03.

Soil Structure
Basic particles such as sand, clay, silt etc. of the soil are often not existed separately and, those
are formed as secondary particles with bonding agents such as organic materials.

Primary Structure

The primary structure of soil refers to the individual soil particles themselves: sand, silt, and
clay. These particles are the basic building blocks of soil and are classified based on their
size:

• Sand: Coarse particles, 0.05 to 2.0 mm in diameter

• Silt: Medium-sized particles, 0.002 to 0.05 mm in diameter
• Clay: Fine particles, less than 0.002 mm in diameter

Secondary Structure

The secondary structure, also known as soil aggregation, refers to how these primary particles
bind together to form larger aggregates or peds. These aggregates can take various forms and
are held together by several types of bonds:

1. Organic Matter: Organic materials, such as decayed plant and animal matter, act as a
glue, binding soil particles together.
2. Clay Particles: Clay has electrostatic charges that attract and hold other soil particles
together.
3. Iron and Aluminum Oxides: These oxides can coat soil particles and act as a
cementing agent.
4. Roots and Fungi: Plant roots and fungal hyphae exude sticky substances that help
bind soil particles into aggregates.
5. Microbial Activity: Soil microbes produce polysaccharides and other organic
compounds that contribute to soil aggregation.

Soil structure is often described based on three key characteristics: shape, size, and grade.
Each of these characteristics provides detailed information about the physical arrangement
and condition of soil aggregates.

1. Shape

The shape of soil aggregates refers to the geometric form or appearance of the soil clumps.
Common shapes include:

• Granular: Aggregates are small, rounded, and resemble breadcrumbs. This shape is
typical of surface soils with high organic matter.
• Blocky: Aggregates are block-like and can be sub-angular (edges are slightly
rounded) or angular (sharp edges). This structure is common in the subsoil.
• Platy: Aggregates are flat and thin, resembling plates stacked horizontally. This shape
can result from compaction or a high clay content.
• Prismatic: Aggregates are vertically elongated, forming column-like structures.
These are usually found in the lower horizons of the soil profile.

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• Columnar: Similar to prismatic but with rounded tops, often found in soils with high
sodium content.

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2. Size

The size of soil aggregates is classified into different categories, often measured in
millimeters (mm). Sizes can range from very fine to very coarse:

• Very Fine: Less than 1 mm in diameter

• Fine: 1 to 2 mm in diameter
• Medium: 2 to 5 mm in diameter
• Coarse: 5 to 10 mm in diameter
• Very Coarse: Greater than 10 mm in diameter

The size of aggregates influences the soil's porosity, permeability, and its ability to retain
water and nutrients.

3. Grade

The grade of soil structure refers to the degree of distinctness and stability of the soil
aggregates. It can be categorized as follows:

• Structureless: No observable aggregation; soil particles are either single grains

(sandy soils) or massive (clayey soils without structure).
• Weak: Aggregates are poorly formed and indistinct. They are fragile and break easily
under slight pressure.
• Moderate: Aggregates are well-formed and moderately distinct. They hold together
under gentle pressure but break apart under moderate pressure.
• Strong: Aggregates are very well-formed and distinct. They are stable and resist
breaking under considerable pressure.

The formation of soil structure

1. Physical Processes

Wetting and Drying

• Expansion and Contraction: Clay particles swell when wet and shrink when dry, causing
cracks and leading to the formation of aggregates.
• Freeze-Thaw Cycles: In colder climates, water in the soil can freeze and expand, creating
cracks and breaking soil particles apart, which then aggregate upon thawing.

Root Growth and Decay

• Root Pressure: As roots grow, they exert pressure on the soil, pushing particles together and
creating space for aggregates to form.
• Organic Matter: Dead roots decompose and contribute organic matter, which binds soil
particles into aggregates.

2. Chemical Processes

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Clay and Organic Matter Interactions

• Clay Flocculation: Clay particles, which are negatively charged, attract positively charged
cations like calcium, magnesium, and iron. These cations act as bridges between clay
particles, causing them to clump together.
• Humus Formation: Decomposed organic matter forms humus, which has a high cation
exchange capacity. This helps bind soil particles together, enhancing aggregate formation.

Iron and Aluminum Oxides

• Cementing Agents: Iron and aluminum oxides act as natural cementing agents, binding soil
particles into stable aggregates.

