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Density is defined as the amount of mass per volume, and is usually given in grams per cubic centimeter (g/cm3). By definition, water has a density of 1 g/cm3; most rocks and minerals have densities from 2-3 g/cm3. Air has effectively/essentially zero density.

The upper portion of most soils is a mixture of living and dead organic material mixed with minerals (crystalline solids, like salt or quartz) and small particles of rock. Soil also contains variable amounts of water and air residing in small holes and pockets, often referred to as pores. The upper portion of soils are often highly aerated and fluffy, containing a high percentage of pore space (a high porosity), and therefore typically have very low bulk densities. Bulk density refers to the density of the soil as a whole, including both the particulate matter and the pore space.

Soil density was measured before and after logging of a mature conifer forest in the foothills of the Cascade Mountains in Washington State (Purser 1988). The sample area was relatively small, confined to one (eventual) clearcut, hence the soil conditions and types were relatively uniform across the sampling area. Only the upper portion of the soil was sampled. Soils sampled before logging show a wide range of values (one order of magnitude), with very low values dominating, as is apparent in the bar chart. A low bulk density implies high porosity and/or high organic content. The distribution of measurements is skew symmetric, not Gaussian/normal. A question for the students; why is that?

Measurements of soil density before and after logging might be able to assess the impact of logging on soil structure and porosity. One could postulate that logging operations might compact soil, due to the weight of heavy machinery, dragging ("skidding") of heavy logs, and/or compaction by direct rainfall impact on the clearcut ground. Compacted soil may hinder natural or artificial reseeding of the clearcut (compacted soils might lack proper aeration), and may increase runoff from the clearcut by lessening infiltration (rainwater can't penetrate compacted soils). Clearcutting is often blamed for increasing the severity of floods, as the trees no longer modulate rainfall, and the soil loses its absorptive capacity.

Students can evaluate these premises by looking at the before-and-after range of values, the means, and/or the shape of the bar graphs. Though the ranges of densities are similar before and after logging, the means are different, and some higher values of bulk density (>1 g/cm3) are recorded. Students can also determine whether the means are statistically different and at what confidence level.

Reference: Purser, M. D. (1988), The Impact of Clearcut Logging with High-Lead Yarding on Spatial Distribution and Variability of Infiltration Capacities on a Forest Hillslope; M. S. Thesis, University of Washington, 127 pp.

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 Source: The Impact of Clearcut Logging with High-Lead Yarding On a Forest Hillslope Pre-logging bulk densities (g/cm^3) Post-logging bulk densities (g/cm^3) 0.265 0.848 0.337 0.157 0.144 0.344 0.452 0.598 0.149 0.378 0.523 0.653 0.101 0.196 0.462 0.393 0.117 1.019 0.100 1.090 0.129 0.701 0.164 0.280 0.639 0.608 0.173 0.145 0.498 0.781 0.114 0.341 0.772 1.097 0.213 1.040 0.125 0.956 0.948 0.852 0.162 0.826 0.187 1.130 0.640 0.608 0.471 0.557 0.186 0.783 0.186 0.899 0.672 0.870 0.642 0.956 0.154 1.144 0.932 1.051 0.580 0.773 0.197 0.895 0.811 0.559 0.728 1.112 0.642 0.756 1.089 mean 0.371 0.733 st dev 0.266 0.294
 pre post Bin Frequency Frequency 0-0.2 16 3 0.2-0.4 3 5 0.4-0.6 6 3 0.6-0.8 5 10 0.8-1.0 3 8 1.0-1.2 0 9

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