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Geochemistry of Soil Ecosystems
Problem Set 3
Winter 1999
- You collect a surface soil sample (0 - 15 cm) from your garden or field. After air
drying the soil you carefully weigh-out a 20 g sample for analysis. You heat the sample in
a muffle furnace at 550 C for 3 h combusting all organic forms of C to CO2.
Your sample now has a mass of 14.23 g.
Given all of the weight lost is the result of combustion, calculate the %OM and %C values
for the soil.
- Use a spreadsheet approach to graph the mathematical model for the binding of H+
by organic matter on p. 63 in Sposito. Size your graph so that it is identical in
dimensions to Fig 3.4. This graph serves as a first order model for the acid/base behavior
of soluble soil organic matter.
What is meant by acid/base buffering?
Highlight the buffering regions on your model graph.
On the same graph plot the buffer intensity (problem 9) for the two major regions of the
titration curve. See Sposito: 2.303[b1csK110-pH/(1
+ K110-pH)2 + 10pH-14 + 10-pH] and
2.303[b2csK210-pH/(1 + K210-pH)2
+ 10pH-14 + 10-pH].
- Work problem 10 in Spostio. Under equilibrium conditions predict the concentration
of PCE sorbed to the soil (mol kg-1) if the soil solution concentration is 0.1
mol m-3 .
CSS 445: Geochemistry of Soil Ecosystems
Instructor: John Baham ( John.Baham@orst.edu
)
Webmaster: Sara Griffith ( Sara.Griffith@orst.edu
)
Last Updated: February 18, 1999
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