soil science test 4

How much CO2 and O2 in the soil air realative to the atmosphere.

soil air has higher CO2 and Lower O2

What is oxygen availavility determined by? (O2 rates of soil) Soil Air

Macroporosity- More macropores= more easily O2can be replenished = more O2 available

Water content- Less water = fewer blocked pores for gas exchange = more O2available

/\ these 2 together make Air-filled porosity

O2 consumption rate- (i.e., number of organisms in soil)

• Fewer organisms = less O2consumption for respiration = more O2available

What 4 things happen with excess water in soil?

-anaerobic conditions

-Some nutrients become less available

-some pollutant toxicity changes

-soil color becomes more grey

Specific heat

-Amount of energy to heat soil

-SpH of water > soil> soil air

-wet soils are harfer to heat than dry ones

-wet soils store more heat

Thermal conductivity

-movement of heat through soil

-(highest) Soil> water> soil air

-particles with close proximety conduct heat

-water conducts heat

-wet compacted soils conduct heat well

Temperature moderation and depth

-The deaper the soil, the less seasonal temp change.

-Depth delays T changes

Out of micropores and macropores what is likly to be filled with what?

Macropores are more likly to be folled with air

since micropores are filled with water

How to improve aeration in soil?

-improve aggrgation ( add calcium for flocculation)

-improve tillage

-reducing compaction

What can be slowed by lower temperature?

Plant growth

plant uptake of water

decomposistion of OM

Nutrient cycling

Hydolic cycle

process of cycling water from warth to atmosphere and back

What is evaporation and transpiration

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

eveotranspiration.

Water balance equation

P = ET + SS + D

P = precipitation (may also represent irrigation)

ET = evapotranspiration- Depends on vegetation, soil cover, temperature, humidity

SS = soil storage- Depends on infiltration rate, soil moisture state, topography, microrelief

D = discharge (drainage or runoff)

What causes run off?

•Depends on

1• Precipitation intensity & duration (timing of snowfall) •Vegetation type

• Soil properties

1• Porosity, BD, compaction, Texture/structure

2• Water holding capacity

3• Temperature/Season

How to encourage infiltration?

• Cover crops

• Conservation tillage

• Strip cropping

• Furrow dikes

• Contour rows

• Terraces

•Any process that increases OM and improves structure

• Subsurface drainage

Potential Evapotranspiration (PET)

Amount of water vapor that woild be lost frim a densly vegetated soil IF soil water content were optimum.

What does PET Potential evapotranspiration depend on?

-temperature

-relative humidity

-cloud cover

[-wind

Transpiration efficiency

Dry matter yeild/unit water lost to T

ET efficency

dry matter yeild per unit water lost to ET

Transpiration ratio

kg water transpired per 1 kg dry matter biomass

How to control ET? evap and transpiration

•Manage crop

•(Irrigation)

•Limit plant nutrients

•Sow fewer seeds

•Plant so there is some kind of shading

•Eliminate weeds

•Terminate cover crops sufficiently early

•Mulch

Factors that affects leaching

-Solubility

-Texture, structure

-Frequency & Amount of percolate

-soil pH

-Soil fertility status

What is a consequence of excess runoff (leaching to water?

Eutrophication

What are the 2 ways to remove excess water? and the pros and cons

Surface drainage

-Handles large volumes

-Low installation cost

-More prone to erosion

-High maintenance cost

Subsurface drainage

-Provides percolation

-low maintenance cost

-high rainfall can lead to leaching and flooding

-high installation costs

Total water potential = ?

Gravitational, matric, and osmotic

What are the 4 types of colloids?

Phyllosilicate clays, Fe/Al oxides, humus, Allophane

What are the properties of a colloid?

-small size

-Large surface area

-typically negatively charged

What 5 minerals are Phyllosilicates?

smectite, vermiculite, mica, chlorite, kaolinite

What Phyllosilicates are 1:1 and 2:1

Kaolinite = 1:1, smectite, vermiculite, mica, chlorite = 2:1

Which Phyllosilicats are expanidible?

