Bulk Density
Author: Anna Houston, Grant Tranter, and Ian Miller

 

Principle:

Bulk density is a measure of a soils mass per unit volume of soil. It is used as a measure of soil wetness, volumetric water content, and porosity. Factors that influence the measurement include; organic matter content, the porosity of the soil, and the soil structure these factors will intern control hydraulic conductivity. The reference mass of the soil is taken after oven drying, and the volume is taken for the fabric which is less than 2mm, including solids and pore space, (Grossman and Reinsch, 2002). The equation used is:

Bulk Density (p) = Mass of oven dried soil / Total volume

A soil that has a well developed structure will become less dense as porosity increases; as a result the bulk density of the soil will decrease. Soils which show massive structures and less porosity will show higher bulk densities ranging from 1.6 to 1.7gcm-3, water movement will be hindered at this point down the profile. Most soil bulk densities will be found in a range form 1 to 2 gcm-3, with the density of soil solids (Quartzite), being 2.65gcm-3. The bulk density

 

In agricultural terms the bulk density of the soil can be used to give an indication of the porosity and structure of the soil which will, govern O2 and H2O movement in the soil.
It is also a measurement of the degree of compaction of the soil, which gives a comparative basis to indicate the strength of similar materials. One of the most important factors agriculturally in terms of bulk density is plant growth, if the soil has a high bulk density (compaction) the seed will be restricted in emergence and root growth which will effect the total plant growth and yield. The use of tractors will directly effect the soils bulk density causing extreme compaction especially if the soil is wet (see image on the left), however lately the use of controlled traffic has decreased these problems. Careful management on the land is required to create an ideal bulk density for optimum plant growth and healthy soil.

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Description of apparatus/method:

A soil core is obtained by clearing any vegetation from the soil surface, then knocking a metal cylinder of known dimensions into the ground.

 

 

The pictures show inserting a metal cylinder into the ground to obtain soil core, then placing soil core in bag, and sealing to preserve for lab analysis.

The soil core is then excavated from the ground assuring that the cylinder is full of soil, while at the same time flush with the end of the cylinder assuring the total volume of the cylinder is full but not overflowing. Once soil core is collected it is “bagged” so that soil does not dry out and loose any moisture.

In total 16 samples were taken from the 8 peg sites (samples from C1 and C2 were not collected). A dry and wet sample were taken from each peg site, the wet sample area had been wetted from either a ponded or tension disk.

Lab analysis of soil core starts with the soil core being weighed as it was found in the field -air dry. This weight is recorded and the core is placed into an oven at 105 deg.C to obtain oven dry (OD) soil whereby soil moisture has been removed. The OD soil is weighed and the water content can then be calculated by the volume of water (cm3) divided by the total volume of soil (cm3). Bulk density will be calculated by the mass of OD soil (g) divided by the total volume of soil (cm3).

 

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Analysis of data:

Analysis of the 16 core samples were conducted on the JMP 5.1 statistical package. The analysis was conducted to determine if there is a significant difference between bulk density due to land use, wet/dry soil, or wetting method in terms of water content.

The landuse and wet/dry soil analysis was preformed by using the "fit x by y" function, whereby the bulk density is placed on the y axis and the variable of interest on the x axis. Once this has been preformed a one way ANOVA t-test was conducted to show the mean of each variable and any significant difference. The significant difference is determined by Prob>F, if <0.05 there is a significant difference.

To determine if there is a significant difference between landuse and wetting method a "Fit Model" analysis was conducted. Where the water content percentage was graphed against landuse, wetting method and landuse and wetting method combined.

