What is Particle Density?
Particle density can be defined as the dry, solid mass of soil particles per unit volume.
More simply, it is how much a certain volume of soil weighs when it is squashed together, without the pore spaces within the soil.
The standard measurement is grams per cubic centimetre, or g cm-3.
The actual density of soil particles vary according to the type of mineral.
For practical purposes an average figure of 2.65 g cm-3 is often used as this is the actual figure for quartz, which is the most significant component of sands and is also a mid-range figure for the more common clay particles as well.
As a comparison water has a density of 1 g cm-3 .
What use is this in Maintaining Turf?
Being aware of particle density allows for an accurate determination of soil porosity. This will indicate how much 'space' there is for roots to grow into, air to circulate within and water to both accumulate in and drain through.
For soils with low (< 3%) organic matter ( these are termed mineral soils) an average value of 2.6 g cm-3 can be used in soil porosity calculations.
For organic soils (those with > 15% organic matter) a figure of 1.4 g cm-3 is sometimes used for the organic matter content, although a figure of 1.3 g cm-3 has also been used for sportsturf calculations, so an average value of 1.35 g cm-3 can be used in practice.
The average particle density for an organic soil can be arrived at as follows:
As an example, 100g of oven dry soil is found to contain 20g (i.e. 20%) of organic matter and 80g of mineral particles.
As Volume = Mass ÷ Density, the following calculation is carried out:
Organic matter = 20g ÷ 1.35 g cm-3 = 14.815 cm-3
Mineral particles = 80g ÷ 2.65 g cm-3 = 30.189 cm-3
Total volume = 45.004 cm-3 .
Average particle density = 100 g ÷ 45.004 cm-3 = 2.222 g cm-3
This figure can now be used within a soil porosity calculation.
References
Gibbs, R.J. & Baker, S.W. (1989),'Making the most of Natural Turf pitches. Case Studies: III. Soil Physical Properties', The Sports Council, 5
Rowell, D.L. (1994),'Soil Science: Methods & Applications', 60-78