Environmental Engineering Reference
In-Depth Information
7.9 Additional Soil Characteristics
7.9.1 Soil pH
Soil pH refers to the pH of water in equilibrium with soil. pH is defined as the
negative logarithm of the hydrogen ion activity in solution: pH
Log (H + ).
Activity and concentration (moles/L) are nearly equivalent in dilute solutions, so
that concentration is often used instead of activity. Soil pH impacts many of the
chemical reactions in soil, determining the rate and or direction of a reaction. pH
tends to move toward the neutral value as a soil becomes anaerobic (Mitsch and
Gosselink 2000 ). Soil pH is routinely measured on a soil slurry (1:1 soil to water
ratio) in the lab with commercially available pH meters and combination
electrodes. The method described below was presented by Thomas ( 1996 ).
Ten g of air-dry soil is weighed out in a 50 ml beaker. After adding 10 ml of
deionized water, the slurry is mixed well. Let stand for 10 min. After gently
swirling the suspension the electrode is inserted. The sample may or may not be
stirred during the readings. If stirred, the electrode is inserted into the suspension. If
not stirred, the electrode is inserted either into the sedimented soil or into the
supernatant above the soil. Readings taken in the supernatant are generally slightly
higher than readings taken in the stirred suspension (Thomas 1996 ). All
3 approaches are acceptable; the important thing is that a consistent approach is
used. The value should be recorded as pH w. . The electrodes should be rinsed with
distilled water between readings.
Alternative methods for measuring soil pH include pH papers and test kits. Both
are colorimetric methods; contact of the soil sample with the pH paper or, in the
case of test kits, reagents causes a color change. That color is compared to a
reference chart that equates pH values to a specific color. The primary limitations
to these methods are a lack of accuracy and a limited pH range. For example, the
Hellige-Truog soil pH test kit has long been used for routine soil investigations.
It has a pH range of 4.0-8.5 and is accurate to within 0.5 pH units. Relatively
inexpensive, rugged pH meters are now available for field work and are more
appropriate for biogeochemical assessment of wetland soils.
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7.9.2 Soil Oxygen Demand
Biochemical oxygen demand (BOD) is a measure of the quantity of O 2 used by
microorganisms in the oxidation of organic matter. Eutrophication is the process
where water bodies receive excess nutrients that stimulate excessive plant growth.
Traditionally, BOD has been used to assess eutrophication in surface waters but a
similar approach can be used to assess nutrient loading in wetlands. For wetlands
that do not exhibit continuous inundation, soil O 2 demand (SOD) can provide an
indication of nutrient loading. Soil oxygen demand can be easily measured in the
lab using a soil incubation procedure and an O 2 electrode. A weighed wet soil
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