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Questions and Answers
What are the three phases generally considered to constitute soil?
What are the three phases generally considered to constitute soil?
- Solid, solution, and air
- Organic, inorganic, and gas
- Solid, liquid, and gas (correct)
- Mineral, liquid, and gas
Soil colloids are chemically unreactive due to their neutral electrical charge.
Soil colloids are chemically unreactive due to their neutral electrical charge.
False (B)
What is the typical size range (in microns) for soil colloids?
What is the typical size range (in microns) for soil colloids?
0.2 to 1 micron
__________ is a complex molecular weight organic product of the biological decomposition of organic residues in the soil.
__________ is a complex molecular weight organic product of the biological decomposition of organic residues in the soil.
Match the following clay types with their silica to alumina sheet ratio:
Match the following clay types with their silica to alumina sheet ratio:
Which of the following describes Kaolinite's expansion characteristics?
Which of the following describes Kaolinite's expansion characteristics?
The basal spacing in kaolinite is relatively large compared to other clay types, allowing for significant water retention.
The basal spacing in kaolinite is relatively large compared to other clay types, allowing for significant water retention.
What type of chemical bond tightly binds the Th and Oh sheets together in Kaolinite?
What type of chemical bond tightly binds the Th and Oh sheets together in Kaolinite?
The general formula for aluminum and silicon in Allophane is __________.
The general formula for aluminum and silicon in Allophane is __________.
Match the clay mineral with the correct CEC range (meq/100g)
Match the clay mineral with the correct CEC range (meq/100g)
Which of the following conditions promotes the formation of kaolinite?
Which of the following conditions promotes the formation of kaolinite?
Montmorillonite clay minerals are stable under high hydronium concentrations and rapid leaching conditions.
Montmorillonite clay minerals are stable under high hydronium concentrations and rapid leaching conditions.
In montmorillonite clay minerals, which ion commonly replaces $Al^{3+}$ in the octahedral (Oh) layer?
In montmorillonite clay minerals, which ion commonly replaces $Al^{3+}$ in the octahedral (Oh) layer?
Unlike montmorillonite and kaolinite, vermiculite does not form by crystallization from solution but instead by __________.
Unlike montmorillonite and kaolinite, vermiculite does not form by crystallization from solution but instead by __________.
Match the soil properties to their correct type of hydrous oxides of Fe and Al.
Match the soil properties to their correct type of hydrous oxides of Fe and Al.
Which property characterizes soils rich in hydrous oxides of iron and aluminum?
Which property characterizes soils rich in hydrous oxides of iron and aluminum?
Amorphous silicate clays are typically associated with old, highly weathered soils.
Amorphous silicate clays are typically associated with old, highly weathered soils.
What are two examples of amorphous (non-crystalline) silicate clays?
What are two examples of amorphous (non-crystalline) silicate clays?
Hydrous mica formation starts when $K^+$ replaces some of the interlayer cations of __________ or vermiculites
Hydrous mica formation starts when $K^+$ replaces some of the interlayer cations of __________ or vermiculites
Match the following clay minerals with one of their defining characteristics
Match the following clay minerals with one of their defining characteristics
What is the main characteristic of chlorites that differentiates them from other 2:1 layer minerals?
What is the main characteristic of chlorites that differentiates them from other 2:1 layer minerals?
Ion exchange is an irreversible process in which ions are permanently bound to soil particles.
Ion exchange is an irreversible process in which ions are permanently bound to soil particles.
What is the primary reason that ions are exchanged in the soil?
What is the primary reason that ions are exchanged in the soil?
Cation Exchange Capacity (CEC) is a quantitative measure of the ability of a soil to exchange __________ with the soil solution.
Cation Exchange Capacity (CEC) is a quantitative measure of the ability of a soil to exchange __________ with the soil solution.
Match the following cations with the correct strength of absorption:
Match the following cations with the correct strength of absorption:
What does the degree of ESP measure:
What does the degree of ESP measure:
Increasing the clay content in soil generally decreases the cation exchange capacity (CEC).
