*BULK DENSITY* Top
The principle behind how bulk density works is best explained through knowledge of soil properties. Factors such as organic matter, soil structure and porosity will influence the levels recorded for bulk density. Soil bulk density is the mass per unit volume of soil. In agriculture, the reference mass is after oven - drying, and the volume is for the <2mm fabric, inclusive of solids and pore space (Grossman and Reinsch, 2002). ‘Variation in bulk density is attributable to the relative proportion and specific gravity of solid organic and inorganic particles and to the porosity of the soil. Most mineral soils have bulk densities between 1.0 and 2.0.’ (www.geology.iupui.edu)
As the structure of a soil develops, the soil becomes increasingly porous. Pore spaces form between soil peds, the soil becomes less dense and bulk density declines. High bulk density measurements, ranging from 1.6 to 1.7gcm-3 are associated with massive, structure less soils that are hydraulically restrictive and disrupt the downward percolation of water.
1. The distribution of water and pores in a soil (www.geog.plym.ac.uk)
Figure 1. The distribution of water and pores in a soil (www.geog.plym.ac.uk)
The equation most commonly used in soil experiments is shown below:
Bulk density = r = Mass of oven dried soil
Bulk density is a measure of the degree of compaction of a soil, which can be used on a comparative basis to indicate strengths of similar materials. Therefore bulk density can be used to measures the amount of pores present or porosity of the soil by taking a core of soil that is weighed and then divided by the total volume.
What does it measure?
Bulk Density can be used for the measurement of wetness, volumetric water content and porosity, these properties can be calculated through the following formula:
q = w * r/rw where rw is the density of water which is taken as 1g cm-3
w is wetness
q is the volumetric water content
f = 1- r/rs where f is porosity
r is bulk density and
rs is solid density
The bulk density of soil in agricultural terms is very important. A large bulk density i.e. highly compacted soil will greatly impede the growth of crops and other plants by restricting root growth. An open friable soil with good organic matter content will have a bulk density of < 1.0 g cm-3. Management practices are directly responsible for changes in the bulk density, firstly through its control of the formation of organic matter over time and secondly because it involves the application of forces that either loosen or compact the soil. The latter is most commonly associated with the effects of farm machinery and trampling by humans and animals. For this reason, bulk density is an important soil property to fertility studies of soils under long-term cultivation. Hence the measurement of bulk density on the farm can greatly aid in the management techniques applied.
Figure 2. A representation of the bulk density of soil columns after the passage of five vehicles (www.geog.plym.ac.uk)
3. Displays a soil that has undergone significant
tractor compaction, such that infiltration is impeded due to high soil bulk
The method requires heating the soil so that it dries out completely. Weighing the soil before and after heating allows gravimetric soil moisture as well as dry bulk density to be determined. Carefully collect a small-undisturbed core of soil adjacent to the points of soil moisture determination using the tins provided. Trim off excess soil and wrap core in labeled polythene bag. Weigh core containing moist soil core on electronic balance to two decimal places (nearest 0.01g). Record core number. Place core containing moist soil in oven and dry for 24 hours at 105oC. Weigh the core containing dried soil. Extrude the soil and obtain weight of empty core. Dispose of the soil as directed.
But simplistically the method should be as follows: A stainless steel cylinder of known volume is driven smoothly into the soil to a depth of 50mm. After extraction, excess soil is trimmed from the ends of the cylinder. The cylinder is weighed before and after oven drying.
Figure 4. Apparatus used for core sampling (www.geog.plym.ac.uk)
The volumetric water content was calculated using the wetness and bulk density of the soil collected from the cultivated and non-cultivated land. The soil samples were taken from the area used in the ponded and tension permeameter experiments. This enabled us to analyze the amount and effects of pores in the profile as the ponded permeameter excluded pores greater than 2cm. Also a one-way ANOVA t-test was used to asses whether or not the two techniques used had significantly different means.
Figure 5. Bivariate Fit of Density By Peg
Figure 6. One-way analysis of volumetric water content
Figure 7. Ponded & Tension method for volumetric water content at site
The Bivariate Fit method was used on both ponded and tension data to demonstrate the changes if any over the survey area in volumetric water content. In the ponded method all pores less than 2cm were available whereas only the pores with a radius of 0.075cm or less were filling and transporting water in the tension experiment. Pegs ranged from 1 – 8 and went through non-cultivated soil (1-4) and cultivated soil (5-8). An analysis of variance calculated the possibility of the means for each peg being significantly different.
Bulk density and wetness were used to calculate the volumetric water content at the site for cultivated and non–cultivated soil. The mean bulk density for non-cultivated land was 1.37g cm-3 and for cultivated 1.24g cm-3. However bulk density values alone only give some basic information about soil structure and should be converted into volumetric water content to give hydraulic properties of the soil. After the tension permeameter experiment the average volumetric water content for the soil over the entire site was found to be 38.8%, whereas the ponded permeameter experiment gave an average volumetric water content of 44.9%. The probability of these means being significantly different (assuming equal variances) were dismissed following a p-value of greater than 0.05 (p=0.36). The values found for the volumetric water content were not unusually large or small but conformed to the properties of the soil profile.
After both experiments the soil would be close to saturation and would demonstrate a large volumetric water content, in cultivated soils the level was found to be higher due to a larger level of porosity. For cultivated soils the mean volumetric content (average of both experiments) was 45.1% and 32.2% for pasture.
Figure 7 demonstrates a linear increase in the volumetric water content from non-cultivated pegs to cultivated pegs. This depicts an increase in the amount of pores available in cultivated land and how cultivation reduces the bulk density and the amount of large pores present in the soil. In the ponded permeameter experiment the larger pores had been excluded and demonstrated that the pore size in the soil at the site were similar across the peg range, this is shown with the volumetric water content being similar for cultivated and non-cultivated land.
Comparisons between the results of the two experiments demonstrated that cultivation had decreased the structure of the topsoil leading to higher volumetric water content after saturation (see figure 7). The agricultural/environmental implications of this result demonstrates that the cultivation of land will increase the volumetric water content of the soil leading to higher efficiency in cropping but in turn will reduce the bulk density of the soil, which has long term structure impacts. Also Compacted soil layers or plough pans might occur with continuous cultivational practices and lead to reduced crop yields.
A problem that might occur with this direct method is that there is little variation of values over the site area. Core sampling for broad acre assessment might lead to the overlooking of localized high bulk densities caused by tillage and traffic and will effect root penetration or water infiltration. Also bulk density data is somewhat limited in soil structure analysis as it is not enough to describe moisture characteristics such as water retention. Another problem is the possible source of error in the core sampling method used at the study site, which can be difficult to quantify. This is soil disturbance by compression during insertion of the sampler. (Smith and Mullins)
· Grossman R.B, Reinsch T.G. 'SSSA Book Series:5 Methods of Soil Analysis' Ch2 pg201, Ed. Dane J.H, Clarke Topp G. Soil Science Society of America, Inc. Madison, Wisconsin, USA 2002
· Smith K.A, Mullins C.E. ‘Soil and Environment Analysis’ Marcel Dekker, Inc New York, 2001