Smectite & vermiculite

Interlayers of each of the phyllosilicates Kaolinite Mica Chlorite Smectite Vermiculite

• Kaolinite = H-bonds

• Smectitite and Vermiculite = hydrated cations

•Mica = K+

• Chlorite = Mg-oxide

Phyllosilicates with a pH dependat souce charge. and how is it caused?

• All colloids

• Caused by broken edges that can protonate based on pH

Phyllosilicates with a permanent source charge? and how is that caused?

• Only 2:1 colloids (smectite, vermiculite, mica, chlorite)

• Caused by isomorphic substation (substitution of one

cation for another in the crystal structure)

Order these in CEC of kaolinite, smectite, mica, vermiculite, Humus Fe/Al oxides, chlorite

Humus > smectite/vermiculite > mica/chlorite > kaolinite >

Fe/Al oxides

CEC defined as

“the sum total of the exchangeable cations that a soil can absorb

(it reprensents the number of charges NOT the number of ions)

Important cations

Al, Ca, Mg, K, Na, H

What is cation adsorption determined by?

1. strengh of adsorption; Al3+ > Ca2+ > Mg+2 > K+ = NH4+ > Na

2. concentration in solution

CEC exchange properties

rapid, reverseable, charge based.

Regional effect of CEC

•Humid regions – higher weathering, more Al and H (acidic)

•Arid regions – lower weathering, more basic cations

CEC of colloids (specific numbers not necessary, but relative to one another)

Which has the most CEC?

•Humus > smectite/vermiculite > mica/chlorite > kaolinite > Fe/Al oxides

CEC conversion rates

1 mol= 100 cmol= 1000mmol

cmolc = cmol * charge of the ion

mg to mmol- mmol = mg / Atomic Weight (AW)mg​

How to estimate the CEC from cations?

add up the cations values to get the estimate CEC

How to estimate the CEC from the mineral composistion of the soil?

Weighted average

How to calculate base sateration from exchangeable cations

%BS= (CEC due to Ca, Mg, K, Na,/CEC) *100

only the bases mentioned above. Add them together. and divide by the CEC of everything including the nonbases.

Anion exchange capacity

Similar to CEC, except with anions

• Represents the exchangeable anions adsorbed on soi

not commonlly used

most soils have low or no AEC

Which clay particles will always exhibit some CEC? Why?

Anything with permeant charge (2;1 clays)

Which clay particles are likely to have AEC when pH is low? Why?

iron alumion oxides

Definition ph

• Negative log of hydrogen ion activity or concentration in solution

•How does pH affect soil

Plant and microbial growth

• Nutrient availability/toxicity

• CEC

• OM formation/decomposition

• Fate of pollutants

• Aggregate stability

• Movement of air/water

pH Dependent Charge only

1:1

• Oxides

• Humus

Dominated by some Permanent Charge

(PC mostly neutralized)

• Mica

– Neutralized by K+ in interlayer

• Chlorite

– Neutralized by Mg-oxide layer in interlayer

Dominated by Permanent Charge

– Vermiculite

• Moderately high permanent charge

• Exchangeable cations neutralize charge

• Shrink-swell

– Smectite

• Moderate permanent charge

• Exchangeable cations neutralize charge

• Shrink-swell

the principal source of negative charge on smectite is due to?

-isomorphic substitution

The majority of the charge on 1;1 aluminosilicates minerals arise from?

hydrogen dissosciation from particle edges

Active acidity

H+ activity in the soil solution

Exchangeable Acidity

• Exchangeable Al3+ and H+

• Released by CEC reactions

Residual acidity

Al and H that are components of soil

particles

• E.g., H+ from pH dependent charge

• E.g., Al released as mineral weathers

pH Buffering

pH > 7

• Carbonates

pH 5.5 to 7

• Surface-bound Al

• Reflected by CEC

pH 3.5 to 5.5

• Exchangeable Al3+ and H+

pH < 3.5

• Free strong acid production (FeS2  H2SO4)

pH > 7

• Poorly availability micronutrients (Fe, Zn, Cu, Mn)

