SoilExam2

What are soil colloids?

Tiny particles (< 0.002 mm) with massive surface areas that influence soil properties like nutrient retention and water holding capacity

How does the surface area vary among clay colloids?

• Kaolinite: Low surface area due to large particle size and simple structure.

• Montmorillonite: High surface area with small particle size and expandable layers.

• Illite: Moderate surface area between kaolinite and montmorillonite.

Which soil colloid has the highest surface area?

Organic colloid (humus) due to its excellent nutrient and water retention properties

How do oxides of iron and aluminum compare in surface area?

Moderate surface area but lack layer-like structure, limiting interaction compared to clay colloids.

What effect does aggregate formation have on soil colloid surface area

What is the structure of smectite clays

Smectite clays have a layered structure with silicon-oxygen tetrahedra and aluminum-oxygen octahedra sheets

How do smectite clays become negatively charged

Negative charge arises due to isomorphic substitution, where:

• Si⁴⁺ is replaced by Al³⁺ in tetrahedral layers.

• Al³⁺ is replaced by Mg²⁺ or other cations in octahedral layers.

Is the negative charge of smectite clays dependent on pH

No, the negative charge is permanent and does not vary with soil acidity or alkalinity

Why is the negative charge important in smectite clays

It attracts positively charged ions (cations) like calcium, magnesium, potassium, and sodium, essential for soil fertility

What is base saturation in soils

Base saturation refers to the percentage of exchange sites in the soil occupied by basic cations (calcium, magnesium, potassium, and sodium) compared to acidic cations (hydrogen and aluminum)

How does base saturation change as soils age

• Young soils: Typically have higher base saturation due to abundant basic cations from parent material.

• Older soils: Base saturation decreases over time as basic cations are leached out by water, replaced by acidic cations.

What factors influence the leaching of basic cations in aging soils

Factors include high rainfall, soil drainage, and the absence of replenishment from new parent material

Why is base saturation important in agriculture

It affects soil fertility, pH, and the ability to support crop growth

What causes soil acidity

1. Rainfall and Leaching: High rainfall leaches basic cations (calcium, magnesium) from the soil, leaving acidic cations (hydrogen, aluminum) behind.

2. Decomposition of Organic Matter: Produces acidic compounds like carbonic acid and organic acids.

3. Parent Material: Some rocks and minerals naturally contribute to soil acidity.

4. Fertilizer Use: Ammonium-based fertilizers release hydrogen ions during conversion to nitrate.

5. Plant Uptake: Uptake of basic cations by plants leaves acidic cations in the soil.

Why is soil acidity a big deal

1. Nutrient Availability: Limits availability of essential nutrients (like phosphorus) while increasing toxic elements (like aluminum and manganese).

2. Microbial Activity: Hinders beneficial microorganisms, reducing soil health and fertility.

3. Crop Growth: Acidic soils can stifle root development and reduce agricultural yields.

4. Soil Structure: Increased acidity can lead to poor soil structure and reduced water infiltration.

How can soil acidity be managed

1. Apply lime (calcium carbonate) to neutralize acidity.

2. Use acid-tolerant crops.

3. Manage fertilizer use carefully to minimize acidification.

What is active acidity?

Active acidity refers to the hydrogen (H⁺) and aluminum (Al³⁺) ions in the soil solution.

• Determines soil pH.

• Directly affects plant growth and microbial activity.

What is exchangeable acidity?

Exchangeable acidity consists of hydrogen (H⁺) and aluminum (Al³⁺) ions loosely bound to soil colloid surfaces.

• Can be released into the soil solution during cation exchange.

• Represents a reserve of acidity that is more dynamic than active acidity.

What is reserve acidity?

Reserve acidity is the hydrogen (H⁺) and aluminum (Al³⁺) ions tightly held by soil colloids.

• Largest pool of acidity in the soil.

• Affects long-term soil buffering capacity.

• Released slowly over time or through significant changes in soil chemistry.

Why are the acidity pools important

Understanding these pools helps in managing soil pH effectively and optimizing soil health for sustainable agriculture.

