Modeling impact of nitrogen carrier and concentration on root substrate pH

We conducted an experiment to quantify the effects on substrate pH from nitrogen (N) carrier and concentration. We used four concentrations of N (3.5–14 mM) and five fractions of ammonium (NH₄⁺) (0–80% NH₄⁺ of total N) that are found in commercially available fertilizers. Fertilizers were applied to fallow 14-cm-diameter pots (1.29 L) filled with a 3 peat:1 perlite (v/v) substrate amended with non-residual powdered calcium carbonate to raise the substrate pH to approximately 6.0. Harvests occurred at 20 and 42 days. Significant effects in the model included main effects of N carrier and N concentration, their squared terms, an interaction effect, and a time × N carrier. The fraction of NH₄⁺ accounted for 45.0% of variation in substrate pH, and N concentration accounted for 1.5% of the total R² of 76.7%. Substrate acidification was likely due to the physiological fertilizer effect and nitrification.