This document discusses soil bulk density, particle density, and porosity and factors that influence them. It defines each term and provides typical values. Bulk density is the weight of soil per unit volume and is affected by mineral content, organic matter, compaction, and depth. Particle density is the weight of soil solids per unit volume and is influenced by mineral and organic matter content. Porosity refers to pore space and is classified by pore size. Factors like texture, moisture, and compaction impact porosity. The relationship between bulk density, particle density, and porosity is expressed through equations calculating percent solid and pore space. An example calculation is provided.

1. Soil Bulk density, Particle
density and Porosity-Factors
influencing-significance
K. Maheshwaran, Asst.Prof. (SS&AC)
Sethu Bhaskara Agricultural College &
Research Foundation

2. Soil bulk density
 Also termed as “Apparent specific gravity”
 Weight per unit volume of soil
Mass/weight of soil
 Bulk density = ------------------------ Mg m-3
Volume of soil
 BD of Heavy textured soil is 1.0 – 1.6 Mg m-3
 BD of Sandy soil is 1.2 – 1.7 Mg m-3
 BD of Organic soil is 0.5 – 0.6 Mg m-3
 BD of Normal soil ranges from 1.4 – 1.6 Mg m-3

3. Factors affecting Bulk density
Factors Affecting
Mineral matter More mineral content BD will be higher
Sandy soil more BD than Clay soil
Organic matter High OM content BD will be lower
Soil compaction Compacted soil have higher BD than loose
and porous soil
Soil depth BD will be increasing with sub surface soil
or increasing with depth lower soil
horizons due to lower OM content
Cultural practices Like OM addition, ploughing, loosening
the soil will lower the BD of soil

4. Particle density
 Also termed as “Absolute specific gravity or True
density”
Mass or weight of soil
 Particle density = ---------------------------------- Mg m-3
Volume of soil solids
 PD of organic matter is 1.2 – 1.7 Mg m-3
 PD of mineral soil is 2.60 – 2.75 Mg m-3
 Normal PD of soil is 2.65 Mg m-3

5. Factors Affecting
Mineral matter More mineral content PD will be higher
such as haematite, limonite, zicon,
magnetite. PD in OM is very low
Organic matter High OM content PD will be lower
Clay and Sesqioxides Presence of high clay content and
sesquioxides decreasing PD by increase
aggregation
Soil depth PD will be increasing with sub surface soil
or increasing with depth lower soil
horizons due to lower OM content
Factors affecting Particle density

6. Porosity
 A soil consist of that portion of the soil volume not
occupied by solids, either mineral or organic
 Also called as “voids”
 Pore space under field condition always occupied with
water and air at all times
 Pore space indirectly influence the amount water & air
and indirectly control the plant growth & crop
production

7. Classification of soil pores based on size
Particulars Characteristics
Macro pores
Non capillary pores
•0.2 mm diameter
•Mostly in coarse textured soil such as
sand soil having more macro pores
than micro pores
•There is no restriction of air and
water movement
•Water holding capacity is very low
Micro pores
Capillary pores
•< 0.2 mm diameter
•Mostly in heavy textured soil such as
•Clay soil having more micro pores
than macro pores
•There is restriction of air & water
movement
•Water holding capacity is high

8. Factors affecting Porosity
Factors Affecting
Texture •Fine textured soil more porosity than
coarse textured soil
•Fine textured soil with 60% porosity, 50%
of micro pores and 10% of macro pores
•Coarse textured soil with 30% porosity,
25% of macro pores and 5% micro pores
Moisture content •Well aerated soil micro pores filled with
moisture and macro pores filled with air
•Water logged soil, micro and macro pores
filled with both water and air
Soil compaction •Compacted soil pores are get reduced due
to tightly arrangement of soil particles
results in poor aeration

9. Relationship between porosity and
densities of soil
Bulk density
% Solid space = X 100
Particle density
Bulk density
% Pore space =100 1-
Particle density
Both are expressed in terms of percentage %

10. Calculation
Soil sample taken from the field for determination
of bulk density. The measurement were: Calculate
the bulk density, solid space and pore space
a) Volume of soil= 73.6
b) Soil weight= 87.8 gm
c) Particle density= 2.65 Mg m-3