Increasing the clay content in soil generally decreases the cation exchange capacity (CEC).
List two cations typically found in the soil solution that can be exchanged with cations on the surface of soil colloids.
List two cations typically found in the soil solution that can be exchanged with cations on the surface of soil colloids.
The process by which $K^+$ replaces some of the interlayer cations of montmorillonites or vermiculites initiates the formation of __________.
The process by which $K^+$ replaces some of the interlayer cations of montmorillonites or vermiculites initiates the formation of __________.
Match the following hydrous oxides of iron and aluminum to their formula:
Match the following hydrous oxides of iron and aluminum to their formula:
Which of the following best describes the typical soil textures in soils rich with hydrous oxides of Fe and Al when assessed by feel?
Which of the following best describes the typical soil textures in soils rich with hydrous oxides of Fe and Al when assessed by feel?
Soil pH is determined by the abundance of sodium and potassium ions in the soil solution.
Soil pH is determined by the abundance of sodium and potassium ions in the soil solution.
What element can become more soluble at a toxic level if pH is too low?
What element can become more soluble at a toxic level if pH is too low?
The process of increasing soil pH by adding lime is known as ___________.
The process of increasing soil pH by adding lime is known as ___________.
Match the liming material with its chemical name:
Match the liming material with its chemical name:
Which of the following accurately describes the importance of soil pH to farmers?
Which of the following accurately describes the importance of soil pH to farmers?
Lowering the pH of soil is generally easier than raising the pH.
Lowering the pH of soil is generally easier than raising the pH.
Name two common materials that can be added to the soil to lower its pH (acidify the soil).
Name two common materials that can be added to the soil to lower its pH (acidify the soil).
___________ refers to the soil's ability to resist drastic changes in pH.
___________ refers to the soil's ability to resist drastic changes in pH.
Match the soil type with their description:
Match the soil type with their description:
Flashcards
Solid Soil Phase
Solid Soil Phase
The solid portion that serves as a skeletal framework of soils.
Liquid phase
Liquid phase
The liquid portion that carries dissolved nutrients.
Soil's Gas Phase
Soil's Gas Phase
The gaseous component mainly composed of N2, O2, and CO2.
Soil Colloids
Soil Colloids
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Humus
Humus
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Crystalline Silicate Clays
Crystalline Silicate Clays
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Kaolinite
Kaolinite
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Vermiculite
Vermiculite
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Montmorillonite
Montmorillonite
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Hydrous Mica (Illites)
Hydrous Mica (Illites)
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Chlorite
Chlorite
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Hydrous Oxide Clays
Hydrous Oxide Clays
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Amorphous Silicate Clays
Amorphous Silicate Clays
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Allophanes
Allophanes
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Ion Exchange
Ion Exchange
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Cation Exchange
Cation Exchange
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Cation Exchange Capacity
Cation Exchange Capacity
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Anion Exchange
Anion Exchange
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Percent Base Saturation
Percent Base Saturation
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Exchangeable Sodium Percentage (ESP)
Exchangeable Sodium Percentage (ESP)
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Soil pH
Soil pH
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Acid Sulfate Soils
Acid Sulfate Soils
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Liming
Liming
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Sources of acidity
Sources of acidity
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Saline Soil
Saline Soil
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Sodic Soil
Sodic Soil
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Buffering Capacity
Buffering Capacity
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Study Notes
- Unit IV covers soil chemistry
- **
Chemical Nature of Soil
- Soil has 3 phases:
- Solid: organic and inorganic, the skeletal framework
- Liquid: soil solution, carrying dissolved nutrients
- Gas: Soil air composed mainly of N2, O2, and CO2
Soil Colloids
- Soil Colloids are very small particles of matter, sized 0.2 to 1 micron
- Soil Colloids are chemically reactive due to electrical charges on their surface
Organic Colloids
- Represented by humus, a complex organic product from biological breakdown of residues
- Humus improves the soil's ability to adsorb and exchange ions
Basic Structure of Aluminum Silicate Clays
- Includes Silica Tetrahedron and Alumina Octahedron
Inorganic Colloids
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Crystalline silicate clays are sheet-structured aluminosilicates
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Various types depending on the ratio of silica sheet to alumina sheet in the crystal structure.
