Nathan Turk
September 15, 2005
FOR 332
Lab Report: Observing the Forested Landscape
Depth to Mottling on an Environmental Gradient
Abstract
Depth to mottling in soil is a possible means of evaluating the effect of slope position on
drainage class. I measured the depth at which mottling (or interspersion of oxidized soil
particles) occurred along a sloped transect in Heiberg Experimental Forest in Tully, NY. I
expected to find mottling deepest beneath the soil surface at the top of the slope, and
most shallow at the bottom. The experiment corroborated this, although rock obstructions
at two of the three slope spots prevented precise depth measurements, underscoring how
the approach can be revised. Drainage class not only correlates with slope position but
also has potential ties with growth rate and species composition. It also poses prospects
for future management endeavors.
Introduction
Soils differ in their ability to drain water. Sometimes those differences can be seen within
a single transect where a high slope position, for example, would experience better
drainage than a lower slope position. Ultimately the steeper the slope, the more
pronounced the differences in the soils’ depth until water or evidence of it is reached
(Cedergren, 1967). One major indicating factor of water presence is mottling, or the
interspersion of oxidized, reddish particles with the soil matrix. This oxidation is caused
by oxygen present in the water, and can be seen to a greater extend beyond the point of
mottling, where soils are often reddish. Where this boundary occurs determines where
ground water sits in relation to the soil profile. The deeper that mottling occurs below the
surface, the greater the soil’s trend toward effective water drainage.
In this study, I used soil depth as indicated by yardstick readings to determine the quality
of drainage along the gradient. At the highest slope position I expected to find mottling
after digging down 16” to 24”, which is the range of moderately well drained soil. At the
lowest position (point seven) I expected to find mottling closest to the surface,
specifically within the first 8” (which would indicate poorly drained soil) or 4” (very
poorly drained).
Methods
Three holes were dug along a sloped transect near Tully, NY at the Heiberg Experimental
Forest. The holes marked point one, at the top of the slope, point seven at the bottom and
point four in between. At each hole I measured the depth at which either mottling was
reached or physical obstructions such as huge rocks prevented me from excavating
further. This depth was then compared to that of common New York state soil groups
according to depth ranges outlined by Ward and Elliot (1995). Depth was related to soil
type as follows:
Lordstown: More than 24” to mottling; well drained
Mardin: Mottling at 16” to 24”; moderately well drained
Volusia: Mottling at 8” to 16”; somewhat poorly drained
Chippewa: Mottling at less than 8” (poorly drained) or at less than 4” (very poorly
drained)
In addition to measuring the depth, I also applied and described the color of soil from
point one using a Munsell color chart to be aware of other soil classification systems
including moist color, texture, structure and consistence.
Results
Soil obstructions prevented me from finding the depth to mottling in holes and points one
and four. But I was able to penetrate 18” and 17” of soil, respectively, so the minimum
threshold of the moderately well drained class (16”) was met at points one and four. Point
seven’s soil boasted mottles within 4” (fig. 1) of the surface in its clay, which means the
lower slope position’s soil qualified as very poorly drained.
Discussions
Though impenetrable, the rocks encountered during digging seemed similarly flat and
shale-like as those in the displaced dirt, just bigger. With more time the pits could have
been expanded, and the large rocks circumvented. The likelihood of the rock barriers
being bedrock is very low, since mottling would probably have occurred much closer to
the surface in the soil profile; water can no further than bedrock. Digging more pits at the
first two points might have proven this.
Standing water was evident where roots were overturned at point seven. This supports the
discovery by Lopez and Ryan that Tsuga canadensis (eastern hemlock) and Acer rubrum
(red maple) were abundant in the lower slope positions, since both species have no
trouble thriving in moist valleys and floodplains (Hardin, Leopold and White, 2001). The
lower slope positions were also correlated with increasing softwood tree diameters as
reported by Team Gray Van, which may imply that less-drained soil classes correlate in
this stand with greater softwood tree vigor. (For hardwoods, the relationship is inverse.)
Species’ proven success at particular slope positions could help future operations aiming
to accommodate optimum growing conditions for timber harvest, seed production,
recreation and other uses.
The methods of this experiment reaped enough soil data to suggest better drainage occurs
at higher slope positions, and thus supported the initial hypothesis. But for more
definitive drainage classifications, more pits would have had to be dug until the exact
mottling horizons in the upper positions were found. Additionally, measurements at
points two, three, five and six may have outlined a better picture of where one soil type
and drainage class gives way to another.
References
Cedergren, Harry R. 1967. Seepage, Drainage and Flow Nets. John Wiley & Sons, Inc.
New York, pp. 340-342.
Ward Andy D, William J Elliot. 1995. Environmental Hydrology. Lewis Publishers. Boca
Raton: 439-445.
Hardin James W, Donal J. Leopold, Fred M. White. 2001. Textbook of Dendrology.
McGraw-Hill Higher Education. Boston: 183 & 453.