Fen Management Handbook - Scottish Natural Heritage

4.4 Classifying water chemistry using solute and oxygen concentrations
In chemical terms, a ‘base’ is a substance whose molecule or ion can combine
with a proton. A greater concentration of base ions is associated with increasing
alkalinity. The most common bases are calcium (Ca 2+ ) and magnesium (Mg 2+ )
ions. Conversely, lower concentrations of base ions are associated with increasing
acidity.
4.4.1 The acidity of solutions (pH)
The measurement of pH is a commonly used method of assessing acidity and
alkalinity. A solution with a pH value of greater than 7.0 is considered alkaline, and
less than 7.0 acidic. A solution at pH 7.0 is considered neutral, i.e. neither alkaline
nor acidic. In practice, pH values of around 5.5. can be regarded as dividing acid
fens from more base-rich fens. Rainwater contains very few bases and is slightly
acidic, as it tends to absorb a small amount of carbon dioxide (CO ) from the
2
atmosphere, resulting in weak carbonic acid (H CO ). Groundwater tends to be
2 3
more alkaline, particularly when it has come into contact with calcareous rocks such
as limestone from which it has picked up base ions such as Ca2+ and Mg2+ and
- their associated bicarbonate ions (HCO ). 3
4.4.2 The electrical conductivity of solutions
Fen water and soil chemistry can be described in terms of electrical conductivity
(EC), which estimates the concentration of dissolved chemical ions. It is measured
using a probe that passes an electric current between two detectors, and is
expressed as Siemens/m (S m-1). For example, the EC of drinking water is 0.005
S/m, compared to seawater that is about 5.0 S/m. Conductivity provides a useful
guide to base enrichment but not macronutrient concentrations.
4.4.3 The reduction – oxidation (‘redox’) potential
Redox potential can provide an estimate of whether soils are aerobic or anaerobic.
Aerobic soils have redox potentials of about 0.6V and anaerobic soils have redox
potentials between 0.4 and -0.2V. Redox potential measurements are made using
redox electrodes (usually made of platinum) and are measured in volts (V), millivolts
(mV) or Eh (where 1 Eh = 1mV).
4.4.4 Ion exchange
Ion exchange is the chemical process by which mineral ions are either lost to soil
solution (i.e. are available for uptake) or held on the surfaces of the soil particles
(i.e. are unavailable for uptake). Peat soils generally have a greater ion exchange
capacity because of their high organic content.
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