SoilMate NutriFact CAM-02
MEASURING EXCHANGEABLE CATIONS
Units
In the laboratory, the concentration of a cation exchanged in the extraction process is measured in mg/kg,
as with other nutrients. However because measurement of exchangeable cations relates to charge in the
soil, not concentration, the measurement is converted to units of charge. To relate the concentration
measured to the likely activity in the soil, the units are modified to indicate the concentration relative to
ionic charge of the nutrient i.e. centimoles per kilogram of soil.
Reporting of measurement of an exchangeable cation is usually in centimoles per kg of dry soil (cmol/kg).
Note: 1 cmol(+)/kg is the same as 1 meq/100g. A centimole is 1/100th of the “molar weight” of an element,
which is the atomic weight of that element divided by the number of charges on the cation. For example
the atomic weight of calcium (Ca2+) is 40.08g. The number of charges on the cation is two so the equivalent
weight is calculated by dividing 40.08 by 2, which equals 20g (rounded off). Dividing this by 100 equals
0.02g or 20mg which is the centimole for calcium.
Calculation of centimole:
= atomic weight / number of charges / 100
Centimole for calcium:
= 40.08 g / 2 / 100 = 0.2g = 200 mg
So a measurement of 4 cmole/kg for calcium means the amount of exchanged calcium was 800mg/kg.
To convert cmol/kg to mg/kg for the individual cations, cmol/kg is multiplied by the following numbers:
Calcium (Ca)
Magnesium (Mg)
Potassium (K)
Sodium (Na)
Aluminium (Al)
200
120
390
230
90
Laboratory techniques
Exchangeable cations are assessed by replacing or exchanging cations with an ion of similar electric charge.
The commonly used extractants for assessing exchangeable Ca, Mg, Na and K are ammonium chloride,
ammonium acetate and barium chloride. Different extractants tend to give different results. Figure 1
shows ammonium (NH4+) being used as an extractant.
Figure 1. Ammonium exchanging with other cations in solution.
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The presence of soluble salts of the cations (e.g. carbonates, sodium chloride, calcium sulfate or gypsum)
can lead to errors in the estimation of exchangeable cations. Some analytical procedures use an alcoholic
pre-wash to remove interfering components from the soil. Alternatively, soluble salts may be measured
directly and subtracted from an estimate of the soluble plus exchangeable bases.
Acidic cations (Al3+ and H+) are usually extracted with potassium chloride, and sometimes with barium
chloride/ammonium chloride.
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