Define bulk density and explain why high bulk density reduces root growth and infiltration.

• A. The ratio of water to soil; high values increase infiltration.
• B. The total pore volume in soil; high values mean more air in soil.
• C. The dry mass per unit bulk volume; high values indicate compaction, reducing pore space for roots and water movement.
• D. The mass of organic matter per unit soil; high values improve drainage.

Answer: (C)

Bulk density is the dry mass of soil per unit bulk volume, including the air and water-filled pores. When bulk density is high, the soil is packed tightly (compacted), so the space available for pore networks shrinks. That reduction in pore space means roots have less room to grow, encounter more physical resistance, and often face poorer oxygen supply, all of which slows or stops root elongation. Infiltration also drops because water must move through connected pores; with compaction, there are fewer large, well-connected pores to carry water inward, so infiltration rates fall and water is more likely to run off. So a high bulk density signals compacted soil with limited pore space, which directly explains both stunted root growth and reduced infiltration.