Soil Science Final Review

These flashcards cover key concepts and terminology relevant to soil science and provide a review tool for students preparing for their final exam.

What does acidity refer to in soil science?

Acidity refers to the concentration of hydrogen ions (H⁺) in a substance, with higher H⁺ indicating more acidity.

What is alkalinity?

Alkalinity refers to the concentration of hydroxide ions (OH⁻) or a substance's ability to neutralize acids, with more OH⁻ indicating greater alkalinity.

What is pH and its range?

pH is the power of hydrogen, measured on a logarithmic scale from 0–14, where low pH (

Why is the pH scale logarithmic?

Each unit in the pH scale represents a 10× change in hydrogen ion activity, meaning pH 5 has 10× more H⁺ ions than pH 6.

Where are acidic soils commonly found?

Acidic soils are common in humid regions, forest ecosystems, and areas with heavy rainfall and leaching.

What causes soil acidity in humid regions?

Heavy rainfall leaches base cations (Ca²⁺, Mg²⁺, K⁺) from the soil, leaving behind H⁺ and Al³⁺, which contribute to acidity.

What are the common locations for alkaline soils?

Alkaline soils are found in arid and semi-arid regions, poorly drained basins, and areas with carbonate-rich parent material.

What process involves the nitrification of ammonium fertilizers and its effect on soil?

Nitrification converts NH₄⁺ to NO₃⁻ and releases H⁺, which increases soil acidity.

What happens when pyrite is exposed to oxygen?

Exposure of pyrite (FeS₂) to oxygen leads to the formation of sulfate (SO₄²⁻) and sulfuric acid (H₂SO₄), causing extreme soil acidity.

What is soil buffering capacity?

Soil buffering capacity is the ability of soil to resist changes in pH.

Describe the three reservoirs of acidity in soils.

1. Active acidity – H⁺ in soil solution, measured by pH. 2. Exchangeable acidity – H⁺ and Al³⁺ on exchange sites, measured by salt extraction. 3. Residual acidity – bound to minerals and organic matter, influencing lime requirement.

What are the impacts of acid rain on soils?

Acid rain can leach nutrients (Ca²⁺, Mg²⁺), mobilize toxins (Al³⁺, heavy metals), acidify water bodies, and harm vegetation.

Why is Al³⁺ a problem for plants at low pH?

Al³⁺ becomes more soluble at pH < 5.5, leading to root tip damage, reduced growth, and inhibited nutrient uptake.

What is the key difference between saline and sodic soils?

Saline soils have high soluble salts, while sodic soils have high exchangeable sodium.

What is the role of gypsum in reclamation efforts for sodic soils?

Gypsum provides Ca²⁺ to replace Na⁺ on exchange sites, promoting flocculation and leaching of sodium.

What is the main source of permanent charge in soil clays?

Permanent charge results from isomorphic substitution within clay mineral layers, independent of pH.

How do nitrifying bacteria like Nitrosomonas and Nitrobacter function in the nitrogen cycle?

Nitrosomonas converts NH₄⁺ to NO₂⁻, while Nitrobacter converts NO₂⁻ to NO₃⁻, both requiring oxygen and occurring in aerobic soils.

What is the significance of Liebig's Law of the Minimum in plant nutrition?

Liebig's Law states that plant growth is limited by the most limiting nutrient, not by the total supply available.

What is the consequence of high C/N ratios for soil microbes?

High C/N ratios mean that microbes will immobilize nitrogen from the soil for cellular needs, leading to temporary nitrogen depletion for plants.

Identify the main benefits of cover crops in soil management.

Cover crops enhance soil health, prevent erosion, improve nutrient cycling, and maintain organic matter.

Explain the significance of the Tyndall Effect in soil science.

The Tyndall Effect describes how colloidal particles scatter light, demonstrating that certain particles can remain suspended in a soil suspension.

What does acidity refer to in soil science?

Acidity refers to the concentration of hydrogen ions (H⁺) in a substance, with higher H⁺ indicating more acidity.

What is alkalinity?

Alkalinity refers to the concentration of hydroxide ions (OH⁻) or a substance's ability to neutralize acids, with more OH⁻ indicating greater alkalinity.

What is pH and its range?

pH is the power of hydrogen, measured on a logarithmic scale from 0–14, where low pH (

Why is the pH scale logarithmic?

Each unit in the pH scale represents a 10× change in hydrogen ion activity, meaning pH 5 has 10× more H⁺ ions than pH 6.

Where are acidic soils commonly found?

Acidic soils are common in humid regions, forest ecosystems, and areas with heavy rainfall and leaching.

What causes soil acidity in humid regions?

Heavy rainfall leaches base cations (Ca²⁺, Mg²⁺, K⁺) from the soil, leaving behind H⁺ and Al³⁺, which contribute to acidity.

What are the common locations for alkaline soils?

