soil Flashcards
(152 cards)
1
Q
What are the chemical properties of soils dependent on?
A
Their composition
Soil composition includes solid mineral particles, organic matter, and ions in the soil solution.
2
Q
What are the main components of soil solids?
A
Soil solids include:
* Clay
* Sesquioxides
* Organic matter
* Ions in the soil solution
3
Q
What do soil minerals typically occur as?
A
Ionic species with a unique and stable electron configuration
Some covalent character also exists in soil minerals.
4
Q
What are the primary minerals in soil?
A
Primary minerals include:
* Feldspars
* Quartz
* Olivine
5
Q
What are secondary minerals in soil?
A
Mineral products of weathering, such as:
* Clay minerals
* Iron oxides
* Al-oxides
6
Q
What are the most common elements in the Earth’s crust by weight percentage?
A
The most common elements include:
* Oxygen (46.20%)
* Silicon (27.72%)
* Aluminum (8.13%)
* Iron (5.00%)
* Calcium (3.63%)
* Sodium (2.83%)
* Potassium (2.59%)
* Magnesium (2.09%)
7
Q
What defines a major element in soils?
A
Concentrations that exceed 100 mg kg−1
All other elements are referred to as trace elements.
8
Q
How can minerals be classified based on their dominant anion?
A
Minerals can be classified into 8 classes:
* Native elements
* Silicates
* Carbonates and phosphates
* Sulphides
* Sulphates
* Oxides and hydroxides
* Chlorides
9
Q
Give examples of primary minerals.
A
Examples of primary minerals include:
* Quartz
* Plagioclase
* Feldspar
* Muscovite
* Amphibole
* Pyroxene
* Olivine
* Biotite
10
Q
What are primary minerals formed from?
A
The solidification of magma and lava
Primary minerals are the building blocks of rock.
11
Q
What characterizes primary minerals in soil?
A
They have not experienced significant chemical alteration since their formation.
12
Q
What defines secondary minerals?
A
End products of the chemical alteration of primary minerals.
13
Q
Name some common types of secondary minerals.
A
Common types include:
* Alumino-silicates
* Oxides and hydroxides
* Carbonates and sulphates
14
Q
What are primary silicates in soil derived from?
A
The physical disintegration of parent rock material.
15
Q
What is the fundamental building block of primary silicates?
A
The silica tetrahedron: SiO4−4
16
Q
What defines a silicate?
A
An ionic compound whose anions consist predominantly of silicon and oxygen atoms.
17
Q
What are the different modes of occurrence of silica tetrahedra in minerals?
A
Silica tetrahedra can occur as:
* Isolated units
* Single chains
* Double chains
* Sheets
* Three-dimensional frameworks
18
Q
What is the silica to oxygen ratio in orthosilicates?
A
1:4
19
Q
What is a notable characteristic of olivine?
A
It features the least amount of covalence in its chemical bonds.
20
Q
What is the silica to oxygen ratio in sorosilicates?
A
2:7
21
Q
What examples of minerals belong to the sorosilicate class?
A
Examples include:
* Epidote
* Allanite
22
Q
What are the repeating units in inosilicates?
A
Single chains: Si2O6^4- and double chains: Si4O11^6−
23
Q
What is the silica to oxygen ratio in cyclosilicates?
A
1:3
24
Q
What are examples of minerals in the cyclosilicate class?
A
Examples include:
* Beryl
* Tourmaline
25
What are tectosilicates characterized by?
Framework silicates where all oxygens are shared, giving Si:O of 1:2
26
What are common minerals in tectosilicates?
Common minerals include:
* Quartz
* Feldspars
27
What distinguishes phyllosilicates?
Consist of two sheets of silica tetrahedra joined to a sheet of metal cation octahedra.
28
What is isomorphic substitution?
The replacement of an atom by another atom in a crystal lattice.
29
What occurs during the weathering of phyllosilicates?
They transform to secondary silicates (clay) by loss of structural K+.
30
What do secondary minerals primarily form from?
The chemical weathering of primary minerals.
31
What factors influence the formation of secondary minerals?
Factors include:
* Rates of weathering reactions
* Rates of crystallization and precipitation
* Rates of leaching
32
What are some examples of cations found in soil?
* Co^2+
* Cu^2+
* Zn^2+
* SO4^2-
* CO3^2-
* PO4^3-
## Footnote
These cations play a significant role in soil chemistry and nutrient availability.