3. Biological Processes

Microbial Activity

• Exudates Production: Soil microbes produce polysaccharides and other sticky substances
that help bind soil particles.
• Decomposition: Microbes decompose organic matter, releasing compounds that contribute
to soil aggregation.

Fauna Activity

• Earthworms and Other Soil Fauna: Earthworms and other soil organisms burrow through
the soil, mixing organic and mineral particles. Their casts (feces) are rich in organic matter
and help form stable aggregates.
• Mycorrhizal Fungi: These fungi form symbiotic relationships with plant roots, extending
hyphae into the soil and secreting glomalin, a sticky protein that helps stabilize soil
aggregates.

4. Human Activities

Tillage and Cultivation

• Positive Effects: Controlled tillage can help incorporate organic matter into the soil,
promoting aggregation.
• Negative Effects: Excessive or improper tillage can break down soil structure, leading to
compaction and erosion.

Organic Amendments

• Adding Compost and Manure: These organic materials increase soil organic matter content,
enhancing microbial activity and aggregate stability.

Crop Rotation and Cover Cropping

• Diverse Root Systems: Different crops have varying root structures that help create and
stabilize aggregates. Cover crops add organic matter and protect the soil surface from
erosion.

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Soil structure can be destroyed or degraded by various physical, chemical, and
biological factors, often exacerbated by human activities. Here's a detailed look at how soil
structure can be destroyed:

1. Physical Factors

Compaction

• Heavy Machinery: The use of heavy agricultural machinery, especially when the soil is wet,
compresses soil particles, reducing pore spaces and destroying aggregates.
• Livestock Trampling: Grazing animals can compact soil, particularly around water points and
feeding areas, leading to a loss of soil structure.

Erosion

• Water Erosion: Runoff from heavy rains can wash away topsoil, breaking down aggregates
and leading to the loss of soil structure.
• Wind Erosion: In dry, bare soils, wind can blow away fine particles, disrupting soil aggregates
and reducing soil structure.

Tillage

• Excessive Tillage: Frequent and deep tillage breaks up soil aggregates, leading to a finer soil
that is more susceptible to compaction and erosion.
• Improper Tillage: Using the wrong tillage methods or tools can disrupt soil structure,
particularly when done repeatedly over time.

2. Chemical Factors

Salinization

• Excessive Salts: High levels of soluble salts in the soil can lead to the dispersion of clay
particles, breaking down soil aggregates and reducing soil structure.
• Irrigation with Saline Water: Irrigating with water high in salts can exacerbate salinization,
further degrading soil structure.

Imbalance of Soil Nutrients

• Fertilizer Misuse: Overuse or improper application of chemical fertilizers can lead to nutrient
imbalances, negatively affecting soil microbial activity and aggregate stability.
• Acidification: Acidic conditions, often caused by excessive use of certain fertilizers, can
dissolve soil organic matter and weaken soil structure.

3. Biological Factors

Loss of Organic Matter

• Reduced Organic Inputs: A decline in organic matter inputs (e.g., crop residues, manure)
reduces the binding agents necessary for aggregate formation.

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 • Overgrazing: Continuous grazing without adequate rest periods can reduce vegetation cover
and organic matter inputs, leading to soil structure degradation.

Decrease in Soil Biota

• Pesticides and Herbicides: Overuse of chemical pesticides and herbicides can harm soil
microorganisms and earthworms that play a crucial role in maintaining soil structure.
• Monoculture Practices: Planting the same crop repeatedly can reduce soil biodiversity,
affecting the biological processes that support soil structure.

4. Human Activities

Deforestation

• Tree Removal: Cutting down trees reduces root systems that help bind soil particles, leading
to increased erosion and loss of soil structure.
• Loss of Leaf Litter: Removal of leaf litter reduces organic matter inputs, negatively impacting
soil aggregate stability.

Urbanization

• Soil Sealing: Construction activities that cover soil with impermeable surfaces (concrete,
asphalt) prevent water infiltration and biological activity, leading to soil degradation.
• Topsoil Removal: Construction often involves removing the topsoil, which is rich in organic
matter and vital for good soil structure.

Importance of soil structure

1. Water Infiltration and Retention

• Infiltration: Well-structured soil has good pore space distribution, allowing water to infiltrate
efficiently. This reduces surface runoff and erosion.
• Retention: Soil aggregates help retain water, making it available to plant roots for longer
periods, especially important in dry climates.

