1. In order to determine the bulk density of a soil, a soil core was taken from

Question

1. In order to determine the bulk density of a soil, a soil core was taken from field 1 and the following information obtained: Volume of soil core = 200 cm3 Mass of the soil (oven dry) = 330 g

a. Calculate the Bulk Density of the soil ; Calculate the percent Pore Space (please show your work including formulas and units)

b. Another sample from a nearby field 2 with the same soil characteristics had the same volume (200cm3), but a dry mass of 250 g. Calculate the Bulk Density. Calculate the percent Pore Space.

c. Can you make conclusions about the differences in bulk densities of samples from field 1 and field 2?

Explanation / Answer

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

% solid space = (bulk density / particle density) x 100

% porosity (% pore spaces) = 100 - (% solid space)

Soil water and air occupy voids in the soil, called pore spaces. The pore system in soil provides the conduits for air and water exchange and houses roots and microbes. Soil porosity is the amount of pore volume (%age of pore space). A medium textured, well-aggregated soil contains about 50% pore space and is in good condition for plant growth when the pores hold an equal distribution of air and water. Pore size affects pore activity. Big pores, macropores, facilitate free-water drainage, aeration, evaporation, and gas exchange. Mesopores, medium-size pores, are essential to capillary water distribution, and micropores provide water storage sites. Macropores are most prevalent in sandy soils and well-aggregated soils, but can be converted to micropores by compaction. Medium-textured soils have an abundance of mesopores. Clays promote aggregation but can also be readily compacted. Clays also increase water storage by providing an abundance of micropores. Thus, texture and structure, plus the level of induced compaction, are the main properties governing amount and type of pore space in the soil. Organic matter affects porosity through its enhancement of soil aggregation.

a. Sample taken from Field 1

A soil core was taken from field 1 and the following information obtained:

Volume of soil core = 200 cm3

Mass of the soil (oven dry) = 330 g

Bulk Density of the soil =

Bulk density = Oven dry soil weight / volume of soil solids and pores

= 330gm/ 200cm3

=1.65gm/cm3

Particle density represents the average density of all the minerals composing the soil. For most soils, this value is very near 2.65 g/cm3 because quartz has a density of 2.65 g/cm3 and quartz is usually the dominant mineral. Particle density varies little between minerals and has little practical significance except in the calculation of pore space.

Particle density for soil = 2.65gm/cm3

% solid space = (bulk density / particle density) x 100

                        = [1.65/2.65]x100

                        =62.26%

  % porosity (% pore spaces) = 100 - (% solid space)

                        =100-62.26

                        =37.74%

b. Sample taken from Field 2

Volume of soil core = 200 cm3

Mass of the soil (oven dry) = 250 g

Bulk Density of the soil= Oven dry soil weight / volume of soil solids and pores

= 250gm/ 200cm3

=1.25gm/cm3.

Particle density for soil = 2.65gm/cm3

% solid space = (bulk density / particle density) x 100

                        = [1.25/2.65]x100

                        = 47.17%

% porosity (% pore spaces) = 100 - (% solid space)

                        =100-47.17

                        =52.83%

c.       Difference in bulk density of two samples taken from field 1 and field 2.

Bulk density of sample taken from field 1 is 1.65gm/cm3

Bulk density of sample taken from field 2 is 1.25gm/cm3

Difference in bulk densities is = 1.65-1.25= 0.4 gm/cm3

Mass of the dry soil in the sample which is taken from field is one is more weight than compare with sample taken from Field 2.

Bulk density increases with depth and tends to be high in sands and compacted pan horizons, and tends to be low in soils with abundant organic matter.

Tillage operations loosen soils and temporarily lower bulk density, while compaction processes raise bulk density.

High bulk densities correspond to low porosity.

Natural soil-forming processes that increase aggregation reduce bulk density, but excessive tillage and raindrop impact on bare soil destroy aggregation and increase bulk density.

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