Density of Soil: Meaning and Types

After reading this article you will learn about the meaning and types of density of soil.  

Meaning of Density:

Density is the weight per unit volume of a substance. It is expressed in gram per cubic centimeter or pound per cubic foot or mega gram per cubic metre (Mg m^-3). Two density measurements—particle density and bulk density are common for soils.

Density (D) = Mass (M)/Volume (V) gm./cc or lb/cft, or Mgm^-3

Types of Density:

1. Particle Density:

The weight per unit volume of the solid portion of soil is called particle density. It is also termed as true density. It is expressed in gm./c.c. (C.G.S. system) or lb/cft (F.P.S. system). It depends upon the accumulative densities of the individual inorganic and organic constituents of the soil. Generally in the normal soils the particle density is 2.65 grams per cubic centimetre or mega grams per cubic metre.

The particle density is higher if large amounts of heavy minerals such as magnetite, limonite, hematite and zircon are present. With an increase in organic matter of the soil, the particle density decreases. When particle density is divided by density of water, a relative weight number is obtained is called specific gravity.

2. Bulk Density:

Bulk density is defined as the mass (weight) per unit volume of a dry soil (volume of solid and pore spaces). It is also expressed in gm./c.c. (G.G.S. system) or lb/cft (F.P.S. system) or mega gram per cubic metre. The bulk density of a soil is always smaller than its particle density. Loose and porous soils have low weights per unit volume and compact soils have high values.

The bulk density of sand dominated soils is about 1.7 gm./c.c, whereas in organic peat soils the value of bulk density is about 0.5 gm./c.c. Bulk density normally decreases as mineral soils become finer in texture. Bulk density is of greater importance than particle density in understanding the physical behaviour of soils.

Generally, soils having low and high bulk densities exhibit favourable and poor physical conditions respectively. Generally in normal soils bulk density ranges from 1-1.60 gm./c.c. or mega gram per cubic metre excepting in very compact sub-soils 2 gm./c.c or mega gram per cubic metre.

Factors Affecting Bulk Density:

(i) Amount of pore space. If the soil containing more pore spaces than that of solid spaces per unit volume, then the value of bulk density will be very low.

(ii) Compactness of the soil. In high compacted soil or waterlogged soils the bulk density will be more.

(iii) Texture of the soil. Textural variations influence the value of bulk density in soils. As for example, clay, clay loam and silt loam surface soils show low bulk density as compared to sands and sandy loam soils which show high bulk density value.

(iv) Organic matter content. Soils containing high organic matter show lower value of bulk density.

(v) Soil structure. Soil structure affects bulk density by influencing the porosity of the soil. More for example, crumb soil structure shows low bulk density than that of platy soil structure.

Problem:

A metal cylinder pushed into a loam soil is removed from the field and the soil it contains is dried in an oven.

The measured data are given below:

Cylinder height – 5.0 cm

Inside diameter of the cylinder – 4.4 cm

Oven dried soil weight – 87.6 g

Calculate the bulk density of the soil.

Solution:

1. The volume of the soil sample equals the volume of the cylinder. A cylinder’s volume (V) equals = Ï€ r² h cubic unit.

2. The bulk density of a soil can be calculated on the basis of dry weight of that soil divided by its volume.

Thus,

Bulk density = Wt. of soil mass/Soil volume

= 87.6 g/76.0 c.c.

= 1.15 g/c.c.