MR 1 Study Guide - Soil Chemistry Flashcards
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
Black soil
Humified (highly decomposed) organic matter
Brown soil
Combination of Fe-oxides, oxyhydroxides, and OM
Red soil
Oxidized iron, hematite (Fe2O3)
Gray soil
Reduction of Fe3+ to Fe2+ and loss of Fe2+, anaerobic conditions WITH leaching,
Grayish colors indicative of iron depletion
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
White soil
Carbonate or evaporite accumulation OR strong eluviation leaving non-pigmented light-colored minerals
How are metamorphic rocks formed?
They are formed when rocks undergo extreme heat, pressure, hot mineral-rich fluids, or any combination of these factors
How are sedimentary rocks formed?
Compacted or cemented particles weathered from any type of preexisting rocks
How are igneous
rocks formed?
Formed from molten magma
What soil properties are determined by the characteristics of the parent material?
Fertility and coarseness of the soil
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
What is a mafic rock? What is a felsic rock?
Mafic is made of heavy elements, felsic is made of light elements
How does element composition in the parent material affect soil properties?
Affects mineral composition, soil fertility, rate of weathering, texture, hydrology, vegetation
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
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
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
What are the three main characteristics of soil colloids?
Small, highly reactive, and massive surface area
The hydrolysis of feldspar is an example of ________ weathering that forms _______, a secondary mineral
chemical, kaolinite
The oxidation of Pyroxene, is an example of __________ weathering that forms, ________, a secondary mineral
chemical, iron oxide
During the processes of chemical weathering from primary to secondary minerals there is generally an ______ (increase/decrease) in Si4+ in the mineral structures
Decrease
The attraction between colloids and water molecules is called:_____
adhesion
The attraction of water molecules to each other is called:______
cohesion
How does the charge of the cations determine how strongly they are attracted to the colloids?
Higher charge = stronger attraction
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
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
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
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
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)
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
