MR 1 Study Guide - Soil Chemistry Flashcards by Keen Chew

What are redoximorphic features, what do they indicate? What are some examples of redoximorphic features?

Variegated color patterns of dull gray and bright orange, indicative of seasonal saturation

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Black soil

Humified (highly decomposed) organic matter

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Brown soil

Combination of Fe-oxides, oxyhydroxides, and OM

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Red soil

Oxidized iron, hematite (Fe2O3)

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Gray soil

Reduction of Fe3+ to Fe2+ and loss of Fe2+, anaerobic conditions WITH leaching,

Grayish colors indicative of iron depletion

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Blue-green soil

Reduction of Fe3+ to Fe2+ and retention of Fe2+, anaerobic conditions WITHOUT leaching

Pale bluish or greenish-yellow colors are indicative of prolonged saturation, but reduced iron is not leached away

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White soil

Carbonate or evaporite accumulation OR strong eluviation leaving non-pigmented light-colored minerals

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How are metamorphic rocks formed?

They are formed when rocks undergo extreme heat, pressure, hot mineral-rich fluids, or any combination of these factors

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How are sedimentary rocks formed?

Compacted or cemented particles weathered from any type of preexisting rocks

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How are igneous
rocks formed?

Formed from molten magma

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What soil properties are determined by the characteristics of the parent material?

Fertility and coarseness of the soil

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What are the two main types of weathering of parent materials? Give some examples of each

Physical and chemical weathering

ex of physical: wind abrasion
ex of chemical: organisms

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What is a mafic rock? What is a felsic rock?

Mafic is made of heavy elements, felsic is made of light elements

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How does element composition in the parent material affect soil properties?

Affects mineral composition, soil fertility, rate of weathering, texture, hydrology, vegetation

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How does grain size in the parent material affect soil properties?

Parent material with larger grain size will generally produce coarser soils while smaller grain size will produce finer soils

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Explain the main properties of granite and how they drive the properties of soil

Granite is a felsic rock

Quartz: resistant to weathering and doesn’t contain minerals

Feldspars and muscovite: more easily-weathered but still contain few nutrients

Soils derived from granitic rock are coarse-textured and have low fertility

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Explain the main properties of basalt and how they drive soil properties

Basalt is a mafic rock

Dominated by small-grained, easily weathered, nutrient-rich minerals including hornblende, augite, biotite.

Parent materials and soils derived from basaltic rock are fine-textured and fertile

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What are the three main characteristics of soil colloids?

Small, highly reactive, and massive surface area

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The hydrolysis of feldspar is an example of ________ weathering that forms _______, a secondary mineral

chemical, kaolinite

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The oxidation of Pyroxene, is an example of __________ weathering that forms, ________, a secondary mineral

chemical, iron oxide

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During the processes of chemical weathering from primary to secondary minerals there is generally an ______ (increase/decrease) in Si4+ in the mineral structures

Decrease

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The attraction between colloids and water molecules is called:_____

adhesion

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The attraction of water molecules to each other is called:______

cohesion

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How does the charge of the cations determine how strongly they are attracted to the colloids?

Higher charge = stronger attraction

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How does the size of the hydrated cations determine how strongly they are attracted to the colloids?

Smaller size = stronger attraction

What is a lyotropic series?

Order of strength of adsorption for common cations

How does the concentration of charge of a cation in the soil solution drive the strength of attraction to the colloid surfaces?

If the concentration of a cation in soil solution is high, there is an increased tendency for that cation to be adsorbed

What is the cation exchange capacity of a soil and how is it expressed (what units)?

Quantifies the capacity of colloids to adsorb cations Units are cmolc/kg

One atom of Ca2+ in the exchange complex will be replaced by one atom of H+ (true/false)

False, it will be replaced by two H+ atoms

What is anion exchange and what type of colloids result in higher rates of anion exchange?

Same as cation adsorption but in positively charged colloid surfaces Positively charged colloids result in higher rates of anion exchange

What type of crystalline structure is shown in the picture below? What type of mineral is characterized by having that structure?

Probably need to refer to study guide for this one, but it looks like a 1:1 silicate clay and given the hydrogens in between the layers, it's probably kaolinite

What is the interlayer space of crystalline clays and why is it important?