2. Root Penetration and Growth

• Ease of Penetration: Good soil structure provides less resistance to root growth, allowing
roots to penetrate deeper and access nutrients and water from a larger volume of soil.
• Anchorage: Strong soil structure offers better support and anchorage for plants, reducing the
risk of lodging (falling over).

3. Aeration

• Oxygen Availability: Proper soil structure ensures adequate air spaces, facilitating oxygen
exchange. This is crucial for root respiration and microbial activity.
• Microbial Activity: Aerobic conditions promote the activity of beneficial soil microbes,
enhancing nutrient cycling and organic matter decomposition.

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4. Nutrient Availability

• Nutrient Holding Capacity: Well-aggregated soil has a higher cation exchange capacity (CEC),
allowing it to hold and supply nutrients to plants more effectively.
• Microbial Interactions: Good soil structure fosters a healthy microbial community, which
helps in nutrient mineralization and availability to plants.

5. Erosion Control

• Aggregate Stability: Stable soil aggregates resist erosion by water and wind, preserving
topsoil and its nutrients, which are vital for crop production.

6. Disease and Pest Management

• Healthy Plants: Plants growing in well-structured soil are generally healthier and more
resistant to diseases and pests.
• Soil Biodiversity: A well-structured soil supports a diverse range of organisms that can
suppress soil-borne diseases and pests through natural biological control.

7. Precision Agriculture

• Soil Mapping: Understanding soil structure allows for precise soil mapping, which is critical
for targeted interventions in precision agriculture.
• Variable Rate Application: Knowledge of soil structure helps in the variable rate application
of water, fertilizers, and pesticides, optimizing their use and minimizing environmental
impact.

8. Sustainable Land Management

• Conservation Practices: Soil structure knowledge guides the implementation of conservation

practices such as no-till farming, cover cropping, and crop rotation.
• Carbon Sequestration: Good soil structure enhances organic matter retention and carbon
sequestration, contributing to climate change mitigation.

9. Irrigation Management

• Efficient Water Use: Understanding soil structure allows for the design of efficient irrigation
systems that match soil water holding capacity and plant needs, reducing water waste.
• Drought Mitigation: Well-structured soils retain water better, helping crops withstand
periods of drought.

10. Soil Health Monitoring

• Biological Indicators: Soil structure can be an indicator of soil health, with well-structured
soils typically supporting higher biological activity.
• Technological Integration: Biosystem technologies like remote sensing and soil sensors can
monitor soil structure changes, aiding in timely decision-making.

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11. Environmental Impact

• Runoff Reduction: Good soil structure reduces surface runoff, minimizing the leaching of
nutrients and chemicals into water bodies.
• Pollution Control: Enhanced soil structure improves the filtration and degradation of
pollutants, contributing to cleaner groundwater and surface water.

12. Innovation and Research

• Soil Amendments: Research in biosystem technology can develop innovative soil

amendments that improve soil structure, such as biochar, compost, and microbial inoculants.
• Modeling and Simulation: Advanced modeling tools can simulate the impact of different
agricultural practices on soil structure, helping in the development of sustainable practices.

Soil Aggregate Stability Drop Test (Simple Explanation)

Purpose

To assess how well soil aggregates withstand impact, indicating the soil’s structural stability.

Materials

• 2 kg of dry soil
• Measuring tape
• Flat cement surface
• Tray or cloth

Procedure

1. Prepare the Soil:

o Collect 2 kg of dry soil, ensuring it has natural aggregates and isn’t too clumpy.
2. Measure the Drop Height:
o Use the measuring tape to measure 2 meters from the cement floor.
3. Drop the Soil:
o Hold the soil sample at a height of 2 meters above the cement floor.
o Release the soil, letting it fall freely to the floor.
4. Observe and Collect:
o Look at the soil after it hits the floor. Check how much stays in aggregates versus how
much breaks into smaller particles.
o Collect the soil using a tray or cloth.

Interpretation

• Good Stability: Many soil aggregates remain intact.

• Poor Stability: Most soil breaks into fine particles.

This simple test helps you understand the soil’s ability to resist physical stress, which is
important for preventing erosion and maintaining healthy soil structure.