Soil, Agriculture, and Environmental Science • Lesson 2
Learning goals: identify soil components; compare sand, silt, and clay; distinguish texture from structure; calculate proportions.
Components and pore space
Mineral particles form a framework. Organic matter includes residues at different stages of decomposition, microbial material, and more persistent compounds. Water and air occupy pores, and their proportions change with wetting and drying. A diagram showing 45% minerals, 5% organic matter, 25% water, and 25% air is an illustrative composition, not a universal measurement. Many real soils differ greatly.
| USDA mineral fraction | Particle diameter | General tendency |
|---|---|---|
| Sand | 0.05–2 mm | Coarser particles; large connected pores often permit rapid drainage. |
| Silt | 0.002–0.05 mm | Intermediate size; smooth feel and susceptibility to detachment when unprotected. |
| Clay | Less than 0.002 mm | Fine particles with high surface area; water retention and chemistry depend on clay type. |
Loam is a textural class, not a fourth particle size
Texture is the relative proportion of sand, silt, and clay in the fine-earth mineral fraction. Loam covers a range of mixtures. The familiar 40% sand, 40% silt, and 20% clay is one example, not the only loam. Texture alone does not establish fertility: nutrient supply, pH, salts, structure, organic matter, and management also matter. Gravel and stones are coarse fragments, usually recorded separately from these texture percentages.
Explore: particle sizes and settling
Choose a hypothetical mixture and follow coarse-to-fine settling. The final layers represent the stated mixture, not a precise field-testing method.
Percentages are sand/silt/clay. Settling time and particle sizes are exaggerated. Real jar tests need proper dispersion and interpretation; layer volumes are not automatically accurate mass percentages. Use the slider or Back/Next to play one stage and pause. Play continues the sequence; Replay starts again after stage 4.
Structure is the arrangement of particles
Structure describes aggregates and pores, not the particle-size percentages. Two soils with the same texture can behave differently because of aggregation, compaction, cracks, and biological channels. Adding compost can improve structure and organic matter without instantly changing the mineral texture into loam.
Worked example: classify the fractions first
A 100 g fine-earth mineral sample has 40 g sand, 40 g silt, and 20 g clay:
\[\text{Clay percentage}=\frac{20\ \mathrm{g}}{100\ \mathrm{g}}\times100\%=20\%.\]
The three fractions sum to 100%. A laboratory texture analysis is needed for reliable classification; dark color alone does not identify loam.
Practice: try before revealing
1. Is loam a mineral particle larger than sand?
Show reasoning and answer
No. It is a mixture-based textural class.
2. Which is smallest: sand, silt, or clay?
Show reasoning and answer
Clay.
3. A sample has 60 g sand, 30 g silt, 10 g clay. Find sand percentage.
Show reasoning and answer
Total = 100 g; sand = 60%.
4. A soil has 35% sand and 45% silt. What is clay percentage?
Show reasoning and answer
100 − 35 − 45 = 20%.
5. Does the label loam guarantee high fertility?
Show reasoning and answer
No. Chemistry, structure, organic matter, and other conditions also matter.
6. Does compaction change sand grains into clay particles?
Show reasoning and answer
No. It changes arrangement and pore space, not the mineral particle-size fractions.