Alkaline soils are found in arid and semi-arid regions, poorly drained basins, and areas with carbonate-rich parent material.

What process involves the nitrification of ammonium fertilizers and its effect on soil?

Nitrification converts NH₄⁺ to NO₃⁻ and releases H⁺, which increases soil acidity.

What happens when pyrite is exposed to oxygen?

Exposure of pyrite (FeS₂) to oxygen leads to the formation of sulfate (SO₄²⁻) and sulfuric acid (H₂SO₄), causing extreme soil acidity.

What is soil buffering capacity?

Soil buffering capacity is the ability of soil to resist changes in pH.

Describe the three reservoirs of acidity in soils.

1. Active acidity – H⁺ in soil solution, measured by pH. 2. Exchangeable acidity – H⁺ and Al³⁺ on exchange sites, measured by salt extraction. 3. Residual acidity – bound to minerals and organic matter, influencing lime requirement.

What are the impacts of acid rain on soils?

Acid rain can leach nutrients (Ca²⁺, Mg²⁺), mobilize toxins (Al³⁺, heavy metals), acidify water bodies, and harm vegetation.

Why is Al³⁺ a problem for plants at low pH?

Al³⁺ becomes more soluble at pH < 5.5, leading to root tip damage, reduced growth, and inhibited nutrient uptake.

What is the key difference between saline and sodic soils?

Saline soils have high soluble salts, while sodic soils have high exchangeable sodium.

What is the role of gypsum in reclamation efforts for sodic soils?

Gypsum provides Ca²⁺ to replace Na⁺ on exchange sites, promoting flocculation and leaching of sodium.

What is the main source of permanent charge in soil clays?

Permanent charge results from isomorphic substitution within clay mineral layers, independent of pH.

How do nitrifying bacteria like Nitrosomonas and Nitrobacter function in the nitrogen cycle?

Nitrosomonas converts NH₄⁺ to NO₂⁻, while Nitrobacter converts NO₂⁻ to NO₃⁻, both requiring oxygen and occurring in aerobic soils.

What is the significance of Liebig's Law of the Minimum in plant nutrition?

Liebig's Law states that plant growth is limited by the most limiting nutrient, not by the total supply available.

What is the consequence of high C/N ratios for soil microbes?

High C/N ratios mean that microbes will immobilize nitrogen from the soil for cellular needs, leading to temporary nitrogen depletion for plants.

Identify the main benefits of cover crops in soil management?

Cover crops enhance soil health, prevent erosion, improve nutrient cycling, and maintain organic matter.

Explain the significance of the Tyndall Effect in soil science?

The Tyndall Effect describes how colloidal particles scatter light, demonstrating that certain particles can remain suspended in a soil suspension.

What is Cation Exchange Capacity (CEC)?

CEC is the total capacity of a soil to hold exchangeable cations, essential for nutrient retention and buffering pH changes. It is typically expressed in centimoles of charge per kilogram of soil (cmol_c/kg).

What is base saturation?

Base saturation is the percentage of the exchange sites occupied by base cations (Ca^{2+}, Mg^{2+}, K^{+}, Na^{+}) versus acidic cations (H^{+}, Al^{3+}). It indicates soil fertility and pH status.

What is the primary purpose of liming acidic soils?

Liming acidic soils involves adding materials like calcium carbonate (CaCO_3) to neutralize acidity, increase pH, and reduce aluminum (Al^{3+}) toxicity, thereby improving nutrient availability and plant growth.

How does soil organic matter influence soil buffering and nutrient retention?

Soil organic matter has a high surface area and contains functional groups that can adsorb cations and protons, significantly increasing the soil's buffering capacity and Cation Exchange Capacity (CEC).

What does acidity refer to in soil science?

Acidity refers to the concentration of hydrogen ions (H⁺) in a substance, with higher H⁺ indicating more acidity.

What is alkalinity?

Alkalinity refers to the concentration of hydroxide ions (OH⁻) or a substance's ability to neutralize acids, with more OH⁻ indicating greater alkalinity.

What is pH and its range?

pH is the power of hydrogen, measured on a logarithmic scale from 0–14, where low pH (

Why is the pH scale logarithmic?

Each unit in the pH scale represents a 10× change in hydrogen ion activity, meaning pH 5 has 10× more H⁺ ions than pH 6.

Where are acidic soils commonly found?

Acidic soils are common in humid regions, forest ecosystems, and areas with heavy rainfall and leaching.

What causes soil acidity in humid regions?

Heavy rainfall leaches base cations (Ca²⁺, Mg²⁺, K⁺) from the soil, leaving behind H⁺ and Al³⁺, which contribute to acidity.

What are the common locations for alkaline soils?

Alkaline soils are found in arid and semi-arid regions, poorly drained basins, and areas with carbonate-rich parent material.

What process involves the nitrification of ammonium fertilizers and its effect on soil?

