Soil density

        Soil particle density and bulk density 

Particle density

              Soil is a three-phase system that consists of a solid phase (organic matter and inorganic matter), gaseous phase and liquid phase (water).

                For particle density, the volume of the solid phase is only considered.

            Particle density is the volumetric mass of the solid soil. It differs from bulk density because the volume used does not include pore spaces.

              * Particle density = oven-dry soil weight / Volume of soil solids

Table Particle density of different soil textural classes

Textural classes	Particle density ( g/ cm3)
Coarse sand	2.655
Fine sand	2.798
Silt
Clay	2.387

Bulk density

             Bulk density is the ratio of the mass of a given soil sample to its bulk volume.  Both volumes of solid particles and pores are considered as bulk volume.    

              Soils that are loose, porous, or well aggregated will have lower bulk densities than soils that are compacted or non-aggregated. This is because pore space (or air) weighs less than solid space (soil particles).

                    * Bulk density= Mass of oven dry soil/ Bulk volume of 

   Sample calculation for particle density and bulk density

          Soil particle density is always larger than soil bulk density.
Bulk the 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

              According to the above table, sandy soils have less total pore than clayey soils, so generally, they have higher bulk densities. Bulk densities of sandy soils usually vary between 1.2 to 1.8 Mg m3. Finetextured soils, such as Clays, silt clays, or clay loams, have bulk densities between 1.0 and 1.6 Mg m-3.

                    Factors affecting soil bulk density

1) Porosity (When the soil has relatively more pores, soils have a lower weight per unit volume therefore soil bulk density relatively is lower.)

2) Organic matter (When soil have more organic matter, soils are well aggregated together so pores are lower, therefore, bulk density is relatively lower.)

 BULK DENSITY INCREASING

           Clayey                                      Loamy                                                         Sandy 

    Well aggregated              moderately aggregated                                 Non-aggregated

    High organic content      Moderate organic content                           Low organic matte content     

3) Aggregation (When soils are well aggregated, pore are larger and the number of pores is increased mass per unit volume is low, therefore bulk density is low)

4) Arrangement of particles(When soil particles are loose bounded together, the soil has more pores so soils have a mass per unit volume, therefore soils have low bulk density.)

5) Compaction (The compaction resulting from cultivation can be detrimental to plant growth. Cropped soils generally have higher bulk densities than uncultivated soils.

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. )

6) Depth of soil profile(lower layers of soils have less organic matter, roots and compact due to the above soil layers, so mass per unit volume is relatively low therefore bulk density is also low.)

Aggregation can decrease some of these effects.