Fundamentals of Soil Science (1+1)

Factors Affecting Bulk Density

Factors Affecting Bulk Density

1. Soil texture and structure:

• Fine textured soils like silt loam and clay loam have lower bulk density than sandy soils. This is mainly because of more granulation/ aggregation. As aggregation and clay content increase, bulk density decreases. Tillage operations do not affect texture, but they do alter structure (aggregation). Primary tillage operations, such as ploughing, generally decreases bulk density and increases pore space, which is beneficial.
• Secondary tillage (cultivation) generally increases bulk density and decreases porosity.

2. Pore space:

• Since the bulk density is related to the combined volume of solids and pore spaces, hence the soils with higher pore space will have lower bulk densities. Thus any factor that influences pore space will affect the bulk density.

3. Compaction:

• It leads to increase in bulk density. The movement of machinery over the field forces solid particles into spaces once occupied by water or air, resulting in less pore space and increased bulk density.
• The fine textured soils have low bulk densities when not compacted but very high when compacted in comparison to sandy soils. In fact sandy soils are less affected by compaction.

4. Organic matter:

• Since organic matter is lighter than an equal volume of solid soil and is more porous, hence a soil with higher organic matter will have lower bulk density.
• Manure additions in large amounts tend to lower the surface bulk density of mineral soils because of the addition of low bulk density material and the consequent promotion of soil aggregation.
• Usually surface soils are rich in organic matter than sub-surface soil and as a result lower bulk density is recorded in surface soil.

5. Crop and soil management:

• The crop and soil management also influences the bulk density. The addition of crop residues and FYM always lower the bulk density of surface soil. This is just similar to the effect of organic matter as discussed above.
• However, the intensive cultivation increases the bulk density because more cultural operations enhance the compaction of the soil. Cropped soils generally have higher bulk densities than uncropped soils.