33
What factors influence the formation of secondary minerals?
* Rates of weathering reactions of primary minerals
* Rates of crystallization and precipitation reactions
* Rates of leaching
## Footnote
These processes are essential in soil development and mineral transformation.
34
What is the process of formation of secondary minerals?
* Alteration
* Neoformation
## Footnote
These processes lead to the transformation of primary minerals into secondary minerals.
35
What are secondary silicates in soil?
Clay minerals formed from the transformation, mineralisation, and reorganisation of primary phyllosilicates
## Footnote
They are crucial for soil structure and nutrient retention.
36
What type of structure do secondary silicates (clays) have?
* Layer structure
* 2:1 Layer silicates
* 1:1 Layer silicates
* 2:1:1 Layer silicates
* Amorphous silicates
## Footnote
The structural classification helps in understanding their properties and behavior in soils.
37
What are the three soil clay mineral groups with 2:1 phyllosilicate structure?
* Illite
* Vermiculite
* Smectite
## Footnote
These minerals have different properties affecting soil behavior.
38
What is the basic structure of 2:1 phyllosilicates?
Two tetrahedral sheets bonded to either side of an octahedral sheet
## Footnote
This structure is characteristic of minerals like mica.
39
What are the characteristics of Illite clay minerals?
* Dioctahedral
* Isomorphic substitution of Al^3+ for Si^4+ in the tetrahedral layer
* Cation exchange capacities range between 10 and 40 cmolc kg−1
## Footnote
These properties influence soil fertility and mineral interactions.
40
What is the range of cation exchange capacity (CEC) for vermiculite?
100 to 200 cmolc kg−1
## Footnote
Higher CEC indicates better nutrient retention capabilities.
41
What are the properties of smectite clay minerals?
* Lower layer charge than vermiculite
* Isomorphic substitution primarily in octahedra
* Exchangeable interlayer cations
## Footnote
These properties contribute to their shrink-swell behavior.
42
What is the significance of hydrous metal oxides in soil?
* Weathering products of anhydrous oxides, hydroxides, and oxyhydroxides
* Hydrous and reactive surfaces
* Can develop either negative or positive surface charge
## Footnote
They play a crucial role in soil chemistry and nutrient availability.
43
What is the most common hydrous iron oxide found in soil?
Goethite [FeOOH]
## Footnote
It is prevalent in cool and wet climates.
44
What is a distinguishing feature of kaolinite clay minerals?
Common in highly weathered soils, produced by the chemical weathering of aluminium silicate minerals
## Footnote
Kaolinite has low cation exchange capacity and low shrink-swell capacity.
45
What characterizes the structure of chlorite clay minerals?
Same basic structure as 2:1 clays, with charge balance by hydroxide sheet
## Footnote
This structure affects their stability and interactions in soils.
46
Fill in the blank: Allophane and Imogolite are _______ and poorly ordered aluminosilicate clay materials.
Hydrated
## Footnote
These materials develop in soils with high concentrations of dissolved Al and Si.
47
What is Fe3+ in the context of soil?
Iron in hydrous Fe oxide structures
## Footnote
Fe3+ is often found in secondary minerals in soils.
48
What are Allophane and Imogolite?
Hydrated and poorly ordered aluminosilicate clay materials
## Footnote
They develop in soils with high concentrations of dissolved Al and Si.
49
In which type of soils are Allophane and Imogolite most commonly found?
Soils of volcanic origin (Andisols)
## Footnote
This is due to the rapid release of Si and Al during the weathering of volcanic glass.
50
What are the major carbonates found in soils?
* Calcite (CaCO3)
* Magnesite (MgCO3)
* Dolomite (CaMg(CO3)2)
* Ankerite ((Ca,Fe,Mg)2(CO3)2)
* Siderite (FeCO3)
* Rhodochrosite (MnCO3)
## Footnote
Carbonates can be inherited from parent material or formed through secondary precipitation.
51
What is the major sulphate mineral found in soils?
Gypsum (CaSO4.2H2O)
## Footnote
Gypsum is a common secondary mineral in soils.
52
What are soil colloids?
Particles larger than individual molecules but small enough (< 2 μm) to remain in suspension
## Footnote
Colloids in soil include clay particles, hydrous oxide particles of Al and Fe, amorphous material, and humic particles.
