The cation-anion balance of a given system is calculated by comparing the total charge of the positive-charged ions (cations) with the total charge of the negative-charged ions (anions).
Calculating the cation-anion balance
In order to find the amount of charges, we have to use a unit that integrates both the concentration (and mass) of the ion and its charge. This unit is called the equivalent (we also use the milliequivalent (meq), which is 1/1000 of an equivalent).
The equivalent is calculated simply by multiplying the number of moles of the ion by its charge. For example, the molecular weight of calcium is 40 grams/mole and it carries a positive charge of +2 (Ca+2). Therefore: 40 grams per liter of calcium = 1 mole x 2 = 2 equivalents.
As another example, the molecular weight of nitrate (NO3-) is 62 grams/mole and it carries a negative charge of (-1), hence 62 grams per liter of NO3- = 1 mole = 1 equivalent.
Cation-anion balance is calculated by comparing the number of equivalents of the cations with the number of equivalents the anions.
Is your irrigation water balanced?
When ionic compounds—minerals, salts and fertilizers—dissolve in water, they are dissociated into ions. According to the principle of electroneutrality, the total charge of an aqueous solution must be zero. Therefore, the number of positive charges must be equal to the number of negative charges. This implies that the irrigation water is always balanced.
So, if water is always balanced, why do we check the cation-anion balance for? The purpose of checking the cation-anion balance in a water analysis is to validate the water test results. If the analysis is accurate, then the sum of milliequivalents of cations and anions should be nearly equal. However, an error of more than 5% in the cation-anion balance might imply that the analysis is not accurate. Also, if the laboratory did not test for one of the major cations or anions, then a correct balance cannot be calculated.
Cation-anion balance in nutrient solutions
All nutrient solutions are always balanced in respect to the cation-anion balance. For example, a typical analysis of calcium nitrate is:
Converting to milliequivalents results in:
1.03 meq NO3
0.08 meq NH4
0.95 meq Ca
Calculate and compare the cations and anions:
Cations (NH4+, Ca+2): 0.08+0.95 = 1.03
Anions (NO3-): 1.03
As we can see, it is balanced.
This same principle applies to all mineral fertilizers. Therefore, addition of mineral fertilizers to the irrigation water always results in a balanced nutrient solution.
Also note that there is a difference between a balanced nutrient solution and a cation-anion balanced solution. The first refers to the ratios, proportions and concentrations of the substances in the water.
For example, we might require specific ratios between ammonium and nitrate in the solution, and we might also require minimum concentrations of certain substances and maximum concentrations of others. Therefore, a nutrient solution that is considered to be balanced for a certain crop might not be balanced for another crop; however, it will be always balanced with respect to cation-anion balance.
Is your soil balanced?
Soil is composed of two phases that are relevant to this discussion: the liquid phase and the soil phase. The liquid phase is the soil solution, and being an aqueous solution, the explanations above are valid for this phase (i.e. cations and ions are balanced). The solid phase, on the other hand, is composed of the soil minerals.
As in the case of the nutrient solution, a balanced soil does not refer to the cation-anion balance, but to the ratios between the substances in the soil or their quantity, in each of its phases. In regards to cation-anion balance, most soil minerals have a negative charge on their surfaces.
So, in order to neutralize this charge, cations are adsorbed to these surfaces. These cations are called exchangeable cations as they are in equilibrium with the soil solution. As such, a soil system is also always naturally balanced in the cation-anion sense.
Also, there are different types of balances and different approaches to determine them. For example, when the balance refers to the ratios between the exchangeable cations (K+, Ca+2, Mg+2, Na+), then it is called base-cation saturation ratio.
So, is your system balanced? The answer to this question depends on the approach you want to take for interpreting your test results and on the crop you are growing. However, as you can probably guess by now that if we referring to the cation-anion balance, the answer to this question would be yes, always.