Terminology

• Subsoil: densely packed layer below topsoil
• Humic matter: the stable part of soil organic matter
• Artificial topsoil: blend of minerals, biosolids, compost, or paper mill sludge
• BS 3882: British specification for topsoil
• C:N ratio: carbon to nitrogen ratio; UK optimum below 20:1

History and demand

Demand trend

• Forming one inch of topsoil takes roughly 500 to 1,000 years; the United States loses almost 3 tons per acre per year
• Erosion-related costs in the United States exceed $45 billion per year; the world has about 60 years of topsoil (2014)

Composition and production

Raw materials

• Mineral particles and organic matter that hold water and air; bacteria, fungi, and insects supply nutrient exchange, decomposition, and aeration
• Commercial topsoil is mostly natural topsoil that has been mined and conditioned

Properties

Physical

• Depth: about 5 to 10 inches (13 to 25 cm), measured to the subsoil

Thermal and moisture

• Drying: dehydrated topsoil shrinks substantially and is more prone to wind erosion

Durability

• pH guidelines: 5.0 to 6.2 (North Carolina) or 5.5 to 8.5 (BS 3882)

Uses in buildings

Building elements

• Landscaping and Planting: lawns, planting beds, and finish grading
• Vegetated Roofs: growing medium, 6 in. or less in extensive roofs
• Site Drainage: finish grading that slopes away from the building

Pros and cons

Pros

• Holds most of a plant's nutrients and roots
• Higher organic matter strengthens structure

Cons

• Slow to form and easily lost to erosion
• Less suitable than potting soil or compost for container planting or food crops
• Not a structural material

Sustainability

Health

• Erosion runoff can pollute waterways and contribute to harmful algal blooms; cover crops are a sustainable way to slow erosion

Sources

• Wikipedia: Topsoil, https://en.wikipedia.org/wiki/Topsoil
• Wikimedia Commons: Topsoil1230.JPG, photo by unknown (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Topsoil1230.JPG