SOCR EXAM 3 Soil Chemistry Flashcards by Nat Poo

list at least three common soil mineral types

oxides / hydroxides
silicates
carbonates
nitrates

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identify major forms of chemical degradation of soils & have a general knowledge of chemical elements active in soil chemistry

major forms of chemical degradation:
nutrient depletion
salinization
dispersion / alkalization
acidification
toxic contamination

variety of chemical elements active in soil chemistry:
aluminum, boron, cadmium, calcium, carbon, chlorine, cobalt, copper, hydrogen, iron, lead, magnesium, manganese, mercury, nickel, nitrogen, oxygen, phosphorus, potassium, selenium, silicon, sodium, sulfur, zinc

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identify major types of chemical reactions in soils

weathering reactions
hydrolysis
dissolution

organic matter decomposition
oxidation
mineralization of nutrients

carbonation reactions
carbonic acid formation & solubilization of soil minerals

nitrogen transformations
nitrogen mineralization
dentrification

cation exchange
adsorption reactions
organic molecules & metals adsorbing to soil minerals & organic matter
oxidation / reduction reactions

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know the basic structure of Phyllosilicates & the difference between 1:1 & 2:1 forms

Phyllosilicates: composed of silicon, aluminum, oxygen & hydrogen
Has silica tetrahedrons & alumina octahedrons, which are organized in sheets
Sheets connect by sharing oxygens

1:1 clay minerals consists of 1 silica tetrahedron sheet & 1 aluminum octahedral sheet

2:1 clay minerals consists of 2 silica tetrahedron sheet & 1 aluminum octahedral sheet

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know the sources of negative charge in clay minerals & soil organic matter

1) upon formation of the mineral, a lower valence cation takes the place of Al3+ or Si4+. This is called isomorphic substitution & results in a permanent negative charge

2) deprotonation of edge OH groups as soil pH increases; this is called pH dependent charge of OM

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define / discuss cation exchange ability

cation exchange capacity CEC: the sum of total exchangeable cations that a soil can absorb
expressed in centimoles of charge per kg of soil, or organic colloid (cmolc/kg)
- the negative charge of the clay mineral is balanced by cations in the soil solution
- if these cations are replaceable by other cations in the soil solution, they are referred to as exchangeable cations
- as pH increases, hydroxyl, phenolic, & carboxyl groups deprotonate & the material becomes more negatively charged so CEC increases

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flocculation vs. dispersion & causes

flocculation: the combining of small particles (clay & humus) as generally caused by adsorption of certain cations (e.g. Ca2+ & Mg2+).
- Ca2+ & Al3+ promote flocculation
dispersion: the process in which aggregates are broken-up into individual soil separates.
- Na+ tends . to cause dispersion of clays (unless at high concentration)

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sources of soil salinity

dissolution of soil minerals
deposition of atmospheric salts
irrigation water & shallow groundwater

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identify effect of high salinity on soil properties & plants

soil: dispersion & decreased hydraulic conductivity
plants: osmotic stress, reduced growth

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Identify metrics used to quantify soil salinity & classification of saline & sodic soils

Electrical conductivity, EC: a measure of dissolved salts, salinity increases with increasing concentration. It is commonly measured in the extract of a water saturated soil paste (symbolized ECe for extract) & reported in ds/m (decisiemens/m)
Sodium Adsorption Ratio, SAR: a measure of the relative amount of sodium to other cations; higher SAR indicates a sodium problem
Exchangeable Sodium Percentage, ESP: the fraction of the total CEC satisfied by sodium

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Briefly discuss requirements for sustainable irrigation in soils susceptible to salinization

sustainability can be achieved by adhering to two basic principles:
(1) salt balance, defined as salts removed must equal salts added
(2) systems must be installed to remove drainage water and dissolved salts when the water table reaches the root zone. These principles can be applied on a field, farm or regional basis.
What can we do about the salinity problem?
- Manage the water so that we don’t get excessively wet soils, need to be able to drain the soil
- We can displace sodium with another cation

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Calculation of irrigation needed to avoid salt accumulation (the leaching requirement)

Approach: adding irrigation in excess of ET losses to prevent salt accumulation. Requires drainage below root zone (rz).
To prevent an increase in the salt concentration in the root-zone with the addition of salts in the irrigation water, we calculate the Leaching Requirement (LR)

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Calculation of the gypsum requirement to affect a change in the ESP. Practice these types of calculations, very likely on the exam.