What is soil pH

Soil pH measures the acidity or alkalinity of the soil on a scale from 0 to 14, with 7 being neutral.

How does pH affect nutrient availability

• Optimal pH Range: Most nutrients are available between pH 6.0 and 7.5.

• Acidic Soils (Low pH): Limits availability of phosphorus, calcium, and magnesium; increases toxicity of aluminum and manganese.

• Alkaline Soils (High pH): Reduces availability of iron, zinc, copper, and manganese

Why is pH management important for plants

Correct pH ensures nutrients are accessible to plants, supporting healthy growth and maximizing yields

How can pH be adjusted to optimize nutrient availability

• To Raise pH: Apply lime (calcium carbonate).

• To Lower pH: Use sulfur or acid-forming fertilizers.

What happens at extreme pH levels

Severe acidity or alkalinity can create toxic conditions, hinder microbial activity, and damage soil structure

What is the general ideal soil pH range for crop production

The ideal pH range is typically 6.0 to 7.5, as most crops grow best in slightly acidic to neutral soils

Why is a pH of 6.0 to 7.5 ideal for most crops

• Nutrients like nitrogen, phosphorus, and potassium are most available in this range.

• Reduces the risk of toxic elements like aluminum and manganese becoming soluble.

What happens to toxic ions in acidic soils (low pH)

Toxic ions like aluminum (Al³⁺) and manganese (Mn²⁺) become more soluble and available, leading to potential toxicity for plants

What happens to toxic ions in neutral to alkaline soils (higher pH)?

• Toxic ions like aluminum and manganese are less soluble and less available.

• Iron and other micronutrients may become less available, but toxicity risks decrease.

How does pH-related toxicity affect plants

Excessive aluminum can inhibit root growth and nutrient uptake.

What is the C:N ratio in organic matter

The C:N ratio represents the proportion of carbon (C) to nitrogen (N) in organic materials. It is a key factor in determining the rate of decomposition

How does a high C:N ratio affect decomposition

• Slows decomposition as microbes lack sufficient nitrogen for their metabolic needs.

• Results in nitrogen immobilization, reducing nitrogen availability for plants.

How does a low C:N ratio affect decomposition

• Speeds up decomposition as microbes have adequate nitrogen to break down organic matter.

• Leads to nitrogen mineralization, releasing nitrogen into the soil.

Why is the C:N ratio important in soil management

• Balances nutrient cycling and soil fertility.

• Impacts how quickly organic amendments (e.g., compost) release nutrients.

What are typical C:N ratios of common materials

Green plant material: Low C:N (around 10:1).

Compost: Balanced C:N (~25-30:1).

What are soil bacteria and their role

• Description: Single-celled microorganisms; most abundant in soil.

• Role: Decompose organic matter, fix nitrogen (e.g., Rhizobium), and break down pollutants.

• Specialty: Thrive in diverse environments; essential for nutrient cycling.

What are soil fungi and their role

• Description: Multi-celled microorganisms with thread-like structures (hyphae).

• Role: Decompose complex organic matter like cellulose and lignin.

• Specialty: Form mycorrhizal associations with plant roots to improve water and nutrient uptake.

What are soil nematodes and their role

• Description: Microscopic roundworms; diverse group in the soil food web.

• Role:

  • Decomposers: Feed on bacteria and fungi.

  • Predators: Control pest populations.

  • Nutrient Cyclers: Release nitrogen as they break down organic matter.

• Specialty: Indicators of soil health and balance.

How do fungi differ from bacteria in soil

• Fungi decompose more complex organic materials, while bacteria break down simpler compounds.

• Fungi can grow in drier and more acidic soils than bacteria.

Why are soil microorganisms important overall

They play a vital role in nutrient cycling, organic matter decomposition, and maintaining soil structure.

Enhance plant growth and contribute to the soil's ecosystem balance

What are the environmental benefits of healthy soils

• Carbon Sequestration: Healthy soils store carbon, reducing greenhouse gases.

