Chapter 5

BULK DENSITY AND PARTICLE DENSITY

Bulk Density (Db)

Soil bulk density, like all density measurements, is an expression of the mass to volume relationship for a given material. Soil bulk density measures total soil volume. Thus, bulk density takes into account solid space as well as pore space. Soils that are loose, porous, or well-aggregated will have lower bulk densities than soils that are compacted or nonaggregated. This is because pore space (or air) weighs less than solid space (soil particles). Sandy soils have less total pore than clayey soils, so generally they have higher bulk densities. Bulk densities of sandy soils vary between 1.2 to 1.8 Mg m^-3. Fine-textured soil, such as Clays, silty clays, or clay loams, have bulk densities between 1.0 and 1.6 Mg m^-3.

Farmers often speak of 'heavy' and 'light' soils in relation to the ease of tillage. 'Heavy' soils are clayey and difficult to till, while 'light'soils are sandy and easy to till. These terms are misnomers in the technical sense because sandy soils are heavier per unit volume than clayey soils. Remember that sandy soils have less pore space than clayey soils, so in a given volume of both soils, the sandy soil has less air (more solid soil particles) and is therefore heavier. The terms 'heavy' and 'light' actually refer to other physical properties of the soil, such as plasticity, cohesion, adhesion, etc. which determine the soil's ease of tillage.

Bulk density is an indirect measure of pore space and is affected primarily by texture and structure. As aggregation and clay content increase, bulk density decreases (Figure 5.1). Tillage operations do not affect texture, but they do alter structure (soil particle aggregation). Primary tillage operations, such as plowing, generally decrease bulk density and increase pore space, which is beneficial. Secondary tillage (cultivation) generally increases bulk density and decreases pore space. The compaction resulting from cultivation can be detrimental to plant growth. Cropped soils generally have higher bulk densities than uncropped 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.

>---------BULK DENSITY INCREASING---------->

>-----------BULK DENSITY INCREASING-------->

Figure 5.1. Relationship of soil bulk density to texture, organic matter content, and aggregation.

The density of water is 1.0 Mg m-3 and mineral soils are usually heavier than water. However, organic soils generally have a bulk density less than water. As the organic matter content of mineral soils increases, the bulk density decreases (Figure 6.1). Manure additions in large amounts tend to lower the surface bulk density of mineral soils because of the addition of low bulk density material and the consequent promotion of soil aggregation. Soil bulk density increases with soil depth primarily because of less organic matter and decreased aggregation.

Measurement of Soil Bulk Density

Measurement of soil bulk density involves the determination of the mass and the volume of a given amount of soil material. These two measurements can be obtained in a variety of ways. Some of these methods are listed below.

Clod Method - Bulk density is best measured on an undisturbed sample. One method of determining Db of soils in their natural state is to use large soil peds or clods. Clod volume is determined by coating with a water repellant substance, such as paraffin or saran, then making use of Archimedes Principle. Archimedes Principle states that a mass immersed in water will displace it's own volume. Db can them be calculated by first weighing the coated clod suspended in water and then in air.

Core Methods - Another method of determining Db of soils under natural conditions is to obtain a core or cylinder of soil, of known volume, by driving or pushing a core sampler into the soil. The soil extracted from the sampler is oven dried and weighed.

Excavation Method - In this method a quantity of soil is excavated, oven dried and, weighed. The volume of the hole left is measured by filling the hole with either sand or water and measuring the volume needed to fill the hole. In the water method, a balloon is placed in the hole and is filled with water from a graduated vessel.

Radiation Method - Measurement of either gamma radiation transmission or scattering can be used to estimate Db. The transmission technique requires the boring of two holes into the soil. A probe containing a source of gamma radiation is lowered into one hole. A second probe containing a geiger counter is lowered into the other hole. With proper calibration, the rate at which gamma radiation passes through the soil can be used to estimate Db. In the scattering method, the gamma radiation source and the geiger counter are lowered into a single hole. The detector then measures the gamma rays that 'bounce back' from the soil.