It is a negatively charged space that can hold cations and water, which can make it expansible

Describe the components of a 2:1 clay mineral

1 octahedral sheet in between two tetrahedral sheets

Describe the structure and properties of kaolinite ## Footnote What layer What stability What shrink-swell potential What level and range of CEC What level surface area

1:1 layer mineral Stable No shrink-swell Low CEC: 4-10 cmolc/kg Surface area: 7-30 m2/g Layers held together by hydrogen bonding, very low isomorphic substitution

Describe the structure and properties of vermiculite ## Footnote What layer What stability What shrink-swell potential What level and range of CEC What level surface area

2:1 layer mineral Relatively stable Low shrink-swell potential High CEC 100-150 cmolc/kg Surface area is 600-800 m2/g Potential issue is K fixation Substitution of Al3+ for Si4+

Describe the structure and properties of chlorite ## Footnote What layer What stability What shrink-swell potential What level and range of CEC What level surface area

2:1 layer mineral Stable No shrink-swell Moderate-low CEC 15-40 cmolc/kg Surface area is 25-150 m2/g

Describe the structure and properties of mica ## Footnote What layer What stability What shrink-swell potential What level and range of CEC What level surface area

2:1 layer mineral Relatively stable Low shrink-swell potential Moderate-low CEC 10-40 cmolc/kg Intermediate surface area 70-100m2/g K resides in interlayer spaces which strongly bonds adjoining sheets Isomorphous substitution in tetrahedral sheets of Al3+ for Si4+ in every fourth tetrahedral site

Describe the structure and properties of montmorillonite (a smectitic clay) ## Footnote What layer What stability What shrink-swell potential What level and range of CEC What level surface area

2:1 layer mineral Not stable High shrink-swell potential Moderate CEC 60-100 cmolc/kg clay Surface area is 600-800 m2/g Highly impermeable to water when wetted

What type of 2:1 clay has a moderate CEC (60-100 cmolc/kg clay), high surface area and strong shrink and swell behavior?

Smectitic clay (Montmorillonite would fall under this category)

What is isomorphous substitution? How does it affect the charge of crystalline clays?

Substitution of one cation for another in a tetra- or octahedron Can change charge depending on what cation was substituted

What is the source of charge in crystalline clays?

The center cation

What is the source of charge in organic matter and amorphous clays?

Lots of negative charge from OH groups for both Negative charge from O- in organic matter, some positive charges if their O- and OH- groups accept H+

How does pH change the charge of colloids?

It can make them positively charged if the pH is low

Under low soil pH, it is more likely to find_______ (positive/negative) charges in the soil colloids

Positive

Do all colloids change the charge when pH changes? Give examples of colloids that will tend to change their charge with changes in pH

Colloids with isomorphous substitution are not really affected by pH changes (like smectite and vermiculite) For crystalline silica colloids, simpler structure means lower percentage of permanent charge and higher percentage of pH-dependent charge. More complex crystalline colloids are the opposite Kaolinite, organic matter, gibbsite, and goethite more pH dependent

Why does CEC increase with pH?

Because negative charges on colloids will increase, thereby increasing CEC

What range of soil pH is ideal for crop production?

5.5-6.5 pH

What type of nutrients become unavailable at low soil pH?

Phosphorus, magnesium, and other plant macro/micronutrients

What type of elements become more concentrated at low soil pH?

Aluminum becomes more concentrated at low soil pH

What are the three main types of sources of acidity?

Active, exchangeable, and residual

What is an acid cation and how does it generate acidity?

Al3+ and Fe3+ ions that have a strong tendency to hydrolyze water and release H+ ions, which make the soil more acidic

How do high precipitation rates drive acidity?

Leaching, and rain is already slightly acidic

At what pH does Al toxicity become a problem?

Below 4

What type of microorganism is more strongly affected by soil pH? Bacteria or fungi?

Bacteria, which are sensitive to low pH

How does plant nutrient uptake change soil pH?

Plant roots release charged ions to take up other ions

What is a basic cation?

A cation with no hydrolyzing power

Explain two human-induced drivers of soil acidification

Nitrogen fertilization: causes acidification of the soil which is not neutralized if plants have been over fertilized Acid deposition from the atmosphere: N and S gasses emitted after the combustion of fossil fuels combine with water and create acidity, causing acid rain Exposure of potential acid materials: Drainage of wetlands can create acid mine drainage, which is extremely acid and toxic

What elements become toxic with high soil pH?

Molybdenum

What nutrients become deficient at high soil pH?

Iron, manganese, copper, zinc

What are the main alkalinizing processes in soils?

Weathering (mineral weathering that consumes H+ may counteract acidification) Production of base-producing anions (carbonate and bicarbonate CO32-, HCO3- may react with water to produce OH- ions) Cations (lack of acid cations, concentration of base cations)

What is reserve and active acidity?

Active acidity is the H+ concentration in the soil solution Reserve acidity is the H+ and Al3+ that is adhered to the cation exchange complex

Soils with high CEC have more reserve acidity than low CEC soils (true/false)

True

How many H + can be neutralized with one molecule of carbonate?

Two