What is Bulk Density?
Bulk density is defined as the dry mass of soil per unit volume.
More simply, it is how much a certain volume of soil weighs, including the pore spaces within the soil.
The standard measurement is grams per cubic centimetre, or g cm-3.
How does this relate to turf areas?
For soils with a high bulk density there will be a higher amount of mineral matter and less spaces (or pores) present for air, water and roots to grow into.
This results in
- more compacted soils,
- a lower drainage rate,
- increased surface layer rooting, and
- lower quality turf surfaces.
What is a good or poor bulk density for a soil?
This depends on a number of factors, such as
- soil texture - Sandy soils are more open than clay soils and are less prone to compaction.
- soil structure - A good crumb structure will have good soil porosity and lower bulk density. Too low a bulk density value can result in an inadequately firmed soil giving poor foot grip for players.
- soil water content - Higher soil water levels make it easier to compress a soil: This reduces the soil porosity and increases the soil bulk density
- organic matter content - This can help aggregate soil particles, producing a good crumb structure.
- root diameter of the plant species - Plants with a fine root diameter can penetrate into smaller soil pores than plant with a large root diameter.
The point at which root growth is effectively stopped is called the 'Critical Bulk Density' and is influenced by the above factors.
What are typical bulk density values for turf areas?
A good starting point to consider the sort of values that may be present within turf areas is provided by the USGA and their recommended bulk density for the topsoil mixture for golf greens. The value should be within the range 1.20 - 1.60 g cm-3, although the ideal range is given as 1.25 - 1.45 g cm-3.
The following table shows some typical values that have been recorded as part of some research projects:
| Facility | Bulk Density g cm-3 | Depth measurements were taken at | Comments |
| Golf Greens (Over 100 greens were tested) |
0.86 - 1.70 | 10 - 90mm | Lower densities were due higher amounts of organic matter and increasing uses of sandy top-dressings/construction materials. |
| 1.06 - 1.86 | 100 - 180mm | ||
| 1.0 - 1.2 | 10 - 90mm | A large frequency of greens fell within these ranges for the given depths. | |
| 1.4 - 1.6 | 100 - 180mm | ||
| 1.44 (0-5 years) | 10 - 90mm | Typical bulk densities also decreased with the age of greens. | |
| 1.27 (5 - 10 years) | |||
| 1.15 (>10 years) | |||
| 1.60 (0-5 years) | 100 - 180mm | ||
| 1.54 (5 - 10 years) | |||
| 1.40 (>10 years) | |||
| Cricket Pitches (Over 300 samples from pitches used for first-class cricket) |
1.19 - 1.68 | 0 - 40mm | The variation in organic matter content had a significant effect on reducing bulk density values. The increasing organic matter content also reduced the pace of wickets. |
| 1.29 - 1.67 | 40 - 80mm | ||
| A Bulk Density of 1.4 - 1.6 might be a suitable value to aim for within a cricket pitch. This value is also based on an organic matter content of 6-8%. | |||
| Football Pitch | 1.26 | 10- 90mm | Only 1 pitch tested for each facility and also based on a slit tine aeration frequency of once every 4 weeks. |
| Rugby Pitch | 1.14 | 10- 90mm | |
This table illustrates the wide range of values that are actually found in practice within turf facilities.
References
Anon, (1993), 'Specifications for a Method of Putting Green Construction', USGA
Baker, S.W., (1994), 'The effect and frequency of slit tine aeration on the quality of Soccer and Rugby pitches', Journal of the Sports Turf Research Institute (Vol. 70), 44-54
Hind, P.D. et al, (1995), 'A Survey of Golf Greens in Great Britain. I. Soil Properties', Journal of the Sports Turf Research Institute (Vol. 71), 9-22
Watts, S. & Halliwell L. (1996), 'Essential Environmental Science', 275-277
Anon, (1997), 'Cricket Pitch Maintenance. Guideline Specification for Local Authorities'. ECB. 72
Baker, S.W. et al, (1998), 'Soil characteristics of first-class cricket pitches and their influence in playing performance', Journal of Turfgrass Science (Vol. 74), 63-79