Soils Unit IV Flashcards
(107 cards)
1
Q
What variable affects a wide range of soil chemical and biological properties?
A
Acidity
2
Q
Soil acidity is a balance of ______ and _____.
A
Hydrogen ions (H+)
Hydroxyl ions (OH-)
3
Q
T/F: more H+ makes soil more alkaline.
A
False. More H+ = more ACIDIC
4
Q
Alkaline soils have a higher presence of ______.
A
OH- (hydroxyl ions)
5
Q
List four effects of acidity in soils:
A
- determines which plants dominate a landscape
- determines whether elements are present at perfect, deficient or toxic levels for plant growth
- Affects aggregate stability and water movement
- Determines fate of soil pollutants
6
Q
T/F: Acidity determines the fate of many soil pollutants, stimulating their destruction or movement from the soil into the groundwater.
A
True
7
Q
T/F: Low pH tends to increase movement of pollutants, it passes through very easily and can leach into groundwater.
A
True
8
Q
Between alkaline and acidic soils, which doesn’t allow pollutants to leach?
A
Alkaline
9
Q
It is common to find acidity in ____ environments and alkalinity in ____ environments. Why is this?
A
Humid and Arid
Less water = less movement of OH- (soils tend to be negatively charged)
10
Q
_____ is the measurement of how acidic or basic soils are.
A
pH
11
Q
In regards to pH, 0-6 is _______
A
acidic
12
Q
In regards to pH, 8-14 is ______
A
Basic
13
Q
Why is 7 neutral?
A
A pH of 7 means that the concentration of hydrogen ions [H^+] is equal to the concentration of hydroxide ions [OH^-], which is the definition of a neutral solution—pure water is the standard example.
14
Q
How do carbonic and organic acids contribute to soil acidity?
A
CO₂ dissolves in water to form carbonic acid, and organic matter produces weak acids during decomposition—both release H⁺ into soil.
15
Q
How do organic matter and cation leaching increase soil acidity?
A
Organic acids dissociate to release H⁺, and leaching of base cations (Ca²⁺, Mg²⁺, K⁺) leaves excess H⁺ and Al³⁺, increasing acidity.
16
Q
How does nitrification contribute to soil acidification?
A
Microbial oxidation of NH₄⁺ to NO₃⁻ releases 2 H⁺ ions per reaction, acidifying the soil.
17
Q
How does sulfur oxidation impact soil pH?
A
Decomposition of plant residues and sulfide minerals produces sulfuric acid (H₂SO₄), releasing H⁺ and lowering pH.
18
Q
: How do acid rain and plant nutrient uptake contribute H⁺ to soil?
A
Rain absorbs CO₂, NOₓ, and SO₂, forming acids. Plants release H⁺ to balance cation uptake (e.g., Ca²⁺), contributing to soil acidity.
19
Q
What is the greatest contributor to soil acidity?
A
Carbonic acids
20
Q
T/F: win order to maintain a balance of cations and anions, roots release H+
A
True
21
Q
What are alkaline soils deficient in?
A
iron, manganese, zinc, copper and boron
22
Q
T/F: Fungi are very tolerant of pH fluctuation but bacteria and actinomycetes are less active in acidic soils
A
true
23
Q
What does aluminum toxicity cause in plants?
A
Stunted root growth and symptoms of drought stress and phosphorous deficiency
24
Q
Why is Al³⁺ a key contributor to soil acidity?
A
A – Aluminum is Attacked by H⁺
→ H⁺ ions break down soil minerals, releasing Al³⁺
L – Locks onto soil (but not helpful)
→ Al³⁺ binds to cation exchange sites but is not exchangeable, so it blocks nutrient access
A – Al³⁺ is Acidic and Harmful
→ It is toxic to organisms and reduces nutrient availability
H – Hydrolyzes to make more H⁺
→ Al³⁺ reacts with water and generates even more acidity
25
What gives water molecules its polarity?
Its asymmetrical shape (V): electronegative is by oxygen and electropositive is by the Hydrogen
26
What is the process where water molecules are attracted to positively charged ions and colloidal surfaces called?