• Water Filtration: Purifies water by filtering pollutants.

• Erosion Control: Maintains soil structure, reducing erosion.

How do healthy soils support plant growth?

• Nutrient Availability: Provide essential nutrients for crops.

• Water Retention: Store water efficiently, reducing drought stress.

• Root Development: Promote strong and extensive root systems.

What are the economic benefits of healthy soils

• Increased Crop Yields: More productive soils lead to higher yields.

• Reduced Input Costs: Healthy soils require less fertilizer and water.

• Sustainable Agriculture: Supports long-term farming profitability.

What is the nitrogen cycle

The nitrogen cycle is the process by which nitrogen moves through the atmosphere, soil, plants, animals, and microorganisms. It ensures nitrogen is available for living organisms.

What is nitrogen fixation

• Conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) or nitrate (NO₃⁻) by soil bacteria or lightning.

• Makes nitrogen usable for plants.

What is nitrification

• The process where ammonia (NH₃) is converted into nitrites (NO₂⁻) and then nitrates (NO₃⁻) by bacteria.

• Nitrates are the form most accessible to plants.

What is denitrification?

• Conversion of nitrates (NO₃⁻) back into nitrogen gas (N₂) by bacteria.

• Releases nitrogen into the atmosphere.

What is ammonification

• The decomposition of organic matter by microorganisms, releasing ammonia (NH₃).

• Part of the nitrogen recycling process.

What is volatilization

• The loss of nitrogen as ammonia gas (NH₃) from soil or fertilizers.

• Happens when nitrogen is exposed to air under certain conditions.

What is leaching in the nitrogen cycle

• The movement of nitrate (NO₃⁻) through soil with water drainage.

• Reduces nitrogen available for plants and can pollute water sources.

Why is the nitrogen cycle important

• It provides nitrogen for plant growth, ensuring ecosystem stability and agricultural productivity.

What is phosphorus (P) in soils

Phosphorus exists in different forms:

• Non-labile P: Fixed, unavailable form.

• Labile P: Easily available for plant uptake.

• Essential nutrient for energy transfer, photosynthesis, and root development in plants.

Who can make non-labile P more available?

• Mycorrhizal Fungi: Enhance P solubility by extending the root network and excreting organic acids.

• Phosphate-Solubilizing Bacteria (e.g., Bacillus and Pseudomonas): Produce enzymes and organic acids to dissolve P.

• Organic Matter Decomposers: Break down organic compounds to release P.

How do microbes release phosphorus

• Secrete organic acids that lower soil pH.

• Produce enzymes like phosphatases to free P from organic matter.

Why is phosphorus important for soil biology?

• Supports microbial growth and activity.

• Drives nutrient cycling and ecosystem productivity.

What can enhance phosphorus availability in soils

• Adding organic matter (compost, manure).

• Promoting microbial activity through sustainable soil practices.

• Balancing pH for optimal P release.

Why is the nitrogen cycle a biological cycle?

It is driven by biological processes involving microorganisms like bacteria and plants that transform nitrogen into usable forms.

What are the biological components of the nitrogen cycle?

1. Nitrogen Fixation: Carried out by nitrogen-fixing bacteria (e.g., Rhizobium) that convert atmospheric N₂ into ammonia (NH₃).

2. Nitrification: Performed by bacteria (e.g., Nitrosomonas and Nitrobacter) that convert ammonia into nitrates (NO₃⁻).

3. Denitrification: Bacteria convert nitrates back into nitrogen gas (N₂), releasing it into the atmosphere.

4. Ammonification: Decomposition of organic matter by microorganisms, releasing ammonia.

Why are plants and soil microbes crucial to the nitrogen cycle?

• Plants absorb nitrates for growth and development.

• Soil microbes drive nitrogen transformations, maintaining ecosystem balance.

Why is the nitrogen cycle important for ecosystems

It ensures the recycling of nitrogen, making it available for living organisms, sustaining plant and microbial life.

What is biological nutrient cycling

The process by which nutrients are recycled through plants, microorganisms, and the soil, maintaining soil fertility and health.

