What is Soil Porosity?
This is the amount of pore spaces within a soil and is primarily influenced by the structure of the soil.
From a turf maintenance perspective, the timing and frequency of operations can influence the soil porosity, as can the usage that takes place on a turf area.
The impact on the soil porosity will be most significant when these activities occur under moist to wet soil conditions, encouraging soil particles to be squashed together and this will result in an increase in compaction within the soil.
Soil porosity is usually divided into Air-Filled Porosity and Water-Filled Porosity.
How is Soil Porosity Calculated?
A fairly straightforward formula is used as follows:
Soil Porosity = ( 1 - (Bulk Density ÷ Particle Density) ) x 100. This will indicate the percentage of the soil that contains pores.
Using some previously calculated figures, if a sandy soil, with organic matter content of 20%, has a bulk density of 1.4 g cm-3, and an average particle density of 2.222 g cm-3 then the soil porosity is as follows:
( 1 - (1.4 ÷ 2.222) ) x 100 =
(1 - (0.63) ) x 100 = 37%.
The remaining 63% is the solid material of the soil itself.
What are Macropores?
These are pores within the soil that are > 50-75µm (0.05 - 0.075mm) in diameter, depending upon which author is consulted.
They are non-capillary pores from which water has drained naturally through the action of gravity.
Aeration Porosity is the term used to describe this situation and typically represents when the soil is at field capacity.
(Russell states that "water only drains out of soil under gravity if it can move through pores larger than 60 to 30 µm". Rowell states pores with a diameter of 60 - 30 µm (5 - 10 kPa) as field capacity, Adams & Gibbs give field capacity as 4 kPa or pores with a diameter of >75µm, whilst the USGA golf green specification also uses 4 kPa as the value for determining porosity, but does not actually define this as field capacity).
By contrast, both Adams & Gibbs and Rowell give permanent wilting point as 1500 kPa, or pores with a diameter of 0.2µm, so at least there is some consensus on this value.
Adequately aerated soils should have a minimum of 10% aeration (air-filled) porosity, however, the USGA provide more detailed recommendations for golf greens.
| Total porosity | 35%-55% | The total amount of pores within the soil |
| Air Filled Porosity | 15%-30% | The amount of air filled pores within the soil |
| Capillary Porosity | 15%-25% | The amount of water filled pores within the soil, where the water is available for use by the plant. |
[Capillary water is water held by surface tension within micropores of ~50µm - 0.2µm in diameter].
For football pitches, research has shown that the maintenance of air-filled porosity of at least 10% in the top 20mm of a pitch is unrealistic, even for a high sand specification pitch.
A figure of 10-15% air-filled porosity at 20-70mm depth can, however, be achieved more readily especially for a high sand specification pitch.
References
Russell, E.W. (1973), 'Soil Conditions and Plant Growth', 10th Edition, 479
Gibbs, R.J. & Baker, S.W. (1989),'Making the most of Natural Turf pitches. Case Studies: III. Soil Physical Properties', The Sports Council
Anon, (1993), 'Specifications for a Method of Putting Green Construction', USGA
Adams, W.A. & Gibbs, R.J. (1994), 'Natural Turf for Sport and Amenity Science and Practice', 17-18
Rowell, D.L. (1994),'Soil Science: Methods & Applications'79-84