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Kaolinite -Silicate Clay Ratio of 1:1 -Non-expanding type
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Illite -Silicate Clay Ratio of 2:1 -Non-expanding type
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Vermiculite -Silicate Clay Ratio of 2:1 -Limited expansion type
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Montmorillonite -Silicate Clay Ratio of 2:1 -Expanding type
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Chlorite
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Crystalline Silicate Clay Ratio of 2:2
Amorphous Silicate Clays
- Represented by allophone and imogolite
- In the Philippines, they are associated with young soils from volcanic ash with high organic matter
Amorphous Non-Silicate Clays
- Occur as amorphous hydrous oxides of iron and aluminum
- Examples include Hematite, Geothite, Limonite, Boehmite, and Gibbsite
Ion Exchange
- Reversible process: ions exchanged between solid/liquid phases or between solid phases in close contact
- Occurs due to electrical charges in the soil
Cation Exchange
- Attraction of cations to colloid surfaces and exchange for ions in the soil solution
- Common cations: NH4+, Ca2+, Mg2+, Na+, H+, K+
Cation Exchange Capacity (CEC)
- Soil CEC is the ability to adsorb and exchange cations with the surrounding soil solution and plant roots
- The sum of all adsorbed cations per unit amount of soil
- CEC increases with more clay and organic matter
- Adsorption strength: (Al3+, H+) > Ca2+ > Mg2+ > K+ > Na+
- In leached soils, strongly adsorbed cations are left in the soil
Calculating Cation Exchange Capacity (CEC)
- Number of cation adsorption sites per unit weight of soil
- Sum total of exchangeable cations that a soil can adsorb.
- cmol Weight Calculation:
- Divide the relative atomic mass of the ion by 100.
- Divide the result from above by the charge on the ion.
- For Ca2+, it is 40 g mol^-1 or 0.40 g cmol^-1/2 or 0.20 g cmol^-1.
Anion Exchange
- Attraction of anions to colloid surfaces and exchange for ions in the soil solution
- Examples include NO3-, PO4-, and SO4-
Anion Exchange Capacity
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Ability to remove anions in the presence of water and store those anions for later exchange.
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Colloids with low CEC tend to have some AEC.
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Levels of AEC are much lower than CEC.
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Phosphates tend to be held at anion exchange sites.
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Compacted soils, such as clay, have more anion exchange than loose soils.
Relative Order of Anion Exchange in Soil Colloid:
F- > H2PO4- > HCO3- > HBO3- > SO42- > Cl- > NO3-
Percent Base Saturation
- Degree to which exchange sites in colloids are occupied by basic cations.
- Formula: %BS = (me of bases/CEC) x 100
Example Calculation
-BS = (10.0+6.0+0.5+1.5) / 27.0 x 100
- %BS = 66.7%
Exchangeable Sodium Percentage (ESP)
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Degree by which exchange sites of colloids are occupied by sodium ions Calculated by taking the ratio of the me of Na+ and that of the CEC.
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Sample Calculations
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CEC = 27 me/100g soil,
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Na+ = 1.5 me/100g soil
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ESP = (me Na+) / CEC x 100 -ESP = (1.5/27) x 100 (%)
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ESP = 5.6%
Soil pH
- Soil pH is determined by the level of acidity or alkalinity
- Also referred to as soil reaction.
- pH = - log [H+]
- Indicator of fertility problems
Soil pH and Nutrient Availability
- Favorable pH for agriculture; pH 6 to 7
- Philippine pH range: pH 5.5-6.5
- If pH is low/acidic (<5.0): Ca, Mg, K, P, Mo, and N availability decreased
- Low/acidic pH; Fe, Al, and Mn become soluble to toxicity
- High/alkaline pH; micronutrients except Mo become unavailable, resulting to Iron deficiency
- **
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