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examBoard: Cambridge
examType: IGCSE
lessonTitle: Soil Particle Sizes and Soil Types
Soil is much more than just "dirt" – it's a complex mixture of minerals, organic matter, water, air and living organisms. The mineral component of soil consists of particles of different sizes, which greatly influence the soil's properties and its suitability for different uses.
Key Definitions:
Soil particles are classified into four main size categories, each with distinct properties that affect how the soil behaves:
Size: Less than 0.002 mm
Properties: Clay particles are microscopic and flat. They have a large surface area relative to their size, which allows them to hold water and nutrients effectively.
Behaviour: Clay soils drain slowly, warm up slowly in spring and can become sticky when wet and hard when dry. They have high nutrient-holding capacity.
Size: 0.002 to 0.05 mm
Properties: Silt particles are medium-sized, smooth and feel like flour when dry.
Behaviour: Silty soils hold moisture well but drain better than clay. They are fertile, easy to cultivate, but can form a crust that prevents water infiltration.
Size: 0.05 to 2.0 mm
Properties: Sand particles are large enough to see with the naked eye and feel gritty.
Behaviour: Sandy soils drain quickly, warm up rapidly in spring and are easy to work. However, they don't hold water or nutrients well.
Size: Greater than 2.0 mm
Properties: Gravel consists of small rocks and stones.
Behaviour: Soils with high gravel content drain very rapidly and have very low nutrient-holding capacity. They can be difficult to cultivate.
Most soils contain a mixture of particle sizes. The relative proportions of sand, silt and clay determine the soil's texture class. Soil scientists use the soil texture triangle to classify soils based on these proportions:
The soil texture triangle shows the 12 major soil texture classes based on the percentages of sand, silt and clay.
Composition: More than 85% sand
Properties: Light, warm, dry, tends to be acidic
Good for: Root vegetables, bulbs, drought-resistant plants
Limitations: Poor water retention, nutrients leach away quickly
Composition: Roughly equal parts sand, silt and clay
Properties: Fertile, well-draining, easy to work
Good for: Most plants, considered ideal garden soil
Limitations: Can become compacted if overworked when wet
Composition: More than 40% clay
Properties: Heavy, nutrient-rich, retains water
Good for: Summer crops, ornamental trees and shrubs
Limitations: Poor drainage, compacts easily, hard to work
Composition: More than 80% silt
Properties: Fertile, retains moisture, compacts easily
Good for: Most vegetable crops and moisture-loving plants
Limitations: Can become waterlogged, forms crust when dry
Composition: High calcium carbonate content
Properties: Alkaline (high pH), often shallow and stony
Good for: Plants that tolerate alkaline conditions
Limitations: Prone to nutrient deficiencies, especially iron
You can determine your soil's texture with some simple field tests:
Method: Rub moist soil between your fingers.
Method: Try to form a ribbon by squeezing a moistened ball of soil between thumb and forefinger.
This test gives a more accurate measure of the proportions of sand, silt and clay in your soil:
The Loess Plateau in northern China contains one of the most extensive deposits of loess soil in the world. Loess is a silty soil deposited by wind over thousands of years.
For centuries, farmers cultivated this fertile soil, but poor management led to severe erosion. By the 1990s, the region suffered from extreme poverty and environmental degradation.
A massive restoration project begun in 1994 transformed the landscape through terracing, planting vegetation and sustainable farming practices. This project demonstrates how understanding soil properties can inform effective land management strategies.
The results were dramatic: soil erosion reduced by 60%, agricultural productivity increased and poverty rates fell significantly. The project is now a model for ecological restoration worldwide.
The composition of soil has profound effects on agricultural practices and productivity:
Sandy soils drain quickly and may require frequent irrigation.
Clay soils retain water but may need drainage systems to prevent waterlogging.
Loamy soils provide balanced drainage and water retention, requiring less intervention.
Sandy soils need frequent, small applications of fertiliser as nutrients leach quickly.
Clay soils hold nutrients well but may lock them up, making them unavailable to plants.
Silty soils are naturally fertile but benefit from organic matter to improve structure.
Understanding your soil type allows you to take appropriate measures to improve it:
Remember that soil is a precious resource that forms very slowly – it can take hundreds of years to form just a few centimetres of topsoil. Understanding soil composition helps us manage this resource sustainably for future generations.
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