The rationale for the gypsum (CaSO4*2H20) reclamation is to use Ca to displace Na from the cation exchange sites & then rinse (leach) Na from the soil
The amount of gypsum needed naturally depends on 1. How much sodium needs to be displaced, which is a function of the △ESP you desire & 2. The d

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Typical soil pH & correlation with rainfall

typical range of soil pH 4-8
Midwestern & Eastern US: 4-8
Arid west region: 5-10
Forest soils: 3-8
A higher amount of rainfall will cause a lower soil pH (more acidic).
Generally, as you travel from the western US to the eastern and southeastern US, soil pH decreases (because rainfall)

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Identify pools of soil acidity & buffering capacity

Active acidity: The free H+ ions in the soil solution, is the measured pH, and is usually the lesser source.
Reserve (or exchangeable ) acidity: H+ and Al3+ bonded to exchange sites on clays and organic matter. The Al3+ is included because when it exchanges into the solution it undergoes an acid forming reaction.
Buffering capacity- pH buffering is resistance to pH change. In soils with a sizable exchangeable %H, this acts as a reservoir to release H+ to the soil solution if the active H+ is reduced.
Buffering capacity (cmolc/kg) = % free CaCO3 * 20

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Identify sources of soil acidity & acid producing reactions

Natural causes
Rainfall leaching base cations (Ca, Mg, etc.) from soil
Organic matter decomposition
Carbonation (H2O & CO2) reactions from plant root and microbial respiration
Agriculturally enhanced causes
Nitrogen transformations from fertilizer use
Removal of base cations with plant harvest
Sources of acid producing reactions:
Carbonation & nitrogen transformation

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Identify plant hazard of low soil pH

limited root development, stunted growth, nutrient deficiency symptoms, discoloration, and poor yields.
2 elements hazardous at low soil pH: aluminum & manganese

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Identify the effect of soil pH on the availability of certain metals

lowering pH increases the availability of some micronutrients beneficial to plants (Fe, Zn), but it can increase Al and Mn to toxic levels.
as pH increases, metal concentration decreases in soil
If pH decreases 1 unit, the plant availability of Zn2+, Cu2+, and Mn2+ increases 100 fold
If pH decreases 1 unit, the plant availability of Fe3+ increases 1000 fold

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Calculation of lime requirements to treat acid soils. Practice these types of calculations, very likely on exam

Summary of benefits of liming:
Elevates pH & hence reduces Al & Mn in the soil solution & toxicity
Increases availability of Phosphorus to plants (up to pH=6.5)
Reducing acidity generally improves conditions for soil microorganisms & plant growth
Increases Ca2+ in soil which can improve soil structure

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soil organic matter

Living: roots, mycorrhizae, & bacteria
Dead: crop residues, dead roots, microbial biomass
Very dead: humus stabilized OM

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Sources of soil organic matter

Plant residues and green manure (plants incorporated when immature)
- Animal manures
- Animal and organism bodies
- Compost
- Sewage sludge (biosolids)
- Mineral soil 1-7% (up to 20%)
- Organic soil >20%
- Average in US about 3%
- Average in CO about 1.5%

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Identify the three most prominent compounds in plant residue

cellulose, lignin, and hemicellulose

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Identify the major pools of decomposing organic matter in soils & briefly describe their characteristics

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passive organic matter pool: 500-5000 years
slow organic matter pool: 15-100 years
active organic matter pool: 1-2 yea
labile carbon and stabilized pool (soil humus) with remains of very slowly decomposing material

Discuss the benefits of increasing soil organic matter

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Increased water retention
Promotes soil aggregation and improves soil structure
Provides nutrients for plant growth and improves conditions for increased microbial activity and population diversity
Helps to buffer soil pH
Increases the cation exchange capacity of soil. From 20-70% of CEC in many souls is caused by soil organic matter
Soil organic matter can adsorb and retain pollutants and slow their movement to groundwater.

Identify at least three fates of organic chemicals in soil

- photodecomposition - volatilization - microbial decomposition - conjugation (plant uptake) - sorption by soil humus

Describe how adsorption is quantified (adsorption isotherm)

- The amount of adsorption to the soil solids is characterized by the adsorption isotherm (adsorption over a range of solution concentration at constant temperature)

Identify the benchmark properties for mobility & persistence

mobility is Kd or Koc and persistence is half life

Identify environmental factors favoring decomposition & the types of compounds most resistant to biodegradation

Which environmental factors favor decomposition? Aeration Warmer temperatures Available inorganic nitrogen Soil pH near 7 Priming: previous history of chemical exposure can enhance populations of microorganisms capable of degrading compound Types of compounds most resistant to biodegradation: Normal Alkanes C10-C19, Straight Chain Alkenes C12-C18, Gases C2-C4, and Alkenes C5-C9. cycloalkanes, aromatics, branches alkenes, alkenes, branches alkanes, alkanes, gases, straight chain alkenes, normal alkanes

Identify four sources of heavy metal enrichment in soils & the heavy metals most commonly found in elevated concentrations at contaminated sites

- Mining and Industrial Waste - Fertilizers - Sewage Sludge - Pesticides - Parent Material the heavy metals most commonly found in elevated concentrations at contaminated sites Lead (Pb), Chromium (Cr), Arsenate (As), Zinc (Zn), Cadmium (Cd), Copper (Cu), and Mercury (Hg)

Name the most common fate of metals in soils. What fate of secondary importance can be used in clean-up (removal) of metal contamination?

Most common fate: adsorption to solids (particularly clay minerals and soil organic matter). Fate of secondary importance: metals taken up by plants and movement by water

Why might metal speciation be an important consideration?

Metals undergo oxidation and reduction reactions that change their form which can alter mobility and toxicity.

