Bulk Density and Total Pore Space

(This is an example of a lab report. Use it as a guide for writing your own reports.)

Joe Agronomo
Soils 101
Lab Section 1
Feb. 21, 1996

Introduction

Soil is an arrangement of solids and voids. The voids, called pore spaces , are important for gas exchange between the soil and atmosphere, root growth, water movement, and water storage. A medium-textured soil that is good for plant growth will have a pore space content of about 0.50. (half solids, half pore space). The total pore space is the space between sand, silt, and clay particles (micropore space), plus the space between soil aggregates (macropore space; Brady & Weil, 1996).

Bulk density is the mass of a given volume of dry soil in its natural condition (the mass of the solids and the pore space). It is determined by removing a block of soil from site, allowing no compaction or crumbling. This is often done by hammering a can or metal ring into the soil and digging the ring or can out when full of soil. The soil is then dried in an oven and weighed. The volume is determined by measuring the volume of the container used to extract the soil. The bulk density is expressed in units grams of oven-dry soil per cubic centimeter (Blake & Hartge, 1986):

Bulk Density = oven-dry soil (g) / soil (cm³)

Bulk densities of soils range from about 1.0 g/cm³ (fine-textured soils) to 1.4 or 1.7 g/cm³ (coarse-textured soils). Tillage that loosens soil can temporarily decrease bulk density, while compaction will increase it (Brady & Weil, 1996).

Particle density is the density of just the solid part of the soil, it does not include any pore space. Particle density varies according to the mineral content of the soil particles. It does not usually vary a lot in most soils. In most soils the particle density is about 2.65 g/cm³; the density of quartz is 2.65 g/cm³ and quartz is the dominant mineral in most soils (Brady & Weil, 1996).

Porosity is a value that expresses the relative amount of pore space in the soil. It is not measured directly but is calculated from the bulk density and particle density (Brady & Weil, 1996):

Porosity = 1 - (bulk density / particle density)

To determine the bulk density and total pore space of two soil samples, we hammered cans into the wall of a soil pit (Hagerstown silt loam). Samples were collected from the Ap horizon and a Bt horizon. A block of wood was placed over the cans so that the hammer did not smash them. The cans, full of soil, were dug out of the horizons and the excess soil trimmed off. The samples were dried in an oven at 105 ^oC for two days and weighed. The volumes of the cans were determined by measuring the height and radius:

volume = ¼r²h

Bulk density and porosity were determined using the formulas above. Particle density was assumed to be 2.65 g/cm³. The textural class of each horizon was determined by feel.

Results

We found both soils to have relatively light bulk densities and large porosities, but the Bt horizon had greater porosity than the Ap. The Ap horizon was determined to be a silt loam, the Bt was a clay.

Table 1. Textural class, bulk density, and porosity of two Hagerstown soil horizons.

Table 2. Data used for bulk and particle density determinations.

Sample calculations (Ap horizon data):

Discussion

Both soils had bulk densities and porosities in the range I would have expected from the discussions in the lab manual and textbook. The Ap horizon is a medium-textured soil and is considered a good topsoil for plant growth, so a porosity around 0.5 is in agreement with that. The Bt horizon is a fine-textured horizon (lots of clay) and the bulk density is in the predicted range.

I was a bit surprised that the subsoil (Bt) was lighter than the topsoil. Plowing should make it lighter than normal. I realize now, though, that it hadn't been plowed for a couple of years, so the aeration effect is probably mostly gone by now. I've read that Hagerstown soils are well-drained even though the subsoil has a lot of clay (Braker, 1981). This is due to the good structure that the subsoil has. This structure (aggregation) is probably the cause of the high porosity we found.

I noticed that some of the other groups in our class had slightly different results than we did. The differences seemed to be in a range of +/- 0.1 g/cm³, although most groups found, as did we, that the Bt horizon had a greater porosity than the Ap. The reasons for the variation in results is probably due to our technique in collecting the soil and perhaps weighing it after drying. Rounding differences in the calculations may also account for some of the differences.

References

Blake, G.R., and K.H. Hartge. 1986. Bulk density. p. 363-375. In A. Klute (ed.)

Methods of soil analysis. Part 1. (2nd ed.). Agron. Monogr. 9. ASA and SSSA, Madison, WI.

Brady, N.C. and R.R. Weil. 1996. The nature and properties of soils (11th ed.). Prentice Hall,

New York.

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