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Results:



Bulk density of each site

Site
Bulk density (g/cm-3)
Wetting method
P2 -wet
1.20
tension
P2 -dry
1.02
non
P3 -dry
1.13
non
P3 -wet
1.09
ponded
P4 -wet
1.21
tension
P4 -dry
0.88
non
P5 -wet
1.08
ponded
P5 -dry
1.00
non
C2 -wet
1.22
tension
C2 -dry
1.25
non
C3 -wet
1.13
ponded
C3 -dry
1.26
non
C4 -wet
1.20
tension
C4 -dry
1.25
non
C5 -dry
1.11
non
C5 -wet
1.14
ponded






One way ANOVA Analysis of bulk density by wet and dry soil samples

The mean bulk density of the samples taken in the wet soil was 1.57(g/cm3) compared to the dry soil with a bulk density of 1.11 (g/cm3). There is no significant difference between the two as the p-value was greater than 0.05, which was to be expected.

The variation of the dry samples is large which could possibly be due to the shrink swell nature of the soil, samples may have included a proportion of a crack. The three bulk densities samples in the bottom range of the graph were taken from the pasture area, in the field this are was observed to contain many cracks, which would strongly suggest this to be true. The variation may also be due to sample method whereby the corer was not filled.

 

 

 

One way analysis of Bulk density (g/cm3) by Landuse

There is a significant difference of bulk density between the cultivated and pasture area, shown in the ANOVA test with a p value <0.02. The mean bulk density of the cultivated land was 1.2 (g/cm3), where the pasture area was 1.0 (g/cm3).

The difference in bulk density between the landuses is caused by the management of the soil, the continual cultivation of the soil has caused the structure to decline. However the pasture area contains a lower bulk density due to the minimal damage to the soil, also the vegetation would add organic matter, pores and higher microbial content all aiding in a healthier structure.

 

 

 

 

One way ANOVA analysis of volumetric water content and landuse.

 

The volumetric water content was obtained by the bulk density cores and the Time Domain Reflectrometry (TDR). The results indicated that the two methods are quite similar however the core sampling has under estimated the soil water content. As previously mentioned this would be due to sample error, whereby the core may not have been filled correctly. The cultivated area had a higher water content compared to the pasture. This is due to the evapotransipiration of plants where the plants are utilizing the water. In the field it was observed that cracks were present in the pasture area, an indication of the evapotranspiration effect.
Mean volumetric water content %
Landuse
TDR
Bulk density samples
Cultivated
35
30
Pasture
15
11

 

 

 

 

 

Land use and wetting method leverage

 

When the fit model was applied it showed a significant difference between volumetric water content, wetting method and landuse, however when landuse and wetting method were combined no significant difference was found. The wetting methods that were used for the samples were tension disk, ponded disk and no wetting (dry). The ponded disk has a higher volumetric water content as all pores are wetted in this method, however in the tension disk infiltrometer bio-pores are excluded, these larger pores will conduct air not water. However there is no significant difference in volumetric water content between ponded disk or tension disk infiltration wetting method.

 

 

 

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The results indicated that cultivation has caused compaction therefore decreasing the structure of the soil, however it does cause a higher volumetric water content due to the fact no plants are using the water. Pasture contains a lower bulk density due to minimal disturbance, however these results would also be influenced by the presence of cracks. It would be interesting for future experiments to obtain the bulk density in the subsurface to study the effects of the landuse down the profile.

 

 

 

 

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Problems with method:

The main problem associated with the bulk density method is that it is totally dependant on an accurate collection of soil in the cylinder, (so that the cylinder’s total volume is occupied but not overflowing).
Care also needs to be taken when analyzing soil, so that no soil is excluded from weighing, and calculations. As the core is driven into the ground it may compact the sample effecting the true bulk density of the soil.

One of the problems we have encounted at this site is the shrink swell properties of the soil, samples may have included cracks in the drier regions, especially in the pasture. This can be seen in the one way analysis where a bulk density was recorded at 0.88 g/cm-3, very low for this site.

 

 

 

 

 

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References:

Grossman R.B, Reinsch T.G. SSSA Book Series: 5 Methods of Soil Analysis Ch2, Ed. Dane J.H, Clarke Topp G. Soil Science Society of America, Inc. Madison, Wisconsin, USA 2002.

 

 

 

 

 

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