• Usually soils are calcareous/have CaCO3

pH 5.5 to 7

• Maximum P availability

pH 3.5 to 5.5

• Exchangeable Al3+ can be toxic

• Plants have difficulty with nutrient uptake

pH < 3.5

• Exchangeable Al3+ can be toxic

• Plants have difficulty with nutrient uptake

Al is more acidic than H

• More Al = more acidity

•Ratio of Al to H

• 10:1 is more acidic than 1:10

acidity and salts

Neutral salts typically lower pH

•Salts can appear by

• Fertilization

• Seasonal differences in leaching

name a couple weak acids

• Carbonic acid

• Rainfall

• Respiration

• Acid rain

• Root/microbial exudates (organic acids)

• Acidic groups on organic matter

•Nutrient sources of acidity

• Nitrification

• Plant uptake of NH4

+ and basic cations

• Leaching of basic cations

What can caise acidity in crop production?

• Crop removal

and fertilizers

What kind of minerals can cause acidity?

sulfur compounds

Reactions that consume acidity

(i.e., promote higher pH)

Nitrate

• Plant uptake

• Denitrification

Weathering of soil minerals

• Release of basic cations

Climate

• When ET > rainfall

Common types of lime

• Calcitic lime

• Dolomitic lime

• Burned lime/quicklime

• Oxides of Ca and Mg

• Hydrated lime

• Hydroxides of Ca and Mg

•Reaction of lime in soil

CaCO3 +H2O → Ca2+ + CO3 2-

• Ca2+ displaces any exchangeable acidity

• Carbonate reacts with H+ and eventually forms CO2 and water and is release

Neutralizing power of CaCO3

• 1000 lb/a CaCO3 will neutralize 1 cmolc/kg acidity (AFS)

• ppmx2 = pp2m = lb/a

• mg/kg = ppm

What is CCE?

Neutralizing power in terms of CaCO3

reflects acid neutralizing strength of material

What is ECCE

ECCE

• Based on CCE and particle size

• Reflects actual neutralizing ability

Lime requirement depends on

• Initial pH

• Target pH

• Buffer capacity of soil (i.e., how much exchangeable/residual acidity need to be

neutralized to actually see a change in pH)

• Depth of pH adjustment

• Properties of liming material

• CCE, particle size (or ECCE)

Application of lime fequency timing

Frequency

• 2-5 yr

Timing

• 6 to 12 mo prior to desired pH change

Incorporation vs surface application Subsurface acidity

• Consider CaSO4 to detoxify Al and add Ca to

Arid/semi-arid soils are usually…

Water limited

• Often carbonate-rich

• Often affected by poor leaching of salts

• ET > rainfall

• Typically have pH > 7

• Potential problems with micronutrient deficiencies (Fe, Mn, Cu, Zn)

• Potential problems with P and B deficiency

• Molybdenum is usually high

• CEC is usually high

• Usually considerable 2:1 clays

• pH increases pH dependent charge

• Usually high %BS, due to lack of leaching of base cations

What is the source of salts?

• Weathering of rocks/minerals

• Fossil deposits

• Irrigation

• Poor drainage

Assessing Soil for Salts

Salinity

• Measures total salts

• Electrical conductivity

Sodium

• Exchangeable Na (ESP)

Saline

Saline

EC 4 or above

• High salts – Ca, Mg salts primarily

• Plants struggle to grow

Sodic

Sodic

ESP 15 or above

• High Na

• Low salinity

• pH > 8.5

• Poor infiltration, plants struggle

with lack of water

Saline-Sodic

Saline-Sodic

EC 4 and ESP 15 or greater

• High Na

• High salinity

• Plants struggle to grow

• Can be transformed to sodic

Reclamation of Saline soils

Leaching

• Ensure water is low Na

Drainage

• Add drainage (stop evaporatiom)

• Manage drainage waters

Monitor EC

• Before, during, and after

Reclamation of Saline-Sodic

& Sodic soils

Apply Ca

• Gypsum

Leaching

• May need ponding for sodic soils

Drainage

• Add drainage

• Manage drainage waters

Monitor EC & ESP

• Before, during, after