Definition
Soil texture
Soil texture refers to the relative proportions of sand, silt, clay, and humus in a soil sample.
- Soil texture influences key soil characteristics such as drainage, aeration, nutrient-holding capacity, structure, and ease of cultivation.
- The classification of soil texture relies on identifying the proportions of these particle types and determining how they interact to affect soil function.
A soil that feels gritty and coarse is typically sand-rich, while a smooth, “flour-like” texture indicates a high silt content.
Components of Soil Texture
| Soil component | Particle size | Characteristics | Examples |
|---|---|---|---|
| Sand | Largest (>0.05 mm) | Drains quickly, poor water retention, good aeration | Beach sand, desert soils |
| Silt | Medium (0.002–0.05 mm) | Retains water better than sand, smooth texture | River sediments, loamy soil |
| Clay | Smallest (<0.002 mm) | High water retention, poor drainage, compact when wet | Mud, wetland soil |
| Humus | Varies | Organic matter, improves fertility, absorbs moisture | Decomposed plant/animal material |
A loamy soil contains a balanced mix of sand, silt, and clay, making it ideal for agriculture due to good drainage, aeration, and nutrient-holding capacity.
Methods of Determining Soil Texture
Feel Test (Field Method)
- Soil is rubbed between fingers to determine whether it feels gritty (sand-rich), smooth (silt-rich), or sticky and plastic (clay-rich).
- This method is quick and useful for fieldwork but does not provide precise percentages.
- When using the feel test, add a small amount of water to the soil.
- This helps you better distinguish between the textures of sand, silt, and clay.
Soil Texture Keys
- Standardized soil texture keys guide users through physical observations and hand tests to identify the most likely soil class.
- Such keys combine visual cues, moisture behaviour, and ribbon-making to assign the soil to a texture group.
Sedimentation / Water-Settling Method (Laboratory Method)
- Soil is suspended in water in a measuring cylinder and shaken thoroughly to separate particles by size.
- Sand settles first within minutes, followed by silt (1–2 hours), and finally clay after 24–48 hours.
- Layer thicknesses are measured to calculate the percentage of each particle type.
- These percentages are then plotted on a soil texture triangle to determine the soil’s classification
Observe layers
- Bottom layer = Sand (heaviest, settles first).
- Middle layer = Silt (lighter, settles next).
- Top layer = Clay (finest, remains suspended longest).
- Floating material = Organic matter (humus).
If a sample settles into 50% sand, 30% silt, and 20% clay, it is classified as sandy loam.
The Soil Texture Triangle
- The soil texture triangle classifies soil types based on their sand-silt-clay composition.
- It shows soil categories such as loam, sandy clay, silty clay loam, clay loam, and others.
- It allows large datasets to be displayed clearly, making it easy to identify soil groups.
- It highlights the dominant particle type, helping determine likely soil behaviour.
- A soil can be classified accurately only when the three percentages total 100%.
Soils located near the centre of the triangle, such as loams, typically represent the most balanced and agriculturally productive textures.
Importance of Soil Texture
- Agriculture: Affects root growth, water retention, and nutrient availability.
- Water Management: Determines drainage capacity (sand drains fast, clay retains water).
- Construction: Clay-rich soils shrink and swell, causing structural damage.
- Erosion Control: Fine-textured soils (clay) are prone to erosion when disturbed.
- Don't confuse soil texture with soil structure.
- Texture refers to particle size, while structure describes how particles are arranged into aggregates.
