Experiments were undertaken to obtain the bulk density, particle density, and soil porosity of four soil samples. Bulk density, particle density, and soil porosity are all interrelated terms. Density is the relationship between the mass (m) and volume (V) of a substance. . Based on this relationship, an object or substance that has high mass in relation to its size (or volume) also has a high density (D). Soil bulk density (Db) is a measure of the mass of soil per unit volume (solids + pore space) and is usually reported on an oven-dry basis. The particles that make up a soil have a specific particle density (Dp), which is defined as the mass of solid particles in a unit volume. This can be expressed as, . The particle density of a soil is not affected by particle size or arrangement; rather it depends on the type of solid particles present in the soils. Unlike the measurement of particle density, the bulk density measurement accounts for the spaces between the soil particles (pore space) as well as the soil solids. Soils with a high proportion of pore space have lower mass per unit volume, and therefore have low bulk density. A bulk density greater than 1.6 g cm-3 may indicate soil compaction, which means these soils have a low proportion of pore space and, therefore, low porosity. The bulk density indirectly provides a measure of the soil porosity which is the ratio of the volume of soil pores to the total soil volume. Clayey soils generally have an abundance of very small pores that give them a higher total porosity compared to sands, which are dominated by larger, but fewer pores.
MATERIALS:
1. Core soil samples
2. Balance (0.1 g precision)
3. Flat-bladed knife
4. Veneer calliper
5. Disturbed soil samples
6. 100 ml measuring cylinder
7. De-aired water
METHOD:
The bulk density of the soil was calculated after following a series of steps. Firstly, excess soil from each of the disturbed soil sample was removed using a flat-bladed knife, ensuring that the bottom of the sample was flat and even with the edges of the ring. Excess material or roots penetrating from the sample was cut using a scissors. The dimensions of the core were then measured using a veneer calliper and the dimensions obtained were plugged into the formula Volume= π * radius 2* height to calculate the volume of the core. This value was then used to obtain the bulk density of the soil (Db = ). Afterwards, approximately 50g of soil was weighed out in a weigh dish for each soil sample and the soil weight was recorded to the nearest 0.1 g. e 50 cm of water was then poured into a 100 ml graduated cylinder and the previously weighed soil was added into the water in small increments. Air bubbles were eliminated by stirring with a rod and the final volume of the soil water suspension was read and the value was recorded. This value was then used to calculate the particle density of each soil sample (Dp = ). Using the measured densities the percentage soil porosity was calculated via the formula, 100-( Db / Dp * 100). Data was recorded and presented in a tabular format.
RESULTS:
SOIL SAMPLE
HEIGHT OF CORE (cm)
DIAMETER OF CORE (cm)
VOLUME OF CORE SAMPLER (cm3)
1
5.548
5.36
125.2
2
3.862
5.58
94.5
3
4.889
5.858
131.8
4
4.798
5.384
109.2
FIGURE 1: TABLE SHOWING THE HEIGHT, DIAMETER AND VOLUME OF THE CORE SAMPLER FOR EACH SOIL SAMPLE.
SOIL SAMPLE
WEIGHT
VOLUME (cm3)
1
319.6g
69
2
276.6g
69
3
273.2g
71
4
343.6g
70
TABLE SHOWING THE WEIGHT AND VOLUME OF THE FOUR SOIL SAMPLES.
SOIL SAMPLE
WEIGHT OF FIELD MOIST SOIL SAMPLE (g) WITH CORE
WEIGHT OF DRY SOIL SAMPLE (g) WITH CORE
WEIGHT OF CORE (g)
WEIGHT OF DRY SOIL SAMPLE (g) (WEIGHT OF DRY SOIL WITHOUT CORE WEIGHT)
1
356.7
319.5
188.5
131
2
277.4
276.6
105.1
171.5
3
390.1
273.2
145.8
127.4
4
245.6
343.6
104.5
239.1
FIGURE 1: TABLE SHOWING WEIGHT OF FIELD MOIST SOIL SAMPLE AND OVEN DRY SOIL SAMPLE.
SOIL SAMPLE
WEIGHT OF SOIL SAMPLE (g)
INITIAL VOLUME OF WATER (cm3)
FINAL VOLUME OF WATER (cm3)
VOLUME OF THE SOIL PARTCLES (cm3)
1
50.00
50
69
19
2
50.00
50
69
19
3
50.00
50
71
21
4
50.00
50
70
20
FIGURE 4: TABLE SHOWING WEIGHT OF SOIL SAMPLE, INITIAL VOLUME OF WATER, FINAL VOLUME OF WATER AND VOLUME OF SOIL PARTICLES.
