(Source SkavenHaug 1972)
Solid matter
The solids in soil materials range from organic substances in pure plant material to mineral matter in pure sands, clays or silts. At the same time the specific gravity of the solids, D_{s}, varies from D_{o} in pure plant material to D_{m} in mineral matter. For the majority of soil materials, containing both organic and mineral matter in the solids, the numerical value of D_{s} is an expression of the ratio organic matter/mineral matter.
The specific gravity D_{o} for pure organic matter is not constant. It varies with habitat, species present and degree of decomposition. The principal constituents, cellulose and lignin, have specific gravities of 1.52 and 1.46 t/m^{3} respectively. Literature on the subject gives a specific gravity of 1.53 for fresh spruce and pine while values for other materials lie between 1.47 and 1.52. As a practical estimate
D_{o} = 1.50 t/m^{3}Also the specific gravity D_{m} for pure mineral matter is not constant. It can vary from 2.3 t/m^{3} for gypsum to 5.2 t/m^{3} for hematite for example. In mineral soils a large number of minerals are present, and the mean specific gravity varies between much narrower limits. A collection of data from many soil mechanics laboratories shows a variation between 2.65 and 2.85, and generally speaking the lower value is found in coarse and uniform sand, with values rising the more finely divided the material becomes. For Norwegian sand and clay sediments the specific gravity lies between the following limits which for practical purposes we also choose:
D_{m} = 2.7 t/m^{3} ± 2% for sandThe dry specific gravity D_{d} is the weight of solids per unit of volume, t/m^{3} For groups of materials with approximately constant specific gravity for their solids, D_{d} is a serviceable expression for both weight and volume ratios. This measure is used in international soil mechanics as a characteristic of the quantity of mineral matter. The Norwegian Bog Association has for many years used the corresponding measure g/dm^{3} in dry peat in its investigations of bogs and D_{d} can be regarded as a suitable measure of solidity compaction in peat and bark material.
D_{m} = 2.8 t/m^{3} ± 3% for clay
For the large group of soil materials with varying specific gravity of solids D_{s}, D_{d} is not suitable as a basis for comparison.
Water
The amount of water that the soil material contains, or in certain circumstances can contain, depends on the physical properties of the material. The water content is therefore used as a basis of comparison of soil parameters and as an expression of their quality. The water content may be expressed as the ratios:
weight of water/weight of dry matter (w)All three ratios are in use, and this has sometimes caused confusion. As explained above the weight expressions are not always reliable as a basis for comparison.
weight of water/total weight (w_{tot})
volume of water/total volume (w_{v})
The weight ratio w has been adopted in international soil mechanics and is in general use in technology. For groups of materials with approximately the same D_{s}, w is a serviceable basis of comparison. For materials with variable D_{s}, w is not a good parameter for reference. This can be illustrated by an extreme example. A cubic metre of saturated Norwegian clay contains 0.5 m^{3} of water, and has w = 0.36 = 36 percent. Peat (agricultural peat in bales) with the same water content, 0.5 m^{3}, has w = 5.0 = 500 percent.
The weight ratio w_{tot} been used for a long time in peat terminology, and for peat with approximately the same weight of dry matter it gives a serviceable basis for comparison. One advantage is that w_{tot }is always less than 100 percent.
The volume ratio w_{v} is determined by weighing a known volume, before and after drying. Volume determinations cause extra work, but they make it possible to determine both w_{v} and D_{d}. When D_{s }is known, the weight and volume relationships can be calculated in the 3phase system of water, solids and air.
Some technical calculations need quantities of water and thus of w_{v}. Artificial drying of materials and the, determination of thermal parameters is an example. Since w_{v} is also a good basis for comparison, independent of the kind of material, its use should be widely advocated.
Air
Except for agricultural literature the air content of soil is rarely used. This is probably because of its insignificant weight, so that it must be stated as a volume. The air content of soil is often directly indicative of certain properties, such as low specific gravity, low thermal conductivity, and a large capacity for absorbing water.
Formulae
Weight and volume ratios can be derived from the unit volume (Fig. 36). Here is a survey of formulae that are suitable for practical use:
Symbols 

