Soils and Geology Flashcards

What USDA soil texture classification contains a balanced mixture of sand, silt, and clay in roughly equal proportions?

Answer: Loam

Loam is the USDA texture class that contains approximately 25-50% sand, 25-50% silt, and 10-25% clay, making it ideal for most plant growth.

Which soil horizon is commonly referred to as 'topsoil' and contains the highest concentration of organic matter?

Answer: A horizon

The A horizon (topsoil) is the uppermost mineral soil layer and contains the greatest accumulation of organic matter and biological activity.

What term describes the maximum amount of water a soil can hold after excess gravitational water has drained away?

Answer: Field capacity

Field capacity is the soil moisture content remaining after gravitational drainage has ceased, typically reached 24-48 hours after saturation.

Which pH range is considered slightly acidic and generally optimal for most landscape plants?

Answer: 5.5–6.5

A pH of 5.5–6.5 allows maximum nutrient availability for most plants, as key nutrients like nitrogen, phosphorus, and potassium are most soluble in this range.

Which soil property describes the soil's ability to resist pH changes when acids or bases are added?

Answer: Buffer capacity

Buffer capacity measures how resistant soil is to pH change; soils high in organic matter or clay tend to have greater buffering ability.

The Unified Soil Classification System (USCS) is primarily used by which professional group for soil classification?

Answer: Geotechnical engineers

The USCS, standardized under ASTM D2487, is the primary classification system used by geotechnical engineers to describe engineering behavior of soils.

What is cation exchange capacity (CEC) in soils, and why is it important in landscape design?

Answer: The soil's ability to hold and exchange positively charged nutrient ions

CEC measures how many cations (like Ca²⁺, Mg²⁺, K⁺) a soil can hold; high CEC soils retain more nutrients and are generally more fertile and resilient.