Characterisation of pore space in soil
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Soil is essential to most life on the earth. It is a relatively thin crust where an even smaller portion contains much of the biological activity. Soil consists of three different phases (figure 1). A solid phase (≈ 50%) that contains mainly minerals of varying sizes as well as organic compounds. The rest is pore space. This space contains the liquid and gas phases of the soil components. These pores are essential to the dynamics of the soil profile. It allows the movement and transmission of water, gas and nutrients to flow and exchange within the system.
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[edit] Measurement of pore space
In order to understand porosity better a series of equations have been used to express the quantitative interactions between the three phases of soil. (Figure 2)
[edit] Bulk Density
The bulk density of soil depends greatly on the mineral make up of soil and the degree of compaction. The density of quartz is around 2.65g/cm³ but the bulk density of a soil may be less than half that density.
Most soils have a bulk density between 1.0 and 1.6g/cm³ but organic soil and some friable clay may have a bulk density well below 1g/cm³
Cores are sampled from the field site by driving a metal core into the earth (figure 3) at the desired depth and horizon. The samples are then oven dried and weighed.
Bulk density = mass of oven dry soil*volume-1
The bulk density of soil is inversely related to the porosity of the same soil. The more pore space in a soil the lower the value for bulk density.
[edit] Porosity (f)
or 
Porosity is a measure of the total pore space in the soil. This is measured as a volume or percent. The amount of porosity in a soil depends on the minerals that make up the soil and the amount of sorting that occurs within the soil structure. For example a sandy soil will have larger porosity than silty sand, because the silt will fill in the gaps between the sand particles.
[edit] Pore space relations
[edit] Capillary
In soil physics, capillary action describes the attraction of water molecules to soil particles and the energy associated with the attraction. Capillary action is responsible for moving water from wet areas of the soil to dry areas. The amount of movement and speed of the movement depends on the matric potential (Ψm) the soil pores.
Capillary rise depends on the radius of the capillary. The same is true for soil pores. In Figure (4) it shows the height of water with varying capillary tubes. The soil next to it demonstrates the same response. Small capillary/pore high rise in water. Large capillary/pore large rise in water.
[edit] Hydraulic conductivity
Hydraulic conductivity (K) is a property of soil that describes the ease with which water can move through pore spaces. It depends on the permeability of the material (pores, compaction) and on the degree of saturation. Saturated hydraulic conductivity, Ksat, describes water movement through saturated media. Where, Hydraulic conductivity has the capability to be measured at any state. It can be estimated by numerous kinds of equipment. To calculate Hydraulic conductivity, Darcy's law is used. The manipulation of the law depends on the Soil saturation and instrument used.
[edit] Infiltration
Infiltration is the process by which water on the ground surface enters the soil. The Water enters the soil through the pores by the forces of gravity and capillary action. The largest cracks and pores offer a great reservoir for the initial flush of water. This allows a rapid infiltration. The smaller pores take longer to fill and rely on capillary forces as well as gravity. The smaller pores have a slower infiltration as the soil becomes more saturated.
[edit] Pore Types
A pore is not simply a void in the solid structure of soil. There are three main categories for pore sizes that all have different characteristics and contribute different attributes to soils depending on the number and frequency of each type.
[edit] Macropore
The pores that are too large to have any significant capillary force. These pores are full of air at field capacity. Macropores can be caused by cracking, division of peds and aggregates, as well as plant roots, and zoological exploration. Size >2 mm
[edit] Mesopore
The pores filled with water at field capacity. Also known as storage pores because of the ability to store water useful to plants. They do not have capillary forces too great so that the water does not become limiting to the plants. These mesopores are ideally always full or contain liquid to have successful plant growth. The properties of mesopores are highly studied by soil scientists to help with agriculture and irrigation. Size 200 µm–0.3 µm
[edit] Micropore
The pores that are filled with water at permanent wilting point. These pores are too small for a plant to use without great difficulty. The water associated is usually adsorbed onto the surfaces of clay molecules. The water held in micropores is important to the activity of microbes creating moist anaerobic conditions. The water can also cause either the oxidation or reduction of molecules in the crystalline structure of the soil minerals. Size <0.2 µm
[edit] See also
[edit] References
- Foth, H.D.; (1990) Fundamentals of soil science. (Wiley: New York)
- Harpstead, M.I.; (2001) Soil science simplified. (Iowa State University Press: Ames)
- Hillel, D.; (2004) Introduction to environmental soil physics. (Sydney : Elsevier/Academic Press: Amsterdam ;)
- Kohnke, H.; (1995) Soil science simplified. (Waveland Press: Prospect Heights, Illinois)
- Leeper GW (1993) Soil science : an introduction. (Melbourne University Press: Carlton, Victoria.)