How do plant roots contribute to nutrient cycling

• Exude organic acids: Help release nutrients like calcium, magnesium, and potassium from minerals.

• Absorb nutrients: Take up essential cations, supporting plant growth and returning them to the soil via organic matter decomposition.

How does biological nutrient cycling maintain base saturation

• Adds basic cations (calcium, magnesium, potassium, sodium) back to the soil via plant residue decomposition and root activity.

• Reduces acidification by buffering soil pH.

How does this process keep soil young

• Prevents nutrient depletion.

• Maintains soil structure and fertility, slowing down the aging process of soil through constant renewal of nutrients.

Why are plant roots essential for young and healthy soils

• They foster microbial activity, which aids in nutrient cycling.

• Enhance soil aggregation, improving water retention and reducing erosion.

Why is denitrification a concern in waterlogged soils

• Waterlogged conditions reduce oxygen levels, favoring anaerobic bacteria.

• These bacteria convert nitrate (NO₃⁻) into nitrogen gases (N₂, N₂O), leading to nitrogen loss from the soil.

• Nitrous oxide (N₂O) is a potent greenhouse gas.

What portions of the nitrogen cycle are impacted in aerated vs. waterlogged soils?

• Aerated Soils: Favor nitrification, where ammonia (NH₃) is converted to nitrate (NO₃⁻).

• Waterlogged Soils: Favor denitrification, where nitrate is converted to nitrogen gases, causing nutrient loss.

How do waterlogged soils affect plant growth?

• Denitrification reduces nitrogen availability for plants, stunting growth.

• Accumulation of N₂O gas can indicate excessive nitrogen loss.

What management practices reduce denitrification in waterlogged soils

• Improve drainage systems to aerate soils.

• Avoid excessive nitrogen fertilizer applications.

• Use cover crops to absorb excess nitrogen.

Why is understanding the nitrogen cycle critical in soil management

It helps identify nitrogen loss processes, optimize fertilizer use, and reduce environmental impacts like greenhouse gas emissions.

What does an epipedon describe in soil

An epipedon describes the uppermost layer of the soil, which includes the surface horizon and sometimes subsurface horizons influenced by organic matter or cultivation

What features define an epipedon

• Color: Indicates organic matter content.

• Texture: Can range from sandy to clayey.

• Organic Matter: Determines fertility.

• Mineral Content: Influences soil productivity.

How does an epipedon relate to soil classification?

Epipedons are used in soil taxonomy to classify soils based on surface horizon characteristics, such as organic content, thickness, and degree of mixing.

What are examples of epipedons?

• Mollic: Thick, dark, rich in organic matter.

• Ochric: Thin or pale, with less organic matter.

• Umbric: Similar to mollic, but more acidic.

• Histic: Organic-rich, formed in wet conditions.

What is Land Capability Class 1?

MOST IDEAL

Description: Best soils with minimal limitations.

Use: Suitable for cultivation due to excellent drainage and fertility.

What is Land Capability Class 2?

• Description: Slight limitations, such as moderate slopes or drainage issues.

• Use: Suitable for cultivation, but requires minor management practices.

What is Land Capability Class 3?

• Description: Moderate limitations like steeper slopes or low fertility.

• Use: Suitable for cultivation, but requires careful soil conservation and management.

What is Land Capability Class 4?

• Description: Significant limitations, such as poor drainage or erosion risks.

• Use: Best for grazing and light cultivation.

What is Land Capability Class 5?

• Description: Unsuitable for cultivation due to wetness or steep slopes.

• Use: Suitable for grazing or timber production.

What is Land Capability Class 6?

• Description: Severe limitations that prohibit cultivation.

• Use: Best for grazing, timber, and natural landscapes.

What is Land Capability Class 7?

• Description: Very severe limitations like extremely steep terrain or rocky soils.

• Use: Suitable for timber production or preserved as natural landscapes.

What is Land Capability Class 8?

• Description: No agricultural use; primarily barren land.

• Use: Preserved as natural landscapes.