Characterisation of pore space in soil

Soil is essential to most animals 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. A solid phase (≈ 20%) 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. In order to understand porosity better a series of equations have been used to express the quantitative interactions between the three phases of soil.

Macropores or fractures play a major role in infiltration rates in many soils as well as preferential flow patterns, hydraulic conductivity and evapotranspiration. Cracks are also very influential in gas exchange, influencing respiration within soils. Modeling cracks therefore helps understand how these processes work and what the effects of changes in soil cracking such as compaction, can have on these processes.

Bulk density

Main article: bulk density

G

$\rho = \frac{M_s}{t_V}$

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 2.65 g/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 1.6 g/cm³ but organic soil and some friable clay may have a bulk density well below 1 g/cm³

Core samples are taken by driving a metal core into the earth at the desired depth and soil horizon. The samples are then oven dried and weighed.

Bulk density = (mass of oven dry soil)/volume

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.

Porosity (f)

$f = \frac{V_f}{V_t}$ or $f = \frac{V_a+V_w}{V_s+V_a+V_w}$

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

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 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.

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 >50 nm.^[1]

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 50 nm–2 nm.^[1]

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 <2 nm.^[1]

Modelling methods

Basic crack modeling has been undertaken for many years by simple observations and measurements of crack size, distribution, continuity and depth. These observations have either been surface observation or done on profiles in pits. Hand tracing and measurement of crack patterns on paper was one method used prior to advances in modern technology. Another field method was with the use of string and a semicircle of wire.^[2] The semi circle was moved along alternating sides of a string line. The cracks within the semicircle were measured for width, length and depth using a ruler. The crack distribution was calculated using the principle of Buffon's needle.

Disc permeameter

This method relies on the fact that crack sizes have a range of different water potentials. At zero water potential at the soil surface an estimate of saturated hydraulic conductivity is produced, with all pores filled with water. As the potential is decreased progressively larger cracks drain. By measuring at the hydraulic conductivity at a range of negative potentials, the pore size distribution can be determined. While this is not a physical model of the cracks, it does give an indication to the sizes of pores within the soil.

Horgan and Young model

Horgan and Young (2000) produced a computer model to create a two-dimensional prediction of surface crack formation. It used the fact that once cracks come within a certain distance of one another they tend to be attracted to each other. Cracks also tend to turn within a particular range of angles and at some stage a surface aggregate gets to a size that no more cracking will occur. These are often characteristic of a soil and can therefore be measured in the field and used in the model. However it was not able to predict the points at which cracking starts and although random in the formation of crack pattern, in many ways, cracking of soil is often not random, but follows lines of weaknesses.^[3]

Araldite-impregnation imaging

A large core sample is collected. This is then impregnated with araldite and a fluorescent resin. The core is then cut back using a grinding implement, very gradually (~1 mm per time), and at every interval the surface of the core sample is digitally imaged. The images are then loaded into a computer where they can be analysed. Depth, continuity, surface area and a number of other measurements can then be made on the cracks within the soil.

Electrical resistivity imaging

Using the infinite resistivity of air, the air spaces within a soil can be mapped. A specially designed resistivity meter had improved the meter-soil contact and therefore the area of the reading.^[4] This technology can be used to produce images that can be analysed for a range of cracking properties.

See also

• Bulk density
• Porosity
• Soil physics
• Soil science
• Particle density
• Soil respiration

References

1. ^ ^{a b c} J. Rouquerol et al. (1994). "Recommendations for the characterization of porous solids". Pure & Appl. Chem 66: 1739–1758. doi:10.1351/pac199466081739. http://media.iupac.org/publications/pac/1994/pdf/6608x1739.pdf. 
2. ^ Ringrose-Voase, A.J. and Sanidad. W.B. (1996) A method for measuring the development of surface cracks in soils: application to crack development after lowland rice. Geoderma 71 245-261 doi:10.1016/0016-7061(96)00008-0
3. ^ Horgan, G.W. and Young. I.M. (2000) An empirical stochastic model for the geometry of two-dimensional crack growth in soil. Geoderma 96 263-276 doi:10.1016/S0016-7061(00)00015-X
4. ^ Samouëlian, A, Cousin. I, Richard. G, Tabbagh. A and Bruand. A. (2003) Electrical resistivity imaging for detecting soil cracking at the centimetric scale. Soil Science Society of America. 67 1319-1326

Further reading

• 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.)