This classification is based on the presence of nutrients in the form of positively (cations) or negatively (anions) charged ions. This determines, for example, whether and to what extent interactions occur with the solid phase of the soil.

Cations: NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Zn²⁺, Mn²⁺, Cu²⁺

Anions: NO₃⁻, HPO₄²⁻/H₂PO₄⁻, SO₄²⁻, Cl⁻, B(OH)₄⁻, MoO₄²⁻/HMoO₄⁻

Uncharged: H₃BO₃ or B(OH)₃

Note: Nitrogen can exist and be absorbed either as a cation (ammonium, NH₄⁺) or as an anion (NO₃⁻).