Everything about Characterisation Of Pore Space In Soil totally explained
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 3 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.
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)
Bulk Density
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.
Pore space relations
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.
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, K
sat, 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.
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.
Pore Types
A pore isn't simply a void in the solid structure of soil. There are 3 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.
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
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 don't have capillary forces too great so that the water doesn't 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
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 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
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