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
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).
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.
One way analysis of Bulk density (g/cm3) by Landuse
One way ANOVA analysis of volumetric water content and landuse.
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.
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).
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.
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.