Name at least three soil factors that affect adsorption of metals. Briefly indicate how alteration of these factors might be used as a treatment strategy

Amount and type of clay minerals - Amount of soil organic matter - Soil pH (as pH increases, so does adsorption)

What is Ex-situ vs. In-situ treatment?

Ex-situ: Excavation and removal of the contaminated soil to a location of containment and treatment. Fast but destructive and expensive. In-situ: Leave the contaminated soil in place and apply a strategy to treat and remove the contaminants without excavation. Slower than excavation, but aims to return site to pre-contamination condition (ex: phytoremediation)

What is phytoremediation & identify at least two types. How might chelates be used to improve phytoremediation?

Phytoremediation: Use of plants (and associated microbes) to reduce the concentration or toxic effects of contaminants. phytoextraction- accumulation of pollutants in harvestable biomass phytofiltration- sequestration of pollutants from contaminated water by plants How might chelates be used to improve phytoremediation: to increase the plant availability of metals. This carries the metal in the solution phase to the plant root.

Briefly explain why illite has a smaller cation exchange capacity than smectite:

The "K bridge" or interlayer K neutralizes many of the negative charges

List a cation (chemical symbol plus valence) that can replace Si4+ in the silica tetrahedron

Al3+

List a cation (chemical symbol plus valence) that can replace Al3+ in the alumina octahedron

Mg2+, Fe2+, Fe3+

List the clay mineral that caused the cracking in the Denver International Airport runways

Smectite

Fort Collins Clay loam contains 26% clay & 0.7% humus. Calculate the cation exchange capacity (CEC) of this soil

Clay: 0.26 * 60 cmolc / kg = 15.6cmolc/kg Humus: 0.007 * 200 cmolc / kg = 1.4 cmolc/ kg Total: (15.6 + 1.4) cmolc / kg = 17 cmolc/ kg

Calculate the buffering capacity of the following soils:

%CaCo3 = 0.15%; pH = 7.8; Cation exchange capacity (CEC) = 12 cmolc / kg Bc ≈ 20 % CaCo3 ≈ 20 0.15 B.C. ≈ 3 cmolc / kg

List the steps necessary to reclaim a sodic soil (place the necessary methods in the correct sequence)

1. Ensure good drainage 2. Add gypsum 3. Leach with good quality water

What is the classification of a soil with the following characteristics? ECe = 6.4 dS/m; pH 7.3; SAR = 19

Sodic-saline

Identify the sources of negative charge in clay minerals & soil organic matter

Clay minerals: isomorphic substitution Soil organic matter: deprotonation of OH groups

In general, soil pH is correlated with what climatic factor & why?

Precipitation. This can leach cations & also change how much organic matter is put into the soil; rainfall decreases pH

A soil with a pH=4.6 has a CEC of 13 cmolc/kg with a hydrogen saturation of 25%. What is the buffering capacity?

13(25/100) = 3.25

Name two chemical reactions that contribute to soil acidity and indicate the role of agriculture in soil acidification

- Carbonation (H20 & CO2) reactions from plant root - Nitrogen transformation Role of agriculture: Nitrogen transformations from fertilizer use Removal of base cations with plant harvest

Explain the difference between dispersed and flocculated soil solids. Which is more desirable as a plant growth medium and why? What can cause flocculated soils to disperse? What can be done to promote soils to flocculate?

Flocculated: combining of small particles (clay & humus) as generally caused by adsorption of certain cations Dispersed: process in which aggregates are broken-up into individual soil separates - Flocculated soil solids are more desirable as a plant growth medium, desirable for plant growth because it provides both large and small pores - Na+ can cause flocculation soils to disperse - Calcium, magnesium aluminum, & hydrogen ions promote flocculation (encourage the soil components to stick together)

Identify two effects of high salinity on soil properties and two effects on plants

plants: osmotic stress, reduced growth Soil: dispersion & decreased hydraulic conductivity

Identify the 2 basic principles of sustainable irrigation in soils susceptible to salinization

Salts removed must equal salts added Remove drainage water & dissolved salts when water table reaches the root zone

Name 2 general pools of soil organic matter

1) labile carbon 2) stable pool (humus)

What is phytoremediation & identify 2 types. How might chelates be used to improve phytoremediation?

Phytoremediation: use of plants to reduce the concentration or toxic effects of contaminants 2 types: phytoextraction, phytofiltration Chelates: to increase the plant availability of metals; this carries the metal in the solution phase to the plant root

What does 2:1 mean?

2:1 clay mineral consists of 2 silica tetrahedral sheets & 1 aluminum octahedral sheet (most expanding clay minerals will have this structure); tend to be more reactive

Identify two metrics used to quantify soil salinity

- Electrical conductivity - Sodium adsorption ratio - Exchangeable sodium percentage

Name two strategies to reclaiming saline-sodic soils

1) Ensure good drainage 2) Add gypsum

- Identify the three most prominent compounds in plant residue - Which of the compounds is most resistant to decomposition?

1) Cellulose 2) Lignin 3) Hemicellulose lignin

Identify 3 benefits of increasing soil organic matter

1) Increased water retention 2) Promotes soil aggregation & improves soil structure 3) Helps to buffer soil pH

SOCR EXAM 3 Soil Chemistry Flashcards

(56 cards)