Hydration
27
________ determines how water moves and is retained in soils.
Capillary action
28
T/F: More capillary action occurs in clay due to small pore size and distribution.
True
29
What is field capacity in soils?
The amount of water soil holds after gravity drains excess, representing the upper limit of water available to plants
30
What is the wilting point?
Water is being held so tightly by the soil, plants cannot pull it up. No turgor potential
31
What is turgidity?
Ability of plant to support its own weight
Stiffness of plant
32
T/F: when a soil is fully saturated with water, gravitational water is lost.
True
33
____________ measures water potential.
tensiometer
34
: What is infiltration capacity in soil?
The maximum rate at which water can enter soil under specific conditions (like soil type, moisture, and vegetation).
35
What does hydraulic conductivity measure?
How quickly water moves through soil due to the water potential gradient; depends on soil texture and structure.
36
What is preferential flow in soils?
Nonuniform water movement where water follows specific paths (like root channels or cracks), bypassing much of the soil matrix.
37
What is percolation in soil science?
The downward movement of water through soil layers, typically into deeper horizons or the water table.
38
What are the three types of Hydraulic Conductivity?
1. Saturated flow
2. Unsaturated flow
3.Vapor movement
39
What is saturated flow mainly influenced by?
Soil type (coarse= fast, fine=slow)
40
T/F: Trapped air will increase hydraulic conductivity.
False, trapped air will REDUCE hydraulic conductivity
41
What's one way plants and animals can raise hydraulic conductivity?
Increasing micropores via roots, tunnels, etc
42
Why does preferential flow increase the risk of groundwater pollution, and when does it occur?
Preferential flow occurs when water moves quickly through large pores (like cracks or root channels), bypassing soil’s natural filtration. This increases pollution risk because contaminants can reach groundwater directly. It wouldn’t occur if soil porosity were uniform, since water would move evenly through the soil matrix.
43
What are micropores filled with in unsaturated hydraulic conductivity?
air lol
44
What is unsaturated flow in soil, and what influences it?
Unsaturated flow is the movement of water through unsaturated soil (where pores are not completely filled with water). It’s not driven by gravity but by matric potential gradients—water moves from wetter areas (high matric potential) to drier areas (low matric potential). It is slower than saturated flow and occurs mainly through small pores, since macropores are filled with air.
45
High matric potential = _______ water and low metric potential = ______water.
High matric potential = more water (e.g., –1 kPa)
Low matric potential = less water (e.g., –10 kPa)
Water always moves from high to low matric potential.
46
What is matric potential in soil?
Matric potential is the force that holds water to soil particles, caused by capillary and adhesive forces. It reflects how tightly water is held by the soil and is always negative in unsaturated soils (e.g., –1 kPa to –1500 kPa).
Water moves from high (less negative) to low (more negative) matric potential — i.e., from wetter to drier zones.
47
: How does soil texture affect unsaturated water flow?
Clay soils have many micropores that hold water tightly, allowing slow but steady unsaturated flow. As soil dries, clay can allow water to move faster than sandy loam, because sandy loam lacks enough small pores to maintain flow under low moisture conditions.
48
What is vapor movement in soils, and how does it relate to hydraulic conductivity?
In very dry soils, water can move as vapor instead of liquid, driven by vapor pressure and temperature gradients. While not part of traditional hydraulic conductivity, vapor movement contributes slightly to water flow when liquid movement is minimal.
49
T/F: Changes in water balance: salts become a problem when too much water is supplied not too little
True
50
Arid farming appy 90cm of water to grow annual crops, even if low in salts will likely add 6 Mg/ha (3 tons/acres) of salt on the land every year results in __________.
Saline Seeps
51
T/f: aridisols have low salinity.
False; high
52
What are two ways to measure salinity in soils?
TDS and Electrical conductivity
53
What is Cation Exchange Capacity (CEC)?
