Most chemical soil tests give the total concentrations of all species of a particular ion. The effective concentration is the measured concentration only when there is no ion-pairing, hydrolysis or disassociation. Ultimately, the activities of major ions rather than their concentrations per se drive most chemical processes in the soil solution. As an example, ≈28.3% of the soluble Ca and SO4 in a 10 mM CaSO4 solution are paired as neutral CaSO40, whereas there are 10 mM Ca2+ and 20 mM Cl– in a 10 mM CaCl2 solution (Adams 1974).
Soil solution ionic strength must be known in order to calculate ionic concentrations, ion-pair concentrations and ionic activities. Knowledge of soil ionic strength also assists in the development and choice of methods for assessing the cation and anion exchange capacity of soils, particularly those that are highly weathered. Ionic strength also influences the types of Al species in soil solution and their proportions, which influences whether or not the root environment may be Al toxic.
Gillman and Bell (1978) measured/calculated the ionic strength (I) and other soil solution characteristics of a range of highly weathered soils from North Queensland. They found that I at 0.1 bar soil moisture (across an ionic strength range of zero to 14 mM) was strongly correlated with EC1:5. That relationship provides the basis for this useful soil test.
Soil ionic strength at 0.1 bar (I0.1) = [0.0446*EC1:5 – 0.000173]
where I0.1 has units of mM, and EC1:5 has units of dS/m @ 25°C.
Report estimated soil ionic strength as mM.