Exam 3 Flashcards
What is a soil colloid?
Particles with a
diameter of 0.1 to
0.001µm (10,000µm=1cm)
What are colloids characterized by?
– Their small size (diameter of 0.1 to 0.001µm)
– High surface area with some internal surface area
– Surface charge
– Adsorption of cations and/or anions
– Adsorption of water
What are Inorganic colloids?
– Clay minerals
– Hydrous oxides
– Are usually crystalline
– Usually make up the bulk of soil colloids.
What are Organic colloids?
– Highly decomposed organic matter called humus.
– Generally, have a greater influence on soil properties per unit weight than the inorganic colloids.
What are phyllosilicates?
layered silicate structure
What are tetrahedral sheets made from?
Si^4+ and O^2- (Si is bonded to 4 Os)
What are Octahedral Sheets made from?
Al^3+ and O^2- (Al is bonded to 6 Os)
What is a 1:1 type mineral?
kaolinite (one tetrahedral to one octahedral sheet)
What is a 2:1 type mineral?
Illite (two tetrahedral to one octahedral sheet)
What are some characteristics of Iron & Aluminum colloids?
- Common in highly weathered soils (like the
Oxisols). - Low cation exchange capacities.
- Non-expansive clays.
Highly weathered = more _______?
Little weathering = more ____?
more oxides, more 2:1 type clays
A 1:1 phyllosilicate clay has:
a) 1 tetrahedral sheet to 1 octahedral sheet
b) 2 tetrahedral sheets to 1 octahedral sheets
c) 1 oxygen to 1 silicon
d) 1 aluminum to 2 oxygen
a) 1 tetrahedral sheet to 1 octahedral sheet
What is soil organic matter made of?
Humus, Living, Fresh Residue, and Active Fraction
What is Anion exchange capacity (AEC)?
sum of total anions that a given soil can absorb
What is Cation exchange capacity (CEC)?
sum of total cations that a given soil can absorb
What is Isomorphic Substitution?
process in which one element substitutes another of comparable size in the crystalline structure. Permeant charge.
What is the isomorphic substitution for 1:1 clay?
Mg substitution for Al in the octahedral lattice
Si2O3OHO2Al2(OH)3 →Si2O3OHO2AlMg(OH)3
What is the isomorphic substitution for 2:1 clay?
2:1 Clay (e.g. montmorillite)
– Mg substitution for Al in the octahedral lattice
– Al substitution for Si in the tetrahedral lattice
Si2O3OHO2Al2OH2 Si2O3 → Si2O3OHO2AlMgOH2SiAlO3
What is a Milliequivalent (meq)?
one milligram of hydrogen or the amount of any other ion
that will combine with it or displace it.
Equivalent weight = molecular wt (g)/ valence or charge
What is CEC expressed in?
milliequivalents
What is Flocculation?
– individual particles stick together to form aggregates of particles or floccules. Such aggregates do not move in the soil solution and form the basis for soil structure.
What is Dispersion?
aggregates do not form. Each particle behaves as an individual.
– Without aggregation, water, air, and root movement in the soil is inhibited.
– Dispersion is not a desirable characteristic of productive soils.
In a 1:1 type phyllosilicate clay, what cation is replaces the aluminum, and what sheet is
it replaced on?
a) Mg2+, the tetrahedral sheet
b) Mg2+, the octahedral sheet
c) Si4+, the tetrahedral sheet
d) Si4+, the octahedral sheet
b) Mg2+, the octahedral sheet
What soil pH has the highest cation exchange capacity (CEC)?
a) pH 4
b) pH 5
c) pH 6
d) pH 7
d) pH 7
What type of clay will have the highest cation exchange capacity (CEC)?
a) 1:1 type
b) 2:1 type
c) Iron oxides
d) Aluminum oxides
b) 2:1 type
Metals such as Fe, Mn, Cu, and Zn can be toxic to plants at __ pH
low
When going from a pH 5 to a pH 6, the pH 5 soil has how much more acidity?
a) 1
b) 10
c) 100
d) Cannot be determined
b) 10
What are the 5 sources of acidity?