IT is the soil's nutrient bank account: total amount of positivity charged ions (cations) a soil can hold on its surfaces
54
What is an example of a soil with high CEC?
A clay rich in organic material.
55
Do sandy soils have low or high CEC?
Low: fewer nutrients = poor fertility = low CEC
56
_________ is the ability of soil to sustain plant growth and optimize crop yield.
Soil Fertility
57
What is the name for the rule that states: "Plant production can be no greater than that level allowed by the growth factor present in the lowest amount relative to the optimum amount for that factor"?
Law of the Minimum
58
Explain the limiting factor.
A limiting factor is any single environmental condition or resource that restricts the growth, distribution, or abundance of an organism when it’s in short supply. It sets the upper limit for performance, regardless of other favorable conditions.
59
What is the result if you increase the limiting facotr?
Leads to increased growth until a new limiting factor is formed.
60
What are the inputs for the Nitrogen cycle?
These add or retain usable nitrogen in the soil:
1. Atmospheric Nitrogen Fixation: Converts N₂ gas → NH₄⁺ (ammonium). Done biologically (by Rhizobia, free-living bacteria) or via lightning
2. Mineralization: Organic N → NH₄⁺ (ammonium). Makes nitrogen available for plants. Net mineralization = Input
3. DNRA (Dissimilatory Nitrate Reduction to Ammonium): NO₃⁻ → NH₄⁺ under anaerobic conditions. Recycles nitrate back into plant-available ammonium
4. Fertilization (external): Addition of NH₄⁺ or NO₃⁻ fertilizers. Increases soil nitrogen pool
61
What are the outputs for the Nitrogen Cycle?
These remove or limit nitrogen availability to plants:
1. Immobilization: NH₄⁺ and NO₃⁻ → organic N. Microbes tie up N in their biomass. Net immobilization = Output
2. Ammonium Fixation: NH₄⁺ becomes trapped in 2:1 clay layers, Still present, but unavailable short-term
3. Volatilization: NH₄⁺ → NH₃ gas lost to atmosphere. Especially in warm, moist, alkaline soils
4. Denitrification: NO₃⁻ → N₂, N₂O gases (lost to atmosphere). Occurs under anaerobic conditions
5. Leaching: NO₃⁻ is water-soluble and repelled by negatively charged soil particles. Easily lost in percolating water
62
What are the major inputs and outputs in the soil nitrogen cycle?
Inputs: Nitrogen fixation, mineralization, DNRA, fertilization
Outputs: Immobilization, ammonium fixation, volatilization, denitrification, leaching
63
What about the Nitrogen cycle changes pH?
1.NH₄⁺ uptake by plants → lowers pH (releases H⁺)
2. NO₃⁻ uptake → raises pH (often releases OH⁻ or HCO₃⁻)
64
Nitrate uptake ______ pH and Ammonium uptake ______ pH.
Raises
lowers
65
What is the difference between inorganic and organic nitrogen?
Organic: derived from organic compounds (most common in soils) vs derived from inorganic compounds. (easier for plants to take up)
66
What is the most common form of nitrogen?
N2: atmospheric nitrogen.
67
_______ is the process of converting inorganic N into organic forms.
Immobilization (output)
68
______ is the process of turning organic nitrogen into inorganic nitrogen ions.
Mineralization (input)
69
_______ occurs readily with Ammonium ions being attracted to the negatively charged surfaces of clay and humus.
Fixation
70
Is fixation an input or an output?
Output
71
Is volatilization an input or output
OUtput
72
Which process creates ammonia gas? (NH3)
Volatilization
73
Volatilization is most likely to occur when ______ is present.
Urea
74
_______ is the conversion of NH4 to NO3-
Nitrificiation
75
Which process generates engird from oxidizing ammonium rather than organic material?
Nitrification
76
What are the two steps of nitrification?
1. Ammonium to nitrite via autotrophic bacteria called nitrosomonas
2. Nitrite to nitrate via autotrophic bacteria called nitrobacter
77
Why does nitrate have a hard time staying in soils?
