Modeling the Effects of Macronutrients on Fallow Soilless Root Substrate pH

The chemical fertilizer effect contributes to the overall acidification of substrate pH. To quantify the chemical fertilizer effect, a multifactorial experiment was conducted to test macronutrient effects. Five nutrient factors, including N carrier ratio (NH₄⁺ versus NO₃^-) and concentrations of P (as H₂PO₄^-), K, combined Ca and Mg, and S, were varied at five levels, each encompassing the proportionate range of these nutrients in commercial greenhouse fertilizers.  A unique statistical method known as the central composite design (CCD) was used to reduce the treatment number to 30. Fertilizers were applied to fallow 14-cm-diameter pots (1.29 L) filled with a 3 peat : 1 perlite (v/v) substrate amended with readily soluble powdered calcium carbonate to raise the substrate pH to approximately 5.4.  Sampling occurred after 34 and 59 days. The resulting statistical model described substrate pH over time, with significant effects including three main effects of day, N carrier ratio, and K; two squared terms of N carrier ratio and K; and an interaction effect.  The resulting model was used to calculate substrate pH levels between 34 and 59 days after planting, and N carrier had the greatest impact on substrate pH contributing 51.9% to the model R² of 71.3%.  At 34 and 59 days, the calculated difference in substrate pH values between the high NO₃^- and high NH₄⁺ levels were 0.41 (5.15–4.74) and 0.51 (5.06–4.55), respectively. The acidification most likely originated from nitrification and proton displacement via cation exchange.