53
What is the surface area range of soil colloids?
From ~10 m2 g-1 to ~800 m2 g-1
## Footnote
Surface area for sand is < 1 m2 g-1.
54
What is the charge nature of colloid surfaces in soil?
Could be permanent charge (-), pH dependent charge (-/+) or edge charge (-/+)
## Footnote
Permanent charge is due to isomorphic substitution, while pH dependent charge is due to hydroxide groups.
55
What is the Charge Double Layer in soil?
A tightly held layer and a less tightly held cloud of ions at the colloid's surface
## Footnote
Successive layers of ions are held at increasing distances from the surface.
56
What factors influence cation selectivity in soil?
* Charge
* Size (hydrated radius)
* Concentration
## Footnote
Selectivity increases with increasing charge and concentration, and decreases with increasing hydrated radius.
57
What is Cation Exchange Capacity (CEC)?
The ability of a soil to adsorb cations
## Footnote
It reflects the soil's ability to supply nutrients like K+, Mg2+, and Ca2+.
58
What is the significance of Percent Base Saturation?
Reflects the proportion of the CEC occupied by basic cations
## Footnote
Base cations include Ca2+, Mg2+, K+, Na+, and NH4+.
59
How are exchangeable cations made available to plants?
H+ from root hairs and microorganisms replace nutrients held on clay surfaces
## Footnote
This process facilitates nutrient availability.
60
What are the two major ways anions are held by soil colloids?
* Anion adsorption mechanisms similar to cation exchange
* Reaction with surface oxides or hydroxides, forming inner-sphere complexes
## Footnote
Anions may include phosphates, molybdates, and sulphates.
61
What is the role of hydrous oxides of Fe and Al in anion exchange?
They provide positive charges that facilitate the adsorption of anions
## Footnote
Anions can be replaced by other anions in the exchange process.
62
What is the formula for the reaction involving Al and H2PO4?
2 + H2PO4 - ⇌ - Al - H2PO4 + H2O
## Footnote
This reaction indicates the interaction between aluminum ions and dihydrogen phosphate.
63
What does soil dispersion refer to?
The process where soil particles separate and spread out, often leading to a loss of structure.
64
Define an acid in terms of proton donation.
A substance that can donate a proton (H+).
65
Define a base in terms of proton acceptance.
A substance that can accept a proton.
66
What are HA and A- in acid-base chemistry?
HA and A- are called conjugate acid-base pairs.
67
How is the strength of acid-base pairs evaluated?
Through dissociation constants.
68
What is the general definition of pH?
pH = -log[H+].
69
What is the relationship between [H+] and [OH-] in water?
10^-14 = [H+][OH-].
70
What are the soil pH ranges for acid soils?
Soil pH < 6.0 indicates potential nutrient deficiencies.
71
What is a common cause of soil acidity?
Leaching of basic cations.
72
What happens to cations when pH < 4.7?
Leached cations will be replaced by Al on CEC sites.
73
What are some sources of H+ in acid soils?
* Carbon dioxide
* Microbes generating organic acids
* Organic matter
* Nitrification
* Acid rain
* Plant uptake of cations
74
What is the effect of liming on acidic soils?
Increases pH and reduces excess soluble Al.
75
What is the purpose of gypsum in acidic soils?
Supplies Ca and alleviates Al toxicity.
76
What characterizes alkaline soils?
Reduced availability of Al, Fe, Mn, Cu, Zn due to hydroxide formation.
77
What is the typical pH of normal neutral and alkaline soils in dry areas?
pH is commonly 7 or above.
78
What is the importance of carbonate and bicarbonate in soils?
They act as bases by reacting with water to form hydroxyl ions.
79
What is the classification of soil acidity?
Active acidity, potential acidity, residual acidity.
80
What does residual acidity refer to?
Soil acidity that remains after active and reserve acidity have been neutralized.
81
How does aluminum contribute to soil acidity?
Through its tendency to hydrolyze and release H+ ions.
82
What is the relationship between pH and nutrient availability?
A pH range of about 5.5 to 7.0 promotes nutrient availability.
83
What is the effect of soil pH on microbial activity?
pH influences the availability of plant nutrients and microbial activity.
84
Fill in the blank: The total acidity of soil is equal to active acidity plus potential acidity plus _______.
residual acidity.
