Soil Science & Amendments
Soil types, pH, CEC, organic matter, amendments for clay and sandy soils, and soil testing methods.
- Classify soils by texture and particle size from sand through silt to clay
- Explain soil pH, nutrient availability, and cation exchange capacity (CEC)
- Select the correct amendments including lime, sulfur, and organic matter for different soil conditions
Lección 1
Soil Types, Texture & Particle Size
Understanding Soil Composition
Soil is the foundation of every landscape project. For the Red Seal exam, you must understand the three primary mineral particles that make up soil and how their proportions determine soil behavior. The three particle types, from largest to smallest, are sand, silt, and clay.
Sand particles are the largest, ranging from 0.05 mm to 2.0 mm in diameter. Sandy soils drain quickly, warm up fast in spring, and have excellent aeration. However, they hold very little moisture or nutrients. When you squeeze wet sand in your hand, it falls apart immediately.
Silt particles are medium-sized, ranging from 0.002 mm to 0.05 mm. Silt feels smooth and powdery when dry, almost like flour. Silt soils hold more moisture than sand but can become compacted easily.
Clay particles are the smallest, less than 0.002 mm in diameter. Clay soil has the smallest particle size of all mineral soils. Clay holds the most water and nutrients but drains poorly, compacts easily, and can become waterlogged. When wet, clay is sticky and can be rolled into a ribbon between your fingers.
The Soil Texture Triangle
The soil texture triangle is a diagram that shows how different proportions of sand, silt, and clay combine to create named soil types. Loam is the ideal soil for most landscaping work because it contains a balanced mix of all three particles - typically about 40% sand, 40% silt, and 20% clay. Loam provides good drainage, adequate moisture retention, and reasonable nutrient-holding capacity.
Other important soil texture classes include:
- Sandy loam - mostly sand with some silt and clay, drains well but needs frequent watering
- Clay loam - higher clay content, holds moisture well but can be heavy and slow-draining
- Silty clay - fine-textured, holds water but compacts easily
The ribbon test is a simple field method for estimating soil texture. Take a moist handful of soil and squeeze it between your thumb and forefinger to form a ribbon. If the ribbon extends more than 5 cm before breaking, the soil has high clay content. If it crumbles immediately, the soil is sandy.
Sandy Soil
Drainage: Excellent - fast
Nutrient holding: Poor
Compaction risk: Low
Workability: Easy in all conditions
Loam Soil
Drainage: Good - moderate
Nutrient holding: Good
Compaction risk: Moderate
Workability: Best overall for landscapes
Clay Soil
Drainage: Poor - slow
Nutrient holding: Excellent
Compaction risk: High
Workability: Difficult when wet or dry
Clay has the smallest particle size of all soil types. When the exam asks about particle size order, remember: sand (largest) > silt (medium) > clay (smallest). Loam is the ideal landscape soil because it balances drainage, moisture retention, and nutrient availability.