharge

How do foliar interception and runoff compare for a deciduous forest versus a corn field?

- Cornfield has greater runoff compared to forest, expecially early in growing season

- Forest has greater interception and takes place over long period of the year and have more complete coverage of soil surface

How does cover cropping help to increase soil water storage?

- Root infiltration which opens pores

- Input of organic residues

- Important for building aggregates

- Surface cover protecting surface from beating of rain drops

What is water use efficiency, and how does it compare for arid versus humid regions?

- Plant dry matter produced per unit of water

- Much lower for arid than humid regions

How can evaporation and transpiration be controlled?

Transpiration

o Weed control

o Fallow in dryland cropping

Evaporation

o Shading soil surface

o Organic mulches

o Plastic mulches

o Stubble tillage

What are furrow, basin (flood), sprinkler, and drip irrigation? Which is most efficient, and why?

- Furrow-ditch, running between rows of crops in field graded for slope

- Basin, grow rice, levies around field to flood

- Sprinkler, applying water through air above ground

- Drip, low flow small emitter lines, water right around plants (MOST EFFICIENT)

How do surface and subsurface drainage differ? Which is more important in Illinois?

- Surface - ditches (mode of drainage targeting water coming in and removing it before it infiltrates)

- Subsurface (tile lines, intercept water moving down profile and take it away) vastly common and important here in IL

How does soil type determine the agricultural need for tile drainage?

- Soil is hydric climatic regime is really wet and water saturated, need to drain in order to farm

- How wet is the soil

How does subsoil permeability affect tile spacing and depth?

- Soil has to be permeable enough to move water into tile line so you'd have to put them close together and shallow

How are soils aerated?

- Diffusion - movement of an individual gas in response to its own partial pressure gradient

- Gain o2 loss co2

How and why does soil air differ in composition from ambient air?

- Three ways

- Less oxygen and more co2 and more water vapor

What changes occur when a soil is waterlogged?

- Dissolved o2 decreases

- Eh decreases

- Anaerobic microbes utilize terminal electron acceptors other than o2

- pH increases

What is redox potential (Eh), and how does it relate to soil aeration?

- Redox potential and its relation to soil aeration - measure of electron availability, great demand for electrons

- Demand is low and its anaerobic

- Measuring electron availability

- Lots of electrons not used is a negative redox potential

How does soil temperature affect aeration status?

- Biological activity and aeration

- Warmer microbes more active, use more 02 and produce more co2

How do plants adapt to poor soil aeration?

- Form aerynchma

- Aerate roots

- Close their stomates (what they do when they have a problem)

What are the two fundamental requirements that define a wetland soil?

- Soils are water saturated near the surface

- Become o2 depleted from plant and microbial growth

What are three properties that can be used as hydric soil indicators?

- Organic matter (wetland soils, histisols)

- Take on distinctive color, low chroma, dull grey

- Formation of nodules due to manganese reduction

How does frost heaving occur?

- Physical where ice forms in upper profile when ice lenses form that grow because they attract liquid water from around to expand

- Ice less dense than liquid water

- Exerting pressure upward

How effective is solar radiation for heating the soil?

- Effective enough

- 10% heats soil, rest is lost

- Only daylight hours

How is soil heating affected by albedo and aspect?

- Albedo refers to reflectivity - high albedo is reflective, light soil, loses more sunlight energy back to atmosphere

- Low albedo - reflects less and keeps more

- Aspect is slope relative to sunlight

o Perpendicular is maximized heating

How do growing plants cool the soil?

- Transpiration

- Shading effect but transpiration is key where they consume energy from soil to transpire so cooler soil from losing energy

How does evaporation cool a wet soil?

Water is returning to the atmosphere

What can be done to manage soil temperature?

1. Cover soil with organic mulch, shading that cools but warms in winter

2. Use of plastic mulch - clear plastic (warm as greenhouse)

3. Drain soil to reduce water content or use ridge tillage and put plants on ridge

What are the two sources of negative charge on clay minerals?

- Surface of OH groups that can lose proton and become negative charge

- Isomorphous substitution - permanent charge

Classify the following as 1:1, 2:1, or 2:1:1 clay minerals:

Kaolinite =1:1

Montmorillonite =2:1

Vermiculite =2:1

Illite =2:1

Chlorite =2:1:1

Is this a 1:1 or 2:1 clay mineral, and where is the isomorphous substitution? - look for parenthesis and x and y's, that's where you have it

(Al2-xMgx)Si4O10(OH)2·nH2O - Montmorillonite Al2 Si4 (2:1 mineral)

Al4Si4O10(OH)8 - Kaolinite Al4 and Si4(1:1)

[K2-x(Ca,Mg)x](Si4-yAly)Al2O10(OH)2·nH2O - Illite tetrahedral is parentheses 2-1 mineral twice silicon

Mgn(H2O)6(Si4-xAlx)(Al2-yFey)O10(OH)2·nHO - Vermiculite 2 parenthesis si4 al2 (2:1 mineral)

Is cation-exchange capacity (CEC) directly related to unit-layer charge?

- Illite vs montmorillonite

- Illite has highest unit layer charge but much lower CEC

- NO - exposed surface area is just as important as charge

For what kind of soils would hydrous oxides be an important source of CEC?

- oxisols, andisols (wetter?)

- occurs in oxisols and utisols but not IL soil

How is CEC measured?

- Don't need all the steps

- Ammonium saturation

Which cation is held most tightly to exchange sites? Which is held least tightly?

- Ca2+

- Mg2+

- K+

- Na+

- H+

o Most is Hydrogen - small size, accumulates over time

o Least is sodium - so much water around it, its so large

Which type of soil colloid is highest in CEC? - Clay minerals, Hydrous oxides, Organic matter

o Organic matter high in CEC

o Histosols

Which type of soil colloid is highest in anion-exchange capacity (AEC)? - Clay minerals, Hydrous oxides, Organic matter

o Hydrous oxides - net pos charge that strongly attract neg ions

o High in Altisols

o Illinois is very limited in AEC

How does soil acidity affect CEC and AEC?

Higher pH increases CEC but decreases AEC

How does soil weathering affect CEC and AEC?

altering mineral composition and pH

AEC increases

CEC decreases

How are nonpolar organic compounds retained in soils?

- Get retained by dissolving into organic matter

- They just want to get away from water

- absorption