85
What is the chemical equation for the reaction involving Al(OH)2+ and H+?
Al(OH)2+ + H+ <=> Al(OH)3 + H+
## Footnote
This reaction illustrates the behavior of aluminum ions in soil acidity.
86
What role does Al play in soil acidity?
Exchangeable Al concentration is a source of reserve acidity, similar to reserve acidity from exchangeable H+ concentrations.
87
What is the role of CO2 in soil acidity?
CO2 contributes to soil acidity through various processes including the use of fertilizers.
88
What is the chemical reaction for the nitrification process involving ammonium fertilizers?
2NH4+ + 3O2 => 2NO2- + 2H2O + 4H+
## Footnote
This process increases soil acidity.
89
What is the reaction involving sulfur and its effect on soil?
2S + 3O2 + 2H2O => 2H2SO4
## Footnote
Sulfur-oxidizing bacteria facilitate this reaction, lowering soil pH.
90
What is the impact of pyrites (FeS2) on soil acidity?
Oxidation of pyrite leads to the production of FeSO4 and contributes to soil acidity.
91
What is acid rain?
Acid rain results from the conversion of nitrogen oxides and sulfur dioxide into strong acids.
92
What are the main gases contributing to acid rain?
* Carbon monoxide
* Nitrogen oxides
* Sulfur oxides
93
What is the chemical reaction for the formation of sulfuric acid from sulfur dioxide?
2SO2 + O2 => 2SO3; SO3 + H2O => H2SO4
94
What happens to cations during cation-exchange in acid rain-affected soils?
H+ trade places with metal ions, leading to leaching of Ca, K, and Mg ions.
95
What is the effect of acid rain on aluminum in soil?
Acid rain mobilizes Al3+, which is toxic to plants at pH 5 or lower.
96
What is the role of limestone in neutralizing soil acidity?
Limestone reacts with acids to reduce acidity and change chemical characteristics of water.
97
What factors influence the effectiveness of soil in neutralizing acid rain?
* Type of soil
* Thickness of soil
* Weather
* Water flow patterns
98
How does hydrangea indicate soil pH?
At pH below 5.0–5.5, blooms turn blue due to aluminum absorption; at pH 6.0 or above, blooms are pink.
99
Define buffering capacity of soil.
The ability of soil to resist changes in pH upon addition of acid or alkali.
100
What are redox conditions in soil?
Redox conditions control nutrient mobility and availability, influenced by various factors including pH and waterlogging.
101
What do redox potential values in soil range from?
-300 to 900 mV
102
What color indicates oxidizing conditions in soil?
Red and brown
103
What color indicates reducing conditions in soil?
Blue green and grey
104
What is salinization?
The accumulation of salts in soil, often in arid and semi-arid regions.
105
What are the main salts found in saline soils?
* NaCl
* Na2SO4
* CaCO3
* MgCO3
106
What is the consequence of salinization on soil productivity?
It renders the soil unproductive for decades or centuries.
107
What is the relationship between salinization and desertification?
Salinization is closely related to the process of desertification.
108
What is salinization?
A problem affecting 28 countries and 955M Ha of soil.
109
How are salts brought to the surface in saline soils?
By the upward movement of water which then evaporates, leaving the salts behind.
110
What salinity level is considered to kill growing plants?
2000 - 5000 ppm
111
What are salt-sensitive plants affected by?
Salinity levels
112
What can specific ions in saline soils cause in plants?
Toxicity due to accumulation in plant tissue.
113
What nutrient imbalances can high sodium levels induce in plants?
* Potassium deficiencies
* Calcium deficiencies
114
What is physiological drought in the context of saline soils?
Roots raise their own solute levels to lower osmotic potential, consuming energy.
115
How does high soil salt levels affect microbial populations?
It inhibits some microorganisms while encouraging others, changing the soil's biological nature.
116
What effect does salinity have on plant roots?
It makes them more prone to root rot infections.
117
What is soil salinity?
A measure of the total salt concentration in soil, commonly expressed in terms of electrical conductivity.
118
What is the exchangeable sodium percentage (ESP)?
A measure of the exchangeable sodium fraction expressed as a percentage of total cation exchange capacity.
119
What does the sodium adsorption ratio (SAR) indicate?
The ratio of sodium concentration to the concentrations of calcium and magnesium in soil solution.
