Soil Composition » Mineral Particles in Soil

What you'll learn this session

Study time: 30 minutes

The different mineral particles that make up soil
How to classify soil textures based on particle size
The properties of sand, silt and clay particles
How mineral composition affects soil characteristics
Methods for determining soil texture
The importance of mineral particles for agriculture and ecosystems

Introduction to Mineral Particles in Soil

Soil is much more than just "dirt" beneath our feet. It's a complex mixture of materials that supports plant growth and countless organisms. One of the most important components of soil is its mineral particles, which form the basic structure of soil and influence many of its properties.

Key Definitions:

Soil: The upper layer of earth containing a mixture of organic matter, minerals, gases, liquids and organisms.
Mineral particles: Tiny fragments of rock that have been broken down by weathering processes.
Soil texture: The proportion of different-sized mineral particles in soil.
Soil structure: How the individual soil particles clump or bind together and arrange themselves.

The Main Types of Mineral Particles

Soil contains three main types of mineral particles, classified by their size. Each type gives soil different properties and affects how it behaves.

♦ Sand Particles

Size: 0.05-2.0 mm

Properties:

Largest soil particles
Visible to the naked eye
Feel gritty when rubbed between fingers
Create large spaces (pores) between particles
Allow water to drain quickly
Warm up quickly in spring
Low ability to hold nutrients

♦ Silt Particles

Size: 0.002-0.05 mm

Properties:

Medium-sized soil particles
Not visible individually without magnification
Feel smooth like flour when dry
Create medium-sized pores
Moderate water drainage
Better nutrient retention than sand
Feel slippery when wet

♦ Clay Particles

Size: Less than 0.002 mm

Properties:

Smallest soil particles
Microscopic in size
Feel sticky when wet
Create tiny pores between particles
Hold water tightly
Slow drainage and can become waterlogged
Excellent at storing nutrients
Can be difficult for plant roots to penetrate

⊕ How Particles Form

Mineral particles in soil come from the weathering of rocks over thousands of years. This happens through:

Physical weathering: Rocks break down due to temperature changes, frost action, plant roots, etc.
Chemical weathering: Minerals in rocks dissolve or change form due to reactions with water, oxygen and acids
Biological weathering: Living organisms like lichens, plant roots and burrowing animals break down rocks

Soil Texture Classification

The relative proportions of sand, silt and clay particles determine a soil's texture. Soil scientists use a triangular diagram called a "soil texture triangle" to classify soils based on these proportions.

Common Soil Texture Classes

Sandy soil: Contains more than 85% sand particles
Loamy sand: 70-90% sand with some silt and clay
Sandy loam: 50-70% sand with more silt and clay
Loam: Roughly equal mixture of sand, silt and clay
Silt loam: 50-80% silt with some sand and clay
Clay loam: 27-40% clay with sand and silt
Clay soil: More than 40% clay particles

☆ Loam: The "Perfect" Soil

Loam soil is often considered ideal for growing plants because it combines the best properties of each particle type:

Good drainage from sand
Water retention from clay
Fertility and workability from silt
Balanced nutrient availability
Good root penetration
Suitable for most plants

How to Determine Soil Texture

There are several ways to determine the texture of soil:

✓ Laboratory Method

Scientists use a process called particle size analysis:

Soil sample is dried and sieved
Particles are separated by size
Each fraction is weighed
Percentages are calculated
Results plotted on texture triangle

✓ Field Method

The "feel method" can be done without equipment:

Take a small amount of soil
Add water until it's like putty
Rub between fingers
Try to form ribbons
Gritty = sandy
Smooth = silty
Sticky = clayey

✓ Jar Test

A simple home test:

Fill jar 1/3 with soil
Add water until nearly full
Add teaspoon of detergent
Shake vigorously
Let settle for 24-48 hours
Sand settles first (bottom)
Silt forms middle layer
Clay forms top layer

How Mineral Particles Affect Soil Properties

♥ Water Movement and Retention

The size of soil particles greatly affects how water moves through soil:

Sandy soils: Large pores allow quick drainage but poor water retention
Clay soils: Tiny pores hold water tightly but drain slowly
Silty soils: Moderate drainage and water-holding capacity

This is why plants in sandy soils often need more frequent watering, while plants in clay soils might suffer from waterlogging.

♥ Nutrient Availability

Mineral particles affect how nutrients are stored and made available to plants:

Clay particles: Have negative electrical charges that attract and hold positively charged nutrients (like calcium, magnesium, potassium)
Sand particles: Have few electrical charges and poor nutrient-holding capacity
Silt particles: Moderate nutrient-holding capacity

This is why clay soils are often naturally more fertile than sandy soils.

Case Study Focus: The Nile Delta

The Nile Delta in Egypt provides an excellent example of how mineral particles affect soil fertility. For thousands of years, the annual flooding of the Nile River deposited fine silt particles across the floodplain. These mineral-rich deposits created some of the most fertile agricultural land in the ancient world.

The silt particles contained:

A balanced mixture of different-sized particles
Weathered minerals rich in nutrients
Good water-holding capacity

This natural process allowed Egyptian civilization to flourish for millennia. Today, the Aswan High Dam prevents these floods and farmers must now add fertilizers to replace the nutrients that were once naturally deposited by the river.

Environmental Challenges Related to Soil Mineral Particles

! Soil Erosion

Different-sized particles are vulnerable to erosion in different ways:

Sand particles: Heavy and require stronger forces to move, but don't bind together well
Silt particles: Easily carried by water and wind
Clay particles: Bind together strongly but can be carried long distances once detached

Soil erosion removes the valuable top layer of soil, reducing agricultural productivity and causing pollution in waterways.

! Soil Compaction

Compaction occurs when soil particles are pressed together, reducing the space between them:

Clay soils are most vulnerable to compaction
Sandy soils are more resistant to compaction
Compacted soils have poor drainage and aeration
Plant roots struggle to penetrate compacted soil

Heavy machinery, livestock and even human foot traffic can cause soil compaction.

Managing Soils Based on Mineral Composition

Understanding the mineral composition of soil helps farmers and gardeners manage their land more effectively:

Sandy soils: Add organic matter to improve water retention and nutrient-holding capacity. Water more frequently but with smaller amounts.
Clay soils: Add organic matter and sometimes sand or grit to improve drainage. Avoid working when wet to prevent compaction.
Silty soils: Add organic matter to improve structure. Avoid compaction as these soils can easily become dense.

By understanding the mineral particles in soil, we can better manage this vital resource for sustainable food production and healthy ecosystems.

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