A new approach to calculate the particle density of soils considering properties of the soil organic matter and the mineral matrix

Abstract

The particle density of soil (ρ_S) represents one of the soil's basic physical properties and it depends on the composition of both the mineral and the organic soil components. It therefore varies for different soils, e.g. within the group of mineral soils, and ranges from 2.4–2.9 g cm⁻³. Hence, awareness of this variability is important for properties estimated by a calculation involving particle density. Because ρ_S depends on both the soil's solid mineral particles and soil organic matter composition, we derived a function based on the mixture ratio of these two soil components. This approach represents a further development of earlier investigations dealing with the influence of organic carbon (C_org) on ρ_S. To parameterise this function, two data sets were used: (1) data from soils with C_org contents between 0% and 54.88% and corresponding values of ρ_S between 1.49 and 2.72 g cm⁻³; and (2) data from soils of 17 German long-term experiments contrasting in soil texture and in soil mineral inventory. Data set 1 was used to quantify the influence of soil organic matter on ρ_S, and data set 2 was used to calculate the influence of mineral matrix on ρ_S. The soil organic matter has two major influences on ρ_S: (1) via a mass effect (expressed as a mixture ratio between organic and mineral soil components); and (2) via a quality effect (expressed as calculated changes in particle density of organic soil components). Here, we calculated that with increasing content of soil organic matter (0–100%), the particle density of organic soil components rose from about 1.10 to 1.50 g cm⁻³, and present possible reasons for this phenomenon. Additionally, we demonstrate that the mineral matrix of the soil affects ρ_S especially via variations in the mineral inventory, but conclude that differences in particle size distribution of soils were to a lesser extent suitable for describing the influence of the mineral matrix on ρ_S. Overall, using our approach should generate more realistic values of ρ_S, and consequently of all calculated parameters which are sensitive to ρ_S.

Introduction

The particle density of soil (ρ_S) represents one of the soil's basic physical properties and is defined as mass per volume unit of solid soil components, i.e. excluding voids and water. The standard value of ρ_S that is frequently given to represent all mineral soils is 2.65 g cm⁻³ (Skopp, 2000), and is identical to the mean density of quartz. However, typical values for mineral soils range from 2.4–2.9 g cm⁻³. Therefore, awareness of this variability is important for properties estimated by a calculation involving particle density (Ball et al., 2000). This was especially so, before pedotransfer functions became a ‘white-hot’ topic for soil science and environmental research (McBratney et al., 2002). By using accurate particle density data (as calculated by our approach presented here), a soil's physical processes can be partially monitored. Moreover, because the specific heat of mineral and organic soil particles is very different, it is essential to know the mixture ratio, in order to describe the heat dynamics of soils accurately (Kuntze et al., 1994). Furthermore, the value of ρ_S enters into calculations of more useful soil properties like the pore volume, which can be calculated from the quotient of ρ_S and bulk density (Danielson and Sutherland, 1986, Riek et al., 1995); this can be used in combination with the pore size distribution to calculate transport processes in soil.

The particle density of soil depends on the composition of both the mineral and the organic soil components. In clay and heavy minerals, the particle density varies between 2.2 and 2.9 g cm⁻³ and 2.9 and 4.0 g cm⁻³, respectively (Schachtschabel et al., 1992). These values are directly affected by the mineral type, e.g. gypsum, biotite and hematite have values of 2.32, 2.80–3.20 and 4.80–5.30 g cm⁻³, respectively (Skopp, 2000). Hence, the particle density of mineral soil components of any given soil is an average for the distribution of the soil minerals present. For organic soil components, the particle density depends on the degree of decomposition, and ranges between 1.0 and 1.5 g cm⁻³ (Young and Spycher, 1979, Leuschner et al., 1981, Golchin et al., 1994, Golchin et al., 1995, Hassink, 1995, Ladd et al., 1995, Skopp, 2000, Ellies et al., 2003).

Here, our aims were (1) to calculate ρ_S based on the ratio between organic and mineral soil components, and (2) to quantify the influence of the composition of both the organic as well as the mineral soil components on ρ_S.