- Reaction of CO2
from atmosphere with the soil solution: CO2 + H2O <–> CO3^2- + 2H^+ - Uptake of basic cations and excretion of H+
– Charge balance must be maintained. - Decomposition of organic residues.
- Oxidation of NH4^+ to NO3^- by microbes (manures and ammonium-based fertilizers).
- Aluminum hydrolysis form Al-hydroxides and H+ ions:
(oxidation of Al^3+ to Al(OH)3)
What characteristics of soil has the greatest buffering capacity?
– High clay
– High organic matter
What characteristics of soil has the least buffering capacity?
– Low clay
– Low organic matter
What soil texture has the lowest buffering capacity?
a) Sand
b) Silt
c) Clay
d) Organic matter
a) Sand
Oxisols are _____ acidic than Ultisols which are ____ acidic than Alfisols
more,more
What are the steps in the lime reaction
- Ca2+ from the lime replaces Al3+ and H+ on the cation exchange complex.
- The carbonate reacts with the H+ ions, removing them from solution thereby raising the pH. (turning H+ into water)
- Al3+ is hydrolyzed to form Al-hydroxides and H+ ions, Carbonate from the lime neutralizes the H+ generated during the Al-hydrolosis (step 2 again)
What is the Lime recommendation Model?
Lime Req. (t/ac) = ((target pH - current pH)/(6.6 - current pH)) * Exchangeable acidity (AC)
What is the equation for Exchangeable acidity (AC)?
𝐴𝐶 = 4 × (6.6 − 𝑏𝑢𝑓𝑓𝑒𝑟𝑒𝑑 𝑝𝐻)
What is the difference between Calcitic limestone and Dolomitic limestone?
Calcitic limestone has calcium carbonate and Dolomitic limestone has calcium & magnesium carbonate
What is Calcium Carbonate Equivalent (CCE)?
Neutralizing value of any liming material compared to pure calcium carbonate (CaCO3)
What soil texture needs the most amount of lime to change the pH from 4 to 5?
a) Gravel
b) Sand
c) Silt
d) Clay
d) Clay
How many essential elements do plants need to complete their life cycle?
17
What are the structural elements?
Carbon, Hydrogen, and Oxygen
What are the Primary Macronutrients?
Nitrogen, Phosphorus, and Potassium
What are the Secondary Macronutrients?
Calcium, Magnesium, and Sulfur
What are the micronutrients?
Iron, Manganese, Boron, Zinc, Copper, Chlorine, Molybdenum, and Nickel
What is Mass Flow?
dissolved nutrients move to the root in soil water that is flowing towards the roots
What is Diffusion?
nutrients move from higher concentration in the bulk soil solution to lower concentration at the root
What is root interception?
roots obtain nutrients by physically contacting nutrients in soil solution or on soil surfaces.
What plant element is needed in the largest quantity?
a) Hydrogen
b) Carbon
c) Oxygen
d) Nitrogen
b) carbon
What are the structural components of plant?
a) Hydrogen
b) Carbon
c) Oxygen
d) All the above
d) All the above
When plant nutrients move along a concentration gradient, it is called what?
a) Root interception
b) Mass flow
c) Osmosis
d) Diffusion
d) Diffusion
Which of the following nutrient are beneficial but not essential?
a) Boron
b) Chlorine
c) Sodium
d) Nickle
c) Sodium
What are plant-available forms of Nitrogen?
Nitrate (NO3^-), Ammonium (NH4^+)
How can Nitrogen can be fixation?
Legumes, Bacteria, N2
What is mineralization?
Biological conversation of unavailable organic N into plant available inorganic N, Organic N to ammonium to nitrate (ammonification then nitrification)
What is Immobilization?
Biological conversation of plant available N into unavailable organic N
What factors favor Ammonification?
Warm temperatures, good soil moisture, and Adequate supply of O2
What factors favor Ammonification?
Well drained soils, warm temps, an acidifying process, and lack of pesticides
What is denitrification?
Transformation of NO3^-
to N2, N2O or NO gases.
