In recirculating hydroponic fertigation, elevated pH ($>6.2$) or unbalanced $Ca^{2+}$ and $H_2PO_4^-$ dosing triggers severe precipitation of insoluble dicalcium phosphate ($CaHPO_4$) and calcite ($CaCO_3$), causing emitter blockage. We articulate the Debye-Hückel activity coefficient and Saturation Index ($SI = \log_{10}(IAP / K_{sp})$) thermodynamic framework.
1. The Debye-Hückel Equation
\log_{10} \gamma_i = -\frac{A z_i^2 \sqrt{I}}{1 + B a_i \sqrt{I}}