Bulk density, Particle density

1.DETERMINATION OF BULK DENSITY, PARTICLE DENSITY AND PORE SPACE

Bulk density of a soil indicates the degree of compactness of the soil and is defined as the mass per unit volume which includes space occupied by solids and pore space. The normal range of bulk density in most soils is from 1.02 to 1.80 Mg m-3. Porosity is that fraction of soil volume not occupied by solid particles

                                  Bulk density Db   

         Bulk density varies with particle size distribution, organic matter content, mechanical composition and depth of soil. The bulk density of coarse textured soils will be on the higher side while organic soils will have lower bulk density.

Particle density refers to the density of the soil solids and is defined as the mass per unit volume of soil solids excluding pore space.

Particle density = Dp=  Mgm-3

The particle density is higher than that of bulk density.

         The particle density of most soils range between the narrow limits of 2.60 to 2.70 gm/ml with an average value of 2.65 gm/ml as most of the mineral particles are constituted of mainly quartz, feldspars and other silicates whose densities also vary within the same limits. If the soil contains high amounts of organic matter or heavy minerals such as magnetite, garnet, epidote etc., the bulk density may be less than the lower limit or more than the higher limit respectively. The particle density of soil does not vary due to the mechanical manipulations. For determining particle density, the mass and volume of the soil solids have to be estimated.

         The per cent pore space is an important soil physical property and indicates the soil volume occupied by soil air or soil water. The total pore space is made up of micropores (capillary pores) and macropores (non- capillary pores). The micropores hold water while macropores are filled with water. In an ideal soil, the total pore space should be equally distributed between the two types of pores.

         The total pore space of a medium textured soil is around 50%. Sandy soils have lower porosity while clay and organic soils have higher porosities. Subsoils have lower pore space than the surface soils due to compaction. In a coarse textured soil, the proportion of micropores is greater.

 A) Cylinder method

Principle

The bulk density and percent pore space are determined from the apparent and true volumes of the soil measured by adding a known quantity of water to a measuring cylinder containing a weighed quantity of soil.

Materials required

 i) Measuring cylinder (100 mL) with glass stopper

ii) Chemical balance or Electronic balance

Procedure

·         Weigh exactly 20 g of soil sample and transfer in small quantities at a time to a 100 mL measuring cylinder with glass stopper by gently tapping the cylinder. After completely transferring the soil, note the volume.

·         Add a known volume of water (50 mL) along the sides of the cylinder using pipette till the entire soil mass is completely soaked.

·         There should be atleast 5 mL of water above the soil surface after the addition of water.

·         Care should be taken to see that the final volume of soil and water is below 100 mL mark.

·         Keep the cylinder with soil and water in an undisturbed condition for at least 30 minutes so that the entire pore space is completely filled with water.

·         Note the final volume of soil plus water after the expiry of time and calculate the percentage of bulk density, particle density and pore space.

Calculation

Weight of the soil taken        =   W   (20) g

Volume of the soil taken          =   V1 mL

Volume of water added           =   V2 mL

Volume of soil + water            =    V1 + V2 mL

Volume of soil water at the end of experiment   =   V3 mL

Pore space volume

=V1 + V2 -V3 mL

Per cent pore space

=Bulk density W/V1

=Particle density

Result 

Inference

Assignment

1.Arrange  the following soils in the order of increasing bulk density

Sandy soil, clay loam, clay and sandy loam.

B) Wax coating method

Principle

A soil clod of known weight is coated with wax and immersed in a known volume of water and from the volume thus arrived at, the bulk density is determined.

Materials required

 i) Balance     ii) 250 mL measuring cylinder iii) Twine    iv) Wax

Procedure

·         Take a small clod from the field, the bulk density of which is to be determined and note its weight (Wg).

·         Then tie the clod with a twine. Immerse the clod in melted wax so that there is a complete coating of the clod with wax.

·         Take a known volume of water (150ml) in a 250 mL measuring cylinder. Immerse the wax-coated clod in the water and note the rise in volume of water in the cylinder.

Calculate the bulk density of the clod from the weight and volume of the cold. Calculation

Weight of the clod    =W   g

Initial water level in the cylinder    =V1 mL

Final water level in the cylinder after Immersing the clod   =V2   mL

Volume of the clod    =(V2  – V1) =VmL

Bulk density of the soil

Result 

Assignment

1. State the reasons for coating the clod with wax

 C) Core sampler method

Principle

         Volume of undisturbed soil core from the field is determined and the mass per unit volume is calculated.

Materials required

1. Core sampler    2. Spade 3.Crow bar  4.  Balance

5.  Aluminum box 6.  Hot air oven 7.  Knife

Procedure

·         Drive a core sampler into the soil gently with the help of a hammer so that the entire ring goes into the soil.

·         Remove the soil surrounding the ring with the help of a spade. Remove the core sampler with the soil intact by using a crow bar.

·         Remove the excess soil on both sides of the ring gently using a sharp knife.

·         Transfer the soil in the ring to an aluminum box and dry it in an oven at 105°C for at least 24 hours.

·         Then cool the sample in a desiccator and record the weight.

·         Calculate the volume of the ring from the height and diameter and calculate the bulk density of the soil.

Calculation

Volume of the ring (V)                                              =        p  r2 h cc

Where ‘r’ is the radius and ‘h’ is height of the ring

Weight of oven dry soil                                           =          W g

Bulk density of the soil                                            =

Result