Pore volume (soil)
Pore volume is a term of Soil Science and refers to the entire air-filled or water voids volume of the soil .
General
The soil can be divided into the volume of the solid soil substance and the pore volume .
The pore volume of a soil is divided into primary pores, which depend on the grain size distribution of the mineral substance as well as the type and composition of the organic components (humus), and the secondary pores, which depend on the soil structure and thus on the chemical properties of the minerals, on the influence of the Plants (roots form root tubes) and animals (form passages, dig, mix, ...) depend.
If the soil is completely saturated with water (pore volume = water volume) and the water is withdrawn from it gradually with known negative pressures ( soil water tension ), then the wide (rapidly draining) and narrow (slowly draining) coarse pores, the central pores and the fine pores can be sequentially drain. The cumulative curve of the pore sizes, plotted as a function of the soil water tension, gives the soil water tension curve. Among other things, it describes the availability of soil water for plants.
Primary pores and secondary pores
Primary pores are grain-related pores between the individual grains. Therefore, the size of the predominant individual grains in terms of quantity also determines certain pore size ranges.
- In sandy soils, these are the coarse pores.
- In the loess soils dominated by coarse silt, it is the central pores.
- In clay soils, it is the fine pores and the transition to the middle pores.
As a rule of thumb, the continuous, channel-like pores correspond to about a third of the dominant grain size.
Because small grains are more difficult to move relative to one another than large ones, the total pore volume also increases as the size of the individual grains decreases; Small individual grains are naturally not as tightly packed as large individual grains.
Secondary pores are structurally influenced coarse pores (also called macropores) with equivalent diameters of over 50 µm between the aggregates (i.e. not between the individual grains) and biopores.
Aggregates are formed ( soil structure ):
- purely physically by repeatedly becoming wet and dry, i.e. by the change in soil moisture , which is accompanied by more pronounced swelling and shrinking with increasing clay content.
- chemically through enveloping iron oxides or cementing carbonates
- biologically through sticking mucilage from the intestines of earth eaters such as earthworms or through the obstructive effect of (fine) roots and fungal hyphae
- as well as a root or earthworm tunnel; in the extreme also corridors of mice and moles or the like.
The distribution of the secondary pores is often uneven ( anisotropic ); their orientation predominantly vertical. The secondary pore system is an important aspect with regard to the air and water balance in particularly fine-grained soils.
Classification of the pore volume according to the suction tension
The following classification of the pore volume shows the relationship between pore size and suction tension ( soil water tension ).
For soils that are not influenced by groundwater or waterlogging, the “narrow coarse pores” can also be added to the air capacity, i.e. subtracted from the (usable) field capacity.
| category | Expressions | |||
|---|---|---|---|---|
| Pore areas | wide pores | narrow coarse pores | Central pores | Fine pores |
| Suction tension in hPa | under 60 | 60 to 300 | 300 to 15,000 | over 15000 |
| pF value | below 1.8 | 1.8 to 2.5 | 2.5 to 4.2 | over 4.2 |
| Equivalent diameter in µm | over 50 | 50 to 10 | 10 to 0.2 | below 0.2 |
| Function of the pores | fast moving | slowly moving | plant- available |
not available for plants |
| Seepage water | Adhesive water | |||
| Characteristic values Abbreviations |
Air capacity LK |
usable field capacitance nFK |
Dead water DEAD |
|
| Field capacity FK |
||||
| Total pore volume GPV |
||||
Notes: 1 hPa corresponds to 1 cm water column.
This is the height up to which water rises in a capillary of the respective equivalent diameter and then hangs under the meniscus.
pF value = log10 (suction tension in hPa)
Pore size ranges for main groups of soil types
For the pore size ranges there are specific parameters for the soil physical parameters air capacity, usable field capacity , field capacity and dead water for each soil type in the Soil Science Mapping Instructions or in DIN 4220. If the soil type or at least the main group of soil types is determined in the terrain with a finger test, then the table “Range of pore size ranges for main groups of soil types” gives a first impression of the pore size distribution on site.
The column of the water conductivity for saturated soils shows an effect of the different pore size distributions; for the water conductivity, fluctuations of one fifth to three times are normal!
| Soil types main groups | large pores in% | narrow coarse pores in% | Middle pores in% | Fine pores in% | Water conductivity in cm / d |
|---|---|---|---|---|---|
| Sandy soils | 10 to 20 | 8 to 20 | 10 to 15 | 2 to 8 | 300 |
| Silt soils | 0 to 10 | 5 to 15 | 10 to 20 | 10 to 20 | 30th |
| Clay soils | 5 to 10 | 0 to 10 | 5 to 15 | 5 to 20 | 30th |
| Clay soils | 0 to 5 | 0 to 5 | 10 to 15 | 25 to 40 | 3 |
| Peat soils | 7 to 30 | 0 to 10 | 30 to 55 | 15 to 25 | 200 |
Key figures of the pore volume
Usually the volume of the pores Vp is related to the total volume of the soil Vges. Then the key figure is:
- the pore volume PV = Vp / Vges × 100 (as in the table above) with the specification of the unit as%,% by volume or% (v / v)
- the porosity ε = Vp / Vges with 1 ≥ ε ≥ 0 and the specification of the unit of measurement as "m³ / m³" or "cm³ / cm³"
The unit of measurement for the PV is often also given as "mm / dm". This reduction from volume percent to length percent becomes clear if, based on the specification "dm³ / dm³", a cube of 1 dm³ is distributed over an area of 1 m²: you get a 1 mm high body. According to this, there are so many pores per dm of depth in the soil that they - in a nutshell - make up 1 mm "height". If you stick to the reference area of 1 m² and consider that 1 dm³ = 1 liter, then the information on the pore volume can not only be given as mm (pore proportion) per dm depth (in the soil), but also for a given depth as mm / m² or as liters / m².
The volume of the pores Vp can also be related to the volume of the solid Vf. Then the key figure is:
- the pore number e = Vp / Vf with e also> 1
The number of pores enables changes in the pore volume to be compared with each other, even if there are significant changes in the total volume of the soil (for example in the case of compaction), because here, in contrast to the key figure pore volume or porosity, the reference value remains the same.
Porosity and number of pores can be converted into one another:
- ε = e / (e + 1)
- e = ε / (1 - ε)
See also
literature
- Ad hoc working group Soil: Soil-Scientific Mapping Instructions. Published by the Federal Institute for Geosciences and Raw Materials in cooperation with the State Geological Services. 5th improved and enlarged edition. Schweizerbart, Stuttgart 2005, ISBN 3-510-95920-5 .
- DIN 4220: 2008-11 - Soil science location assessment - Marking, classification and derivation of soil parameters (normative and nominal scaling).
- Winfried Blum : Soil Science in Brief. 6th completely revised edition. Borntraeger, Berlin a. a. 2007, ISBN 978-3-443-03117-6 ( Hirt's index books ).
- Fritz Scheffer , Paul Schachtschabel: Textbook of soil science. 15th edition, reprint. Spectrum Akademischer Verlag, Heidelberg 2008, ISBN 978-3-8274-1324-6 ( spectrum textbook ).