The bulk density of a soil

 Bulk density

 Dry bulk density - It is the ratio of the mass of oven-dried soil (Ms) to total volume (Vt)  of soil.

• Units: g/cm3

• Depends upon texture, structure, and OM content

• May vary from 1.1 to 1.4 g/cm3 in fine and 1.4 to 1.75 g/cm3 in coarse-textured soils.

The bulk density of soil is always smaller than its particle density. The bulk density of sandy soil is about 1.6 g / cm3, whereas that of organic matter is about 0.5. Bulk density normally decreases, as mineral soils become finer in texture. The bulk density varies indirectly with the total pore space present in the soil and gives a good estimate of the porosity of the soil. Bulk density is of greater importance than particle density in understanding the physical behavior of the soil. Generally, soils with low bulk densities have favorable physical conditions.

Bulk density of different textural classes

Textural class                                        Bulk density (g/cc)                          Pore space (%)

Sandy soil                                                      1.6                                                  40

Loam                                                              1.4                                                  47

Silt loam                                                         1.3                                                  50

Clay                                                                1.1                                                  58

Factors affecting bulk density

1. Pore space: Since bulk density relates to the combined volume of the solids and pore spaces, soils with a high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space.

2. Texture: Fine textured surface soils such as silt loams, clays, and clay loams generally have lower bulk densities than sandy soils. This is because the fine-textured soils tend to organize in porous grains especially because of adequate organic matter content. This results

in high pore space and low bulk density. However, in sandy soils, organic matter content is generally low, the solid particles lie close together and the bulk density is commonly higher than in fine textured soils.

3. Organic matter content: More organic matter content in the soil results in high pore space thereby show the lower bulk density of soil and vice-versa.

Particle density

• It is the ratio of the total mass of soil solids (Ms) to the total volume of soil solids (Vs).

• Units: g/cm3

• PD= Ms/Vs

• Depend on chemical/mineralogical composition.

• 2.65 g/cm3

Particle Density:

✓ The weight per unit volume of the solid portion of soil is called particle density.

✓ Generally, the particle density of normal soils is 2.65 grams per cubic centimeter.

✓ The particle density is higher if a large number of heavy minerals such as magnetite; limonite and hematite are present in the soil.

✓ With an increase in organic matter of the soil the particle density decreases.

✓ Particle density is also termed true density.

Significance and manipulation of densities

✓ Changes in bulk density for a given soil can alert soil managers to change the soil quality and ecosystem functions.

✓ Increases in bulk density indicate a poor environment for root growth, reduce aeration, and water infiltration.

✓ Particle and bulk densities are used to calculate the porosity of the soil.

✓ Bulk density is used to calculate total water storage capacity per soil volume and evaluate soil layers to determine root penetration and adequate aeration.

✓ Tillage may temporarily loosen the soil surface, in the long term tillage increases bulk density.

✓ The effect of cultivation on bulk density can be minimized by adding crop residues or farm manure in large amounts and rotating cultivated crops with grass sod.

✓ Repeated use of heavy machinery can form a plow pan below the plowed layer.

✓ Large chisel-type plows can be used in subsoiling to break up dense subsoil layers, thereby permitting root penetration by lowering the bulk density of soil.