Question

How would you describe bulk density of soil to a non-scientist? Include some examples of bulk densities and your interpretation of those values.

Answer 1

The bulk density of a soil is defined as the weight per unit volume of soil. A unit volume of soil includes both the solids and the pore space. Bulk density reflects the porosity of a soil. Loose, porous soils have lesser bulk densities than tight, compacted soil. The bulk density of a soil increases with compaction. Bulk density indicates how easily a soil will till, how easily water will infiltrate, how it will hold water and its suitability for growing plants.

It is calculated as the dry weight of soil divided by its volume. This volume includes the volume of soil particles and the volume of pores among soil particles. Bulk density is typically expressed in g/cm3.

Density = mass (weight) / volume

Bulk density relates weight of solids to total volume of soil including solids and pores.

It is affected by both the nature of solids and the volume of pores.

Bulk Density = weight of oven dry soil divided by the soil volume

If a 100 cubic cm soil sample has an oven-dry weight of 150 grams then

soil Bulk density = Overdry weight / volume

= 150 g / 100 cm³

= 1.5 g / cm³

Bulk density is affected by the solids and porespace

High pore space = low bulk density

Low pore space = high bulk density

1. fine textures silt loams, clay, clay loam - generally lower bulk density 1-1.6 g/cc
2. sandy soils may range 1.2 - 1.8 g/cc
3. strong structure (well granulated topsoil) - low bulk density
4. high organic matter = low bulk density

As bulk density increases

The soil strength increases –

The pore space decreases

The soil becomes more compact.

The greater the total pore space the lower will be the bulk density

As bulk density increases the amount of pore space decreases.