Particle Density (Dp)

Soil particle density is a measure of the mass per unit volume of the soil solids only. Texture and structure do not affect particle density. However, organic matter, which is a soil solid, readily influences particle density. Organic matter weighs much less per unit volume than soil minerals. Soils high in organic matter have lower particle densities than soils similar in texture that are low in organic matter. Soil particle density generally increases with soil depth because of the concurrent decrease in organic matter.

Particle density varies with the type of soil minerals present as well as the amount of organic matter. The particle density of most mineral soils is in the range of 2.60 to 2.75 Mg m^-3. Particle density is used in the calculation of pore space and bulk density on a coarse fragment free basis. When unknown, particle density of mineral soils is assumed to be 2.65 Mg m^-3. Generally quartz, feldspars, and colloidal silicates dominate the mineral fraction of soils. The particle density of these minerals averages about 2.65 Mg m^-3. When large amounts of heavy minerals, such as hornblende or magnetite, are present, the soil particle density is greater than 2.65 Mg m^-3. Soils formed in volcanic parent materials, such as pumice or ash, generally have particle densities less than 2.65 Mg m^-3. Soil particle density is determined using Archimede's Principle.

Laboratory Exercise

The instructor will demonstrate measurement of soil bulk density by the excavation method and the core method. The laboratory exercise also includes a determination of soil particle density.

A. Soil Bulk Density: Excavation Method

1) In the laboratory: Weigh a clean, dry beaker empty.

2) In the field: Excavate a hole in the soil using a soil bucket auger and empty all the soil from the auger into a clean, dry bucket.

3) Tamp down the soil at the bottom of the hole so that it is smooth and flat.

4) Measure the depth and the diameter (for calculation of the volume) of the hole and record on the data sheet.

5) Fill the hole to the top with clean sand, keeping track of the volume of sand needed to fill the hole. Record the volume of sand used to fill the hole on the data sheet.

6) In the laboratory: Weigh the soil and bucket and record on the data sheet.

7) Transfer soil to container for oven drying and place in the oven.

8) Obtain the data for the oven-dry weight of soil from the laboratory instructor the following day.

B. Soil Particle Density

1) Weigh a clean, dry pyconometer with the ground glass stopper to the nearest 0.01 g.

2) Transfer about 10 g of soil to the pyconometer, replace the stopper, and reweigh. This allows the exact weight of soil to be determined.

3) Add water to the pyconometer until it is about 2/3 full. Leaving the stopper out, heat the pyconometer carefully on a hot-plate. Gently boil the water to remove any air entrapped in the soil. Do not leave unattended.

4) Cool the pyconometer to room temperature, fill to the brim with water, and insert the stopper so that water shoots out the hole in the stopper. This ensures that no air is trapped in the pyconometer. The entire volume, including the stopper, should now be free of air.

5) Carefully dry the outside of the pyconometer and weigh to the nearest 0.01 g.

6) Remove all the soil from the pyconometer and fill completely with water only. Insert the stopper as before, wipe the outside dry, and reweigh.

7) Calculate particle density (Dp).

DATA SHEET

Chapter 5

A. Soil Bulk Density: Excavation Method

1. Soil core weight

a) Weight clean, dry beaker, empty _________g

b) Weight beaker and soil _________g

c) Weight soil (by difference) (b-a) _________g

d) Oven-dry weight soil _________g

e) Gravimetric moisture _________%

2. Soil volume

Geometrically Directly

a) Depth of hole ________ cm a) Volume sand used

to fill hole

b) Diameter of hole ________ cm ________ cm³

c) Volume of hole ________ cm³

(Note: Volume = ( ) (radius)² (depth)

3. Soil bulk density

a) Geometrically ________ g cm^-3 (Mg m^-3)

b) Directly ________ g cm^-3 (Mg m^-3)

DATA SHEET
CHAPTER 5

B. Soil Particle Density

1. Weight empty, clean, dry pyconometer ________g

2. Weight pyconometer and soil ________g

3. Weight pyconometer, water, and soil ________g

4. Weight pyconometer and water ________g

5. S = (line 2 - line 1) ________g

6. W = (line 4) ________g

7. X = (line 3) ________g

8. Particle density = __S__ _{________} g cm^-3

W-X+S (Mg m^-3)