What soil physical property most directly indicates compaction and how is it measured?

• A. Bulk density or penetration resistance; measured with a soil core sampler or penetrometer
• B. Soil pH; measured with a pH meter
• C. Water holding capacity; measured by saturation
• D. Soil color; measured with a color chart

Answer: (A)

Compaction changes how tightly soil particles are packed, which directly reduces pore space and increases resistance to movement of air, water, and roots. The soil physical property that best indicates compaction is bulk density or, alternatively, penetration resistance. Bulk density is the dry mass of soil per total soil volume, and you measure it by collecting a known-volume soil core, drying the soil, and calculating dry mass divided by the core volume. A higher bulk density means less pore space and more compacted soil. Penetration resistance, often measured with a cone penetrometer, provides the same idea in a direct way: as soil becomes more compacted, it offers greater resistance to penetration. Other properties like pH, water-holding capacity, or soil color don’t directly reflect how tightly the soil is packed, so they’re not as good indicators of compaction.