Soil Bulk Density

Introduction

Soil bulk density, r_b, is defined as the ratio of dry soil mass to bulk soil volume (including pore spaces). The SI unit for density is megagrams per cubic meter (Mg m^-3), which is numerically equivalent to grams per cubic centimeter.

Just as soil is a combination of soil minerals, organic matter, and air- or water-filled pores, so soil bulk density is a weighted average of the densities of its components:

r_b = f_ar_a + f_pr_p + f_or_o + ...

where

f is the volume fraction of a component

a = air (pores)

p = soil mineral particles

o = organic matter

Typical densities:

r_a = 12 x 10^-4 Mg m^-3

r_p = 2.60 - 2.75 Mg m^-3 (2.65 is a commonly used average value for "particle density")

r_o = 0.9 - 1.4 Mg m^-3

r_b = 0.9 - 2.0 Mg m^-3

Bulk density is primarily a function of relative pore space and OM content.

% pore space = (1 - r_b/r_p) * 100

Bulk density is an important soil parameter in its own right, influencing water infiltration and plant root health. It is also an essential piece of information for converting laboratory chemical data, which is commonly expressed on a per unit mass basis (e.g., mg Ca per kg soil), to volume (or area x depth) units (kg Ca per ha in the 0-30 cm layer).

When calculating bulk density, it is necessary to know whether the mass of soil (the numerator of the fraction) should include the total soil or just the <2 mm (or “fine earth”) fraction; the same is true for soil volume in the denominator. Total mass / total volume would be most useful for construction or engineering applications, where the total mass of material is important. Since all chemical analyses are done on the fine fraction, fine mass / total volume would allow you to convert directly to an area basis for watershed or landscape studies. Fine mass / fine volume, the “true soil bulk density,” would tell you what plant roots are experiencing. To determine fine soil volume, coarse fraction volume (CFV) must be subtracted from total volume, which requires assuming a rock particle density (usually 2.65 Mg m^-3):

CFV (cm³) = Coarse fraction mass (g) / Particle density (g/cm³)

To convert soil concentration of an element (for example, 100 mg of exchangeable calcium per kilogram of soil) to soil content (e.g., grams per square meter in the 0-30 cm depth), multiply by the bulk density (with appropriate conversion factors). For example, assuming a bulk density of 1.1 Mg / m³,

(Don’t confuse “Mg” as “megagrams” with “Mg” as the symbol for magnesium!)

Forest floor is often considered as a whole, rather than as a specific depth. Thus, its bulk density may be expressed in Mass / Area units rather than Mass / Volume (i.e., kg m^-2).

Methods

Core method

A cylindrical metal or plastic coring tool of known volume is driven into the soil to a desired depth. The intact core is removed, dried in an oven at 105°C, and weighed.

Advantages - relatively simple equipment

- undisturbed core

Disadvantages - small sampling area of core

- stones

- compression of soil inside corer

A larger diameter core diminishes all these disadvantages, except when rocks are large but closely spaced.

Excavation method

Level soil surface; dig a hole to the desired depth. Line hole with plastic, then fill it with measured volume of water. Excavated soil is dried and weighed.

Advantages - can be done in stony or gravelly soils

Disadvantages - excavated soil is no longer undisturbed

- water gets heavy to lug around

If done properly, this method will usually give you more accurate numbers than core methods.

Clod method

Coat a clod (a large soil aggregate) with paraffin or other water-repellent substance. Weigh it in air, then in water to determine its volume; or measure the volume of water displaced by the clod in a graduated cylinder.

Radiation methods

Measure radiation transmitted or scattered by soil. Requires a value of soil water content, and a calibration curve derived from soils with a range of known bulk densities.

Equations

Within a geographical region and for soils of a similar genesis, the close relationship between soil organic matter content and bulk density may allow use of regression equations to calculate r_b. For example, Federer 1983) derived an equation {ln BD = -2.314 – 1.0788 ln OM – 0.1132 (ln OM)²} for northern New Hampshire till soils. The equation was tested with good results on soils from sites in Maine, southern NH, and Connecticut.

References

Brady, N.C.. 1990. The Nature and Properties of Soils, 10th ed. New York: Macmillan Publishing Company. pp. 103-110.

Blake, G.R., and K.H. Hartge. 1986. Methods of Soil Analysis, Part 1. A. Klute, ed. Madison: American Society of Agronomy. pp. 374-390.

Federer, C.A. 1983. Nitrogen mineralization and nitrification: depth variation in four New England forest soils. Soil Sci. Soc. Am. J. 47:1008-1014.

Soil Bulk Density Data Sheet

Site _____________________________ Name ___________________ Date ______________

Core

Core volume, Soil Tare Moist Rock Dry

Sample type cu. cm can ID wt. Soil wt. Soil