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Practical Use of Soil Analysis Results

Table 3.4.6 Sodium Hazard Classifications Based on Sodium Adsorption Ratio and Electrical Conductivity

Classification

Sodium Adsorption Ratio

(SAR)

Electrical Conductivity

(dS/m) 1

Soil pH Soil Physical Condition

Sodic > 13 < 4.0 > 8.5 Poor Saline-Sodic > 13 > 4.0 < 8.5 Normal High pH < 13 < 4.0 > 7.8 Varies Saline < 13 > 4.0 < 8.5 Normal

1 dS/m = mS/cm

Source: Kryzanowski et al. 1988

Cation Exchange Capacity (CEC)

Ion exchange in soils is one of the most important processes influencing crop nutrition. CEC is an estimate of the capacity of soil to hold (or adsorb) positively charged (cation) nutrients. The major soil cations include: calcium (Ca 2+ ), magnesium (Mg 2+ ), potassium (K + ), sodium (Na + ), hydrogen (H + ) and aluminum (Al 3+ ).

The unit of measurement used to commonly express CEC is centimoles of positive charge per kilogram of soil (cmol/kg) and is equivalent to the units formerly used to express CEC; milliequivalents per 100 grams (meq/100g).

How Much is a Mole?

A mole is a quantity used in chemistry to describe 6 x 10 23 atoms of a particular element. An element’s atomic weight, found in a periodic table of the elements, is the equivalent mass, in grams, of one mole of that substance. For instance, the atomic weight for sodium is 22.989770 grams per mole.

One mole of positive charge refers to the equivalent positive charge on 6 x 10 23 monovalent (+1 charge) cations.

Basing CEC on centimoles (0.01 moles) of positive charge rather than mass (as the older milliequivalent measure did) makes more sense since the mass and charge of the various exchangeable cations in a soil sample changes, while the number of negatively charged exchange sites do not. Cation exchange capacity in cmol/kg remains the same regardless of which ions occupy the exchange sites in a soil sample.