Nitrification converts NH₄⁺ to NO₃⁻ and releases H⁺, which increases soil acidity.

What happens when pyrite is exposed to oxygen?

Exposure of pyrite (FeS₂) to oxygen leads to the formation of sulfate (SO₄²⁻) and sulfuric acid (H₂SO₄), causing extreme soil acidity.

What is soil buffering capacity?

Soil buffering capacity is the ability of soil to resist changes in pH.

Describe the three reservoirs of acidity in soils.

1. Active acidity – H⁺ in soil solution, measured by pH. 2. Exchangeable acidity – H⁺ and Al³⁺ on exchange sites, measured by salt extraction. 3. Residual acidity – bound to minerals and organic matter, influencing lime requirement.

What are the impacts of acid rain on soils?

Acid rain can leach nutrients (Ca²⁺, Mg²⁺), mobilize toxins (Al³⁺, heavy metals), acidify water bodies, and harm vegetation.

Why is Al³⁺ a problem for plants at low pH?

Al³⁺ becomes more soluble at pH < 5.5, leading to root tip damage, reduced growth, and inhibited nutrient uptake.

What is the key difference between saline and sodic soils?

Saline soils have high soluble salts, while sodic soils have high exchangeable sodium.

What is the role of gypsum in reclamation efforts for sodic soils?

Gypsum provides Ca²⁺ to replace Na⁺ on exchange sites, promoting flocculation and leaching of sodium.

What is the main source of permanent charge in soil clays?

Permanent charge results from isomorphic substitution within clay mineral layers, independent of pH.

How do nitrifying bacteria like Nitrosomonas and Nitrobacter function in the nitrogen cycle?

Nitrosomonas converts NH₄⁺ to NO₂⁻, while Nitrobacter converts NO₂⁻ to NO₃⁻, both requiring oxygen and occurring in aerobic soils.

What is the significance of Liebig's Law of the Minimum in plant nutrition?

Liebig's Law states that plant growth is limited by the most limiting nutrient, not by the total supply available.

What is the consequence of high C/N ratios for soil microbes?

High C/N ratios mean that microbes will immobilize nitrogen from the soil for cellular needs, leading to temporary nitrogen depletion for plants.

Identify the main benefits of cover crops in soil management?

Cover crops enhance soil health, prevent erosion, improve nutrient cycling, and maintain organic matter.

Explain the significance of the Tyndall Effect in soil science?

The Tyndall Effect describes how colloidal particles scatter light, demonstrating that certain particles can remain suspended in a soil suspension.

What is Cation Exchange Capacity (CEC)?

CEC is the total capacity of a soil to hold exchangeable cations, essential for nutrient retention and buffering pH changes. It is typically expressed in centimoles of charge per kilogram of soil (cmol_c/kg).

What is base saturation?

Base saturation is the percentage of the exchange sites occupied by base cations (Ca^{2+}, Mg^{2+}, K^{+}, Na^{+}) versus acidic cations (H^{+}, Al^{3+}). It indicates soil fertility and pH status.

What is the primary purpose of liming acidic soils?

Liming acidic soils involves adding materials like calcium carbonate (CaCO_3) to neutralize acidity, increase pH, and reduce aluminum (Al^{3+}) toxicity, thereby improving nutrient availability and plant growth.

How does soil organic matter influence soil buffering and nutrient retention?

Soil organic matter has a high surface area and contains functional groups that can adsorb cations and protons, significantly increasing the soil's buffering capacity and Cation Exchange Capacity (CEC).

What naturally causes high alkalinity in soils?

High alkalinity often results from the presence of carbonates (like calcium carbonate, CaCO_3) in parent material, leading to a buffering system that maintains high pH, especially in arid and semi-arid regions.

How does soil pH affect nutrient availability for plants?

Optimal nutrient availability for most plants is at pH 6.0-7.0. At low pH, nutrients like phosphorus (P), calcium (Ca), and magnesium (Mg) become less available, while at high pH, micronutrients like iron (Fe), manganese (Mn), and zinc (Zn) become less soluble and thus less available.

What are common methods to lower pH in alkaline soils?

Methods to lower pH in alkaline soils include adding elemental sulfur (which forms sulfuric acid upon microbial oxidation), gypsum (CaSO_4 \cdot 2H_2O for sodic soils), or acidic organic materials.

What is pH-dependent charge in soil?

pH-dependent charge arises from the protonation or deprotonation of functional groups (like carboxyl and hydroxyl groups) on the surfaces of organic matter and certain clay minerals. It becomes more negative at higher pH values.

How does soil pH impact microbial activity?

Different microbial groups thrive at different pH ranges. Fungi generally tolerate a wider pH range (more acidic) than bacteria, which prefer neutral to slightly alkaline conditions. Extreme pH can reduce overall microbial diversity and activity essential for nutrient cycling.