What are the conditions that denitrification occurs at?
–Under low oxygen
(anaerobic) conditions,
– Under warm conditions
– Where there is readily
available source of
organic matter for
microbial energy
source.
What process converts organic N is converted to inorganic N?
a) Volatilization
b) Mineralization
c) Immobilization
d) Fixation
b) Mineralization
When soil bacteria turn atmospheric nitrogen to plant available forms of nitrogen, it
is called what?
a) Fixation
b) Volatilization
c) Immobilization
d) Denitrification
a) Fixation
Explain the Phosphorus cycle
The phosphorus cycle is mainly based on equilibrium chemistry (very little available about 4 ibs/ac) with Phosphorous cycling between Stable, unavailable organic P to soil solution, available P to Stable, unavailable inorganic P
How does pH affect phosphorus in the soil
Acidic soil will make the unavailable bubble more inorganic, Fe and Al oxides can affect phosphorus. most available at 6.5
What are the plant available forms of P?
H2PO4^- and HPO4^2-
What are environmental issues relating to P?
The large use of P from fertilizers from swine and poultry is used on fields that over saturate soil. Phosphorus additions to natural waters can cause eutrophication (algal blooms)
Which plant nutrient leaches the fastest through the soil?
a) Nitrogen
b) Potassium
c) Calcium
d) Phosphorus
a) Nitrogen
Which plant nutrient leaches the slowest through the soil?
a) Nitrogen
b) Potassium
c) Calcium
d) Phosphorus
d) Phosphorus
Sulfur is considered:
a) Structural component
b) Macronutrient
c) Micronutrient
d) Not essential but beneficial
b) Macronutrient
Plant micronutrients include:
a) Carbon
b) Potassium
c) Iron
d) Sulfur
c) Iron
Which of the following plant nutrients are only in anion form?
a) Potassium
b) Phosphorus
c) Carbon
d) Nitrogen
b) Phosphorus
What does 10-10-10 mean?
10% N, 10% P2O5, 10% K2O (NPK)
A bag of fertilizer contains 5-10-15. How much phosphorus is in the fertilizer for
every 100 lbs?
a) 5 lbs
b) 10 lbs
c) 15 lbs
d) Cannot be determined with the data
b) 10 lbs
A bag of fertilizer contains 5-10-15 and we’ve determined it has 10 lbs phosphorous.
What form is the phosphorus in?
a) Elemental P
b) P2O5
c) H2PO4^-
d) HPO4^2-
b) P2O5
What type of clays do Mollisols/Histosols, Alfisols, ultisols, and oxisols have?
Mollisols/Histosols have mostly 2:1 type clays, Alfisols have a mix of 2:1 and 1:1 type clays, Ultisols have moslty 1:1 type clays, and oxisols have mostly Fe, Al oxide clays
What is pH dependent charge?
Broken ends of clays carry a charge. Charge can change based on the pH. Lower pH = more H+ will attach to the broken ends (aka more positive).
What is Active acidity?
H+ or Al3+ in the water of soil and a measure of how acidic the soil is.
What is Exchangeable acidity?
H+ and Al3+ that are on
exchange sites of clay
or OM negative sites.
What is Residual acidity?
H+ or Al3+ that are bound to
clay or OM and NOT
exchangeable.
What are the bacteria that drive nitrification?
Nitrosomonas, Nitrobacter
What are the main questions to ask about nutrient management?
Amount?, Timing?, Source?, Placement?
What depth should you sample soil for field crops?
collect samples to the same depth that the field is plowed (usually about 8 inches)
What depth should you sample soil for perennial crops?
samples taken to a depth of 4 inches will best represent the crop’s lime and fertilizer needs.
The lower the cation exchange capacity of a soil, the higher is its buffer capacity. (T/F)
False
Which of the following is not a characteristic of soil colloids?
A. electronegative charges
B. ability to exchange ions with the soil solution
C. low water holding capacity
D. high external surfaces
E. very small size
C. low water holding capacity
A 2:1 type silicate clay has one octahedral sheet sandwiched between two tetrahedral sheets. (T/F)
True
The acidity of “acid rain” can be partially traced to the reaction of CO2 in the atmosphere forming carbonic acid (H2CO3).
