Ugrás a tartalomhoz

Soil science

Prof. Blaskó Lajos (2008)

Debreceni Egyetem a TÁMOP 4.1.2 pályázat keretein belül

Porosity

Porosity

Soil porosity, or pore space, is the volume percentage of the total soil that is not occupied by solid particles. Pore space is commonly expressed as a percentage:

% pore space = 100 - [bulk density ÷ particle density x 100]

Figure 33 Soil porosity (http://www3.gov.ab.ca/env/water/GWSW/quantity/learn/evaluation/EV3_pumptest.html)

Bulk density is the dry mass of soil solids per unit volume of soils, and particle density is the density of soil solids, which is assumed to be constant at 2.65 g/cm3. Bulk densities of mineral soils are usually in the range of 1.1 to 1.7 g/cm3. A soil with a bulk density of about 1.32 g/cm3 will generally possess the ideal soil condition of 50% solids and 50% pore space. Bulk density varies depending on factors such as texture, aggregation, organic matter, compaction/consolidation, soil management practices, and soil horizon.

Figure 34 Sketch of soil sample to show solid particle and void space distribution. Particles shown in white, voids in black. http://www.geology.iupui.edu/research/SoilsLab/procedures/bulk/Index.htm

Under field conditions, pore space is filled with a variable mix of water and air. If soil particles are packed closely together, as in graded surface soils or compact subsoils, total porosity is low and bulk density is high. If soil particles are arranged in porous aggregates, as is often the case in medium-textured soils high in organic matter, the pore space per unit volume will be high and the bulk density will be correspondingly low.

The size of the individual pore spaces, rather than their combined volume, will have the most effect on air and water movement in soil. Pores smaller than about 0.05 mm (or finer than sand) in diameter are typically called micropores and those larger than 0.05 mm are called macropores.

Macropores allow the ready movement of air, roots, and percolating water. In contrast, micropores in moist soils are typically filled with water, and this does not permit much air movement into or out of the soil. Internal water movement is also very slow in micropores. Thus, the movement of air and water through a coarse-textured sandy soil can be surprisingly rapid despite its low total porosity because of the dominance of macropores.

Fine-textured clay soils, especially those without a stable granular structure, may have reduced movement of air and water even though they have a large volume of total pore space. In these fine-textured soils, micropores are dominant. Since these small pores often stay full of water, aeration, especially in the subsoil, can be inadequate for root development and microbial activity. The loosening and granulation of fine-textured soils promotes aeration by increasing the number of macropores.