Exchangeable Cations – Range Table

(with calculation to convert between mg/kg and cmol(+)/kg)

Use this table to determine how low or high your Exchangeable Cations levels are in your soil.

In order to use this table, use the calculation instructions below to convert your results from mg/kg to cmol(+)/kg.

 Exchangeable Cation Unit Very low Low Moderate High Very High Calcium (Ca) cmol(+)/kg <2 2-5 5-10 10-20 >20 Magnesium (Mg) cmol(+)/kg <0.3 0.3-1 1-3 3-8 >8 Potassium (K) cmol(+)/kg <1 1-2 2-5 5-8 >8 Sodium (Na) cmol(+)/kg <0.1 0.3-3 0.3-0.7 0.7-2 >2
How to Convert your Exchangeable Cations from mg/kg to cmol
• Calcium (cmol(+)/kg ) = Exchangeable Ca (mg/kg) / 200 *
• In other words, divide your Exchangeable Ca result by 200 for your units to be in cmol(+)/kg.
• Magnesium (cmol(+)/kg) = Exchangeable Mg (mg/kg) / 120
• Potassium (cmol(+)/kg) = Exchangeable K (mg/kg) / 390
• Sodium (cmol(+)/kg) = Exchangeable Na (mg/kg) / 230

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.

EXAMPLE

Calculation of centimole:

= atomic weight / number of charges / 100

* Centimole for calcium:

= 40.08 g / 2 / 100 = 0.2g = 200 mg