Because soil is (-) and so is nitrate, leads to more leaching
78
________ is an anaerobic process where heterotrophic bacteria reduce nitrate to gasses.
Denitrification
79
Where is it most likely to find sulfate (So4)?
Regions with low rainfall--arid regions
80
Where are you most likely to find sulfides (s2)
Humid regions: has to be oxidized before uptake; increases acidity
81
_________ is the creation of sulfuric acids by autotrophic bacteria.
Oxidation
82
________ is the process of turning a sulfate to a sulfide.
Reduction
83
What are the four ways sulfur can 'move"?
Mineralization, Immobilization, Oxidation and reduction
84
What is the second most important nutrient to ecosystem productivity and health?
Phosphorous
85
T/F: Phosphorous is more common in the atmosphere than in soils.
False
86
What are the six ways that phosphorous can move through it's cycle?
1. Immobilization: organic to inorganic (input)
2. Mineralization: Inorganic to organic (output)
3. Precipitation: formation of solid phosphate minerals from a solution (output)
4. Weathering (input)
5. Adsorption: Phos molecules bind to the surface of soil particles (output)
6. Desorption: Release of P molecules back into the solution (input)
87
What is found in the largest quantity of any nutrient element?
Potassium
88
What are the three forms of potassium?
1. Mineral: slow to be uptaken
2. Fixed: slow release w/ water
3. Exchangeable K: fast and easiest to be taken up
89
Where are Calcium and Magnesium stored? (3)
1. Ca/Mg containing minerals (slow release)
2. Ca/MG complexed with soil humus
3. Ca/mg held by cation exchange on the clay and humus colloids (easiest/fastest)
90
T/F: 95% of roots are in the upper 2m of soil.
True
91
What are some secondary functions of roots?
1. store carbs
2. synthesize organic compounds
3. transport water and noots
4. secrete chemical substances
5. generate vegetative shoots
92
What are the greatest issues that roots face?
1. Compaction: reduced metric potential and aeration, crushed macropores, finer micropores increases wilting point and decreases water content.
93
What are two main advantages for roots to be in r3elation with mycorrhizae?
1. increased availability of water and noots
2. Protect from fungal and bacterial pathogens
94
which mycorrhizae is more common?
ectomycorrhizal
95
Why are soil organisms important?
1. Species diversity = number of different organisms
2. Functional diversity = variety of roles/processes they perform
96
What is functional redundancy and why is it important to soils?
A: Multiple organisms perform the same function, making the ecosystem more stable and resilient to disturbances.
97
Define stability and resilience in soil ecosystems.
* Stability: processes continue under changing conditions
* Resilience: ability to bounce back after disturbance
98
What do heterotrophs and autotrophs do in the soil food web?
1. Heterotrophs: get energy from other organisms (decay)
2. Autotrophs: use CO₂ or minerals for carbon
99
What are primary consumers in the soil food web?
Herbivores and detritivores that feed on plants and dead matter
100
Who are secondary consumers, and what do they do?
Predators and microbial feeders (mites, centipedes, fungi) that regulate microbial populations and influence nutrient cycling
101
What are tertiary consumers in soil?
Predators of predators—e.g., ants, birds, moles—release nutrients by consuming secondary consumers
102
Who are the “ultimate decomposers”?
Microbial decomposers—bacteria and fungi that break down organic matter directly and in other organisms’ digestive systems
103
What are saprophytic microorganisms?
Fungi and bacteria that feed on dead plant/animal tissue and detritus
104
What is decomposition in soil?
Breakdown of large organic molecules into simpler compounds, releasing nutrients and forming microbial biomass
105
What 4 factors drive decomposition rate in soil?
1. Temperature
2. Water availability
3. Organic matter quality
4. Decomposer community
106
What is the order of organic matter decomposition by speed?
Sugars > Starches > Simple proteins > Crude proteins > Hemicellulose > Cellulose > Fats/waxes > Lignin/phenolics
107
What is lignin and why does it decompose slowly?
A complex structural component in woody plants; provides strength and resists microbial breakdown.
