About Soil Density and Logging
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,