w 
ratio of weight of water/dry matter 
w_{tot} 
ratio of weight of water/total weight 
w_{v} 
ratio of volume of water/total volume 
D 
bulk density of wet material 
D_{d} 
specific weight of dry material (dry density) 
D_{s} 
specific gravity of solids 
s_{v} 
ratio of volume solids/total volume 
n 
porosity 
s_{r} 
degree of saturation 
D_{o} 
specific gravity of organic matter 
D_{m} 
specific gravity of mineral matter 
o_{v} 
ratio of volume of organic matter/total volume 
m_{v} 
ratio of volume of mineral matter/total volume 
l_{v} 
ratio of volume of air/total volume 
o 
ratio of weight of organic matter/dry matter 
m 
ratio of weight of mineral matter/dry matter 
a 
ratio of weight of ash/dry matter 
w_{v} = (D x w) ÷ (w + 1) 
(1) 
D_{d} = D ÷ (w + 1) 
(2) 
w_{v} = D_{d} x w 
(3) 
D_{d} = D  w_{v} 
(4) 
w_{v} = D x w_{tot} 
(5) 
D_{d} = D (1  w_{tot}) 
(6) 
w_{v} = (D_{d} x w_{tot}) ÷ (1 
w_{tot}) 
(7) 
s_{v} = D_{d} ÷ D_{s} 
(8) 
l_{v} = (1  w_{v})  s_{v} 
(9) 
s_{r} = w_{v} ÷ n = (w_{v}
x D_{s}) ÷ (D_{s}  D_{d}) 
(10) 
Then s_{v} = 1  w_{v}
If the volumes are measured, D_{s} can be calculated. We shall come back to this later.
The relation between w_{tot} and w_{v}, equation (7), is shown in Figure 36 for a number of organic materials with D_{s} = 1.55 t/m^{3} and known values for D_{d}. The highest two curves are related to slightly decomposed sphagnum peat in bogs. The curve for which D_{d} = 0.10 t/m^{3} corresponds to the same peat in agricultural bales for protection against frost under railway tracks. The bottom curve, with D_{d} = 0.25 t/m^{3} relates to bales of bark under roads and railways. For bark matting in situ with measured w_{tot} = 0.72, we may read off, in round figures, w_{v} = 0.65, l_{v} = 0.19 and s_{v} = 0.16.
Fig. 36. Relationship of weight and volume ratios for a range of organic materials with known specific gravities
Fig. 37. Cubic unit of a soil material with four phases: organic substance, mineral substance, water and air
Proportions of organic and mineral substances
The dry material may contain both organic and mineral matter, and D_{s} is then the mean specific gravity. It remains to determine the quantitative relations between organic and mineral substances.
By considering Figure 37 and setting up weight and volume equations we get:
o_{v} = D_{d}(D_{m}  D_{s})
÷ D_{s}(D_{m}  D_{o}) 
(11) 


o_{v} + m_{v} = s_{v} 

m_{v} = D_{d}(D_{s}  D_{o})
÷ D_{s}(D_{m}  D_{o}) 
(12) 
o = D_{o}(D_{m}  D_{s}) ÷
D_{s}(D_{m}  D_{o}) 
(13) 


o + v = 1 

m = D_{m}(D_{s}  D_{o}) ÷
D_{s}(D_{m}  D_{o}) 
(14) 
The Specific Gravity of Solid Matter
There are several methods of determining D_{s}.
a. 
The pycnometer method can in principle be used for all
soil materials, but is timeconsuming, especially when it comes to removing the
last vestiges of air in organic matter, and is not suitable for routine
investigations. 


b. 
For cohesive soils saturated with water, e.g. silt, clay and
mud, commensurable volumes can easily be prepared and D_{s} can be
calculated. The measurement of volume can also be carried out by weighing in air
and when submerged in water. This method is suitable for routine investigations,
but is limited to the above mentioned saturated materials. 


c. 
The weight ratio can be found by chemical means, and then
D_{s} calculated from formula (13). A distinction is made between direct
and indirect methods. The direct methods consist of removing the organic matter
and weighing what is left. The best method is by ignition, which will be
described later. Indirect methods are based on the assumption that a particular
element is contained in the organic substance in a constant proportion, so that
the organic substance can be calculated for this element by means of a
conversion factor. These methods, like the direct ones, are not fully accurate,
but must be regarded as the most reliable for soil materials with only moderate
contents of organic matter. 