Soil compaction increases which property?

• A. Bulk density
• B. Porosity
• C. Water holding capacity
• D. Infiltration rate

Answer: (A)

Compaction squeezes the soil particles closer together, reducing the pore spaces between them. Bulk density is the dry mass of soil per unit total volume, so when the soil is compacted, the same amount of dry material occupies a smaller volume, and bulk density increases. This occurs even though the solids don’t gain mass; the volume they occupy decreases as pores collapse. The loss of pore space also slows water movement, so infiltration rate drops, and the overall pore volume available to store water tends to decrease, lowering water holding capacity. Porosity, the fraction of voids in the soil, decreases with compaction, so it does not increase. In short, the property that increases is bulk density.