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).
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 the "Equivalent".
Different ions may carry different charges.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).
40 grams of Calcium = 1 mole X 2 = 2 equivalents.
The molecular weight of Nitrate (NO3-) is 62 grams/mole and it carries a negative charge of (-1), hence 62 grams of NO3- = 1 mole = 1 equivalent.
A Milliequivalent (meq) is 1/1000 of an equivalent.
Cation-anion balance is calculated by comparing the number of equivalents of the cations with the number of equivalents of anions.
When ionic compounds dissolve in water, they are dissociated into ions. Ionic compounds are minerals, salts and fertilizers (all minerals fertilizers are salts).
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?
Why when calculating the cation-anion balance we sometimes find imbalances?
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.
An error of more than 5% in the cation-anion balance might imply that the analysis is not accurate.
However, if the laboratory did not test for one of the major cations or anions, then a correct balance cannot be calculated.
Any nutrient solution is always balanced, with respect to the cation-anion balance.
We should note that all mineral fertilizers, being salts, are also balanced.
For example, a typical analysis of Calcium Nitrate is 14.4% N-NO3-, 1.1% N-NH4+, and 19% Ca+2. Converting to milliequivalents results in 1.03 meq NO3, 0.08 meq NH4 and 0.95 meq Ca.
Making the balance:
Cations (NH4+, Ca+2): 0.08+0.95 = 1.03
Anions (NO3-): 1.03.
And we can see it is balanced.
The same applies to all mineral fertilizers. Therefore, addition of mineral fertilizers to the irrigation water always results in a balanced nutrient solution.
Many confuse between "a balanced nutrient solution" to the cation-anion balance of the solution.
A "balanced nutrient solution" refers to the ratios, proportions and concentrations of the substances in the water, not to the cation-anion balance.
For example, we may require specific ratios between ammonium to nitrate in the solution or between calcium to magnesium etc. We may also require minimum concentrations of certain substances and maximum (threshold) concentrations of others.
Therefore, a nutrient solution may be considered to be balanced for a certain crop, but not balanced for another crop. However, it will be always balanced with respect to cation-anion balance.
The soil is a more complex system.
Actually, the 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. Being an aqueous solution, the explanations above are valid for this phase, i.e. cations and ions are balanced.
The solid phase is composed of the soil minerals. Most of the soil minerals have a negative charge on their surfaces.
Therefore, 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. See article on Cation Exchange Capacity.
We can see that the soil system is also always naturally balanced, when referring to cation-anion balance.
Same 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.
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 soil balanced? The answer to this question depends on the approach you want to take for interpreting your soil test results and on the crop you are growing.
You can probably guess by now that if we refer to the cation-anion balance, the answer for this question would be "yes, always".