120
What are the three groups of soils recognized based on ESP and SAR?
* Non-saline soils
* Saline soils
* Saline-sodic soils
121
What characterizes saline soils?
EC > 4,000 µS/cm, ESP < 15%, pH usually < 8.5.
122
What happens to saline soils at an ESP of 15%?
Dispersion of saline soils occurs.
123
What defines saline-sodic soils?
EC > 4,000 µS/cm, ESP > 15%, SAR at least 13.
124
What happens when saline-sodic soils are leached?
Free salt content decreases, and the soil may become sodic.
125
What is the characteristic of non-saline sodic soils?
EC < 4,000 µS/cm, ESP > 15%, SAR > 13.
126
What can develop due to irrigation in non-saline sodic soils?
Strongly alkaline conditions with pH values reaching 10.
127
What is the effect of sodium on soil structure?
Destruction of soil structure causing poor physical condition.
128
What happens to soil organic matter in sodic soils?
It may disperse and/or dissolve, leading to black coloration on the surface.
129
What can plants take up from saline soils that may injure their tissues?
Sodium
130
What are the reclamation measures for saline soils?
* Establish internal drainage
* Flush salts with water
131
What is a rule of thumb for flooding a unit depth of soil?
Flood with the same depth of water to decrease salt content by 20%.
132
What may limit evapotranspiration in reclamation efforts?
A sand cover
133
What is a potential challenge when reclaiming saline soils?
If the salt content is above 10%, reclamation might be too expensive.
134
What is the impact of sodium salts on soil colloids?
They separate colloids and disperse soil aggregates.
135
What physical condition do sodic soils often exhibit?
Very bad physical condition due to deflocculating action of sodium.
136
What is a key factor in evaluating saline soils?
Salt content above 10% might be too expensive to reclaim.
## Footnote
Evaluating the salt content is crucial for determining reclamation feasibility.
137
What can a sand cover do in saline soil management?
Limit evapotranspiration.
## Footnote
This can help in managing moisture levels in the soil.
138
What is one method to control sodicity in saline sodic soils?
Use of gypsum to control sodicity.
## Footnote
Gypsum provides calcium which helps in replacing sodium on cation exchange sites.
139
What effect does flushing have on saline sodic soils?
Flushing will not leach out excess Na.
## Footnote
It is important to use good quality water to avoid converting saline soils to sodic.
140
What happens to soil pH when neutral ions are leached out?
Soil pH is likely to increase.
## Footnote
This occurs unless leaching water contains sufficient Ca and Mg ions.
141
What occurs when mineral colloids become dispersed in saline soils?
Deterioration of the soil structure.
## Footnote
This can negatively impact soil health and productivity.
142
What should be done first when reclaiming saline sodic soils?
Replace Na on CEC sites with Ca.
## Footnote
The procedure varies with soil texture, clay content, and water quality.
143
What is a recommended action after replacing Na with Ca?
Leach out the excess Na.
## Footnote
This helps to improve soil quality.
144
How can high pH in soils be reduced?
Use sulfur to reduce pH.
## Footnote
This is important for improving soil conditions for plant growth.
145
What is the consequence of applying good quality water to flush saline sodic soil?
It may become sodic.
## Footnote
Care should be taken to avoid this transformation.
146
What is the role of gypsum in managing sodic soils?
Calcium replaces sodium on cation exchange sites.
## Footnote
This process helps in flocculating soil structure.
147
What is a method to maintain high water content in saline soils?
Keeps salts diluted.
## Footnote
This helps plants tolerate higher salt levels.
148
What should be done before planting in saline soils?
Leach soil to move salts below the root zone.
## Footnote
This is critical during early plant development.
149
Where should plants be positioned to avoid salt buildup?
On the side of ridges.
## Footnote
This positioning helps mitigate the effects of salt movement.
150
What is a benefit of using sprinkler irrigation in saline soils?
Keeps salt washed into the soil profile.
## Footnote
This helps manage salinity levels effectively.
151
What happens to soil topography in the presence of impermeable layers?
Creates a low point for water with dissolved salts to seep out.
## Footnote
This can lead to saline seeps which require management.
152
What is a strategy to utilize water in saline seeps?
Add plantings to help utilize the water.
## Footnote
This can assist in managing excess moisture and salinity.