(T/F)
True
Iron and aluminum hydrous oxides are characteristic of soils that are highly weathered chemically. (T/F)
True
The higher the cation exchange capacity of a soil, the greater is its buffer capacity. (T/F)
True
Humus has a much higher cation adsorption capacity at pH 5.5 than at pH 8.0. (T/F)
False
How does the amount of lime needed to neutralize the residual acidity in a clay loam soil compare to that needed to neutralize the active acidity in that soil?
A. Residual requires 5,000 times greater.
B. Residual requires about the same.
C. Residual requires 10 times greater.
D. Residual requires considerably less.
E. Residual requires about double.
A. Residual requires 5,000 times greater.
Among the soils of which order would you most likely find the highest buffering capacities?
A. Alfisols
B. Oxisols
C. Ultisols
D. Spodosols
E. Vertisols
E. Vertisols
The high cation adsorption capacity of a 2:1 clay is due primarily to the substitution of silicon atoms for aluminum in the tetrahedral sheet. (T/F)
False
Most micronutrients play essential roles in plant metabolism as enzyme activators. (T/F)
True
The conversion of plant-available forms of nitrogen into microbial or plant biomass is an example of mineralization. (T/F)
False
Plants can only obtain nitrogen from the soil by taking up soluble anions. In general cations can not be absorbed by plants. (T/F)
False
To be practical, a nitrification inhibitor should kill or inhibit ________.
A. Nitrosomonas
B. Nitrobacter
C. either A or B
D. none of the above
A. Nitrosomonas
Potassium is more readily leached from a neutral sandy soil than from an acid clayey soil. (T/F)
True
Wetlands and forests growing along the banks of rivers remove much nitrogen from groundwater by the process of ________.
A. nitrification
B. ammonification
C. immobilization
D. denitrification
E. none of the above
D. denitrification
In order for nitrate to form from soil organic matter, ________ must be present in the soil.
A. Nitrobacter
B. Nitrosomonas
C. either A or B
D. both A and B
E. none of the above
D. both A and B
Most nitrogen leaching to groundwater is in the form of ammonium. (T/F)
False
In what range of soil pH is phosphorus generally most available to plants?
A. 4.5-5.5
B. 5.5-7.0
C. >8.0
D. 3.5-4.5
E. 7.0-8.0
B. 5.5-7.0
In most soils, mineralization of soil organic matter is an important source of plant available ________.
A. phosphorus
B. potassium
C. both A and B
D. none of the above
A. phosphorus
What soil has the most available phosphorus? Select all that apply.
a. Clayey topsoil
b. Sandy subsoil
c. Sandy topsoil
d. Clayey subsoil
a. Clayey topsoil
What soil elements are cations?
Boron (depending on pH), Calcium, Copper, Hydrogen, Magnesium, Manganese, Nickel, Nitrogen (Ammonium form), Potassium, Zinc
Is Boron a Cation or an Anion?
It depends on pH
Is Calcium a Cation or an Anion?
Cation
Is Copper a Cation or an Anion?
Cation
Is Hydrogen a Cation or an Anion?
Cation
Is Magnesium a Cation or an Anion?
Cation
Is Manganese a Cation or an Anion?
Cation
Is Nickel a Cation or an Anion?
Cation
Is Nitrogen (Ammonium form) a Cation or an Anion?
Cation
Is Potassium a Cation or an Anion?
Cation
Is Zinc a Cation or an Anion?
Cation
What soil elements are Anions?
Boron (depending on pH), Chloride, Molybdate, Nitrogen (Nitrate form), Phosphate *, Sulfate
Is Chloride a Cation or an Anion?
Anion
Is Molybdate a Cation or an Anion?
Anion
Is Nitrogen (Nitrate form) a Cation or an Anion?
Anion
Is Phosphate a Cation or an Anion?
Anion (not mobile in high Fe/Al soils)
Is Sulfate a Cation or an Anion?
Anion
