The mass of soil per unit bulk volume. The bulk volume consists of mineral and organic materials, water, and air. Bulk density is affected by external pressures (e.g., weight of tractors and harvesting equipment, mechanical pressures from cultivation machines), and by internal pressures (e.g., swelling and shrinking due to water-content changes, freezing and thawing, and by plant roots). The bulk density of cultivated mineral soils ranges from 1–1.6 mg/m³.