SOIL SAMPLE
BULK DENSITY (g/cm-3)
PARTICLE DENSITY (g/cm-3)
SOIL POROSITY (%)
1
1.9
6.9
72.5
2
2.49
9.03
72.4
3
1.8
6.1
70.5
4
3.42
12
71.5
FIGURE 2: TABLE SHOWING THE BULK DENSITY, PARTICLE DENSITY AND SOIL POROSITY OF EACH SOIL SAMPLE.
To calculate how many grams of soil are in a hectare plow layer:
Depth = 15cm =0.15m
Hectare = 10,000m2
Volume of HPL = Area of Hectare * Depth of plow
= 10,000 * 0.15
= 1500m3
Db = 1.4g/cm3
=
1kg = 1000g
∴1400kg=1400 Ã-1,000
= 1,400,000g
Therefore, there are 1,400,000g of soil in a hectare plow layer.
DISCUSSION:
Soil porosity and bulk density are interrelated terms. Soil bulk density (Db) is a measure of the mass of soil per unit volume while soil porosity is the ratio of the volume of soil pores to the total soil volume. Bulk density (Db) is closely related to the soil porosity through the following relationship:. The bulk density indirectly provides a measure of the soil porosity (amount of pore space). The value obtained from the measurement of soil bulk density can be used to calculate pore space. For soils with the same particle density, the lower the bulk density, the higher the percent pore space or total porosity. Soil porosity values range from 0 to 1. Soils with a high bulk density have low total porosity because empty pores do not have any mass. When the bulk density is zero, porosity equals 1, meaning there are no particles. If the bulk density is equal to the particle density, then there are no pores and porosity is zero.
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Soil porosity can be affected by changes occurring in the soil. The porosity of the soil decreases in compacted subsoils and increases in well- aggregated, high organic matter surface soils. The values range from as low as 25% to more than 60% respectively. Soil management can also alter soil porosity values such that, cultivated soils tend to have a lower total pore space as a result of a decrease in organic matter content and a consequent lowering of granulation, in comparison to uncultivated soils.
Bulk density and porosity are also affected by changes in soil texture. Fine-textured soils including clay, clay loam and silt loams typically have lower bulk densities as compared to sandy soils, due to the presence of pores between and within the granules. As a result, these fine-textured soils containing a high proportion of pore spaces to solids tend to have low bulk densities. Sandy soils, on the other hand, though larger in particle size have larger yet fewer pore spaces, resulting in a higher bulk density. In terms of porosity, fine-textured soils are very porous. This is because of the large amount of micropores in fine-textures soils which allow water to be held. These micropores prevent the water from being drained away and therefore, result in high total porosity. In contrast, sandy soils have less total porosity due to the relative absence or lack of fine, within-ped pores. This allows water to be drained away easily through the macropores present in sandy soils. Additionally, bulk density and porosity are affected by changes in the size class of the soil particles. Bulk density is lower in sandy soils or well-sorted sand where the particles are generally of one size class whereas a mixture of different size particles (well-graded sand) will cause a high bulk density. In well-graded sand the smaller particles partially fill in the spaces between the larger particles causing a reduction in the pore spaces and a resultant increase in bulk density. Soil porosity, on the other hand, is decreased in well-graded sand as compared to well-sorted sand.
Another factor impacting bulk density and porosity is compaction. Soil compaction refers to the disruption and reduction of the large pores within the soil. It can be caused in a variety of ways such as, the practice of poor agricultural practices, use of machinery on soil, etc. Compaction increases the mass in a given volume, which increases the measured bulk density. Bulk density increases because both the micropores and macropores in a soil are significantly reduced after compaction has taken place. Therefore, with the removal of these empty pores, the mass of soil increases in a given volume. With regards to porosity, compaction reduces the total pore spaces and hence, the total porosity.
In conclusion, bulk density and porosity are two interrelated soil properties which are altered by various factors including soil texture, compaction and soil structure. Calculations into the bulk density and porosity were carried out. Some limitations presented were the fact that bulk density and porosity values would differ from place to place due to certain factors, for example, the level of compaction in an area. However, these figures can be useful in determining the crop yield of a soil.
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