Page 637 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

AQuantitativeSoluTion Soil Density and Porosity
Soil density is a mass per unit volume and can be measured either as bulk density or particle density, both of which are described below.
Calculation of bulk density is based on the naturally existing volume of soil (volume of the soil as it would be in the field) after it has been dried. This measure includes the pore spaces that may be filled with either air or water, and organic material. Bulk density is a mass per unit volume, calculated as the dry weight divided by volume. Typically bulk density for mineral soils ranges from 1.0 g∙cm–3 to 1.8 g∙cm–3, but can exist outside that range. Bulk density of terrestrial soils is used as an indicator of soil compaction.
Particle density is the density of the solid soil particle and excludes pore spaces (that may be filled by air or water). Typically particle density for mineral soil is approximately
–3 equalto2.65g∙cm .
Given the data for the two soil samples in Table AQS 18.1, calculate the porosity of each. To calculate the porosity of each sample, first calculate the bulk density.
For Sample A: DB = 88.8 g / 83.6 cm3 = 1.06 g∙cm–3 For Sample B: DB = 101.4 g / 63.6 cm3 = 1.59 g∙cm–3
Then calculate the porosity:
For Sample A: P = 100 - a 1.06 b * 100
Pores in the soil control the movement of water. Porosity, with units of percent, is the measure of the volume of soil that is air or water, and can be calculated using the bulk density of the soil.
1.59
2.65
P=100-aDB b*100 2.65
Sample B has a porosity of 40%.
where P is porosity and DB is bulk density.
A 83.6 115.7 88.8
moisture through the soil, but Sample B is more compacted than Sample A.
2.65
= 100 – (40) = 60
Sample A has a porosity of 60%. ForSampleB:P = 100 - a
b * 100
Each sample has adequate pore space for the movement of
= 100 – (60) = 40
Table aQS 18.1 Volume and Weights of Two Soil Samples
Sample
Volume (cm3)
Wet Weight (g)
Dry Weight (g)
B
63.6
121.2
101.4
concepts review
KEy lEArninG
■ Define soil and soil science, and list four components of soil.
Soil is a dynamic natural mixture of fine materials, in- cluding both mineral and organic matter. Soil science is the interdisciplinary study of soils involving physics, chemistry, biology, mineralogy, hydrology, taxonomy, cli- matology, and cartography. Pedology deals with the origin, classification, distribution, and description of soil. Edaph- ology specifically focuses on the study of soil as a medium for sustaining the growth of plants. Soil is composed of about 50% mineral and organic matter, and 50% water and air contained in pore spaces between the soil particles.
soil (p. 572)
soil science (p. 572)
1. Soils provide the foundation for animal and plant life and therefore are critical to Earth’s ecosystems. Why is this true?
2. What are the differences between soil science, pedol- ogy, and edaphology?
■ Describe the principal soil-formation factors, and describe the horizons of a typical soil profile.
Environmental factors that affect soil formation include parent materials, climate, vegetation, topography, and time. To evaluate soils, scientists use a soil profile, a ver- tical section of soil that extends from the surface to the deepest extent of plant roots or to the point where regolith or bedrock is encountered. Each discernible layer in a soil profile is a soil horizon. The horizons are designated O (contains humus, a complex mixture of decomposed and synthesized organic materials), A (rich in humus and clay, darker), E (zone of eluviation, the removal of fine particles and minerals by water), B (zone of illuviation, the deposi- tion of clays and minerals translocated from elsewhere), C (regolith, weathered bedrock), and R (bedrock). Soil horizons A, E, and B experience the most active soil pro- cesses and together are designated the solum.
soil profile (p. 573) soil horizon (p. 573) humus (p. 574) eluviation (p. 574) illuviation (p. 574) solum (p. 574)
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