Context: The fertilizer calculation program and its results regarding the calculated pH value.

What the program does not do:

• No dynamic chemical equilibrium (no pKa model)
• No exact activity calculation
• No EC/buffering influences

The program currently calculates the pH purely from the net charge of the ion balance , especially from OH⁻ excesses , but:

A real nutrient solution such as Steiner's solution has a complex buffering effect and cannot be calculated solely by the cation/anion balance .

The model calculates:

• Charge balance = cation charge – anion charge
• → if > 0 → interpreted as OH⁻ concentration → basic
• pOH = –log₁₀([OH⁻]), pH = 14 – pOH

BUT:
Just because the net charge is positive does not mean that OH⁻ is present.

Example:

• Ca²⁺, Mg²⁺, K⁺ → no basic ions
• NO₃⁻, PO₄³⁻ → no “strong” acids
• KH₂PO₄ has a buffering acidic effect
• Chelates such as Fe-EDTA have a slightly acidic effect

"We"  suspect OH⁻ , although there is none.

No acid-base reaction in this model

It does not model any of these real effects:

• Proton release / uptake (H⁺, OH⁻)
• Dissociation constants (pKa) of:
  • H₂PO₄⁻ ↔ HPO₄²⁻ + H⁺
  • BOH₃ (boric acid)
  • EDTA complexes
• Buffer systems (e.g. phosphate buffer)