Quantifying the Impact of Macronutrients on Substrate pH

While many factors that influence substrate pH have been quantitatively measured, the effect from fertilizers continues to be elusive.  A multifactorial experiment was conducted to test macronutrient effects using the unique statistical method known as the central composite design.  Five nutrients—nitrogen [ammonium (NH₄⁺)] vs. nitrate (NO₃^-), phosphorus [as phosphate (PO₄^3-)], potassium (K), calcium+magnesium (Ca+Mg), and sulfur (S)—were varied at five concentrations encompassing the proportionate range of these nutrients in commercial greenhouse fertilizers.  Concentrations included N at 98 mg·L^-1 ppm where the ammonium : nitrate ratios ratios were 0:100, 15:85, 40:60, 65:35, and 80:20; PO₄^3- as P₂O₅ at 10, 25, 50, 85, and 100% of N; K as K₂O at 35, 55, 100, 150, and 170% of N;  calcium+magnesium (Ca+Mg) in a 3:1 ratio at 25, 35, 50, 65, and 75% of N; and sulfur (S) at 10, 17, 30, 43, and 50% of N.  While a typical factorial experiment would have resulted in 5⁵ = 3125 treatments, the central composite design reduced the number to 30 fertilizer treatments.  Two replications of the same experiment were conducted where Salvia farinacea ‘Evolution White’ was grown in 15 cm-diameter pots (1.29 L) in a 3 peat : 1 perlite (v/v) substrate amended with non-residual powdered calcium carbonate to raise the substrate pH to approximately 5.6-5.8.  Two harvests occurred for each experiment after three and six weeks of growth.  Five reps. per harvest occurred in the first experiment and three reps. per harvest in the second experiment.  From the data a model was generated that allowed the prediction of substrate pH over time.  Significant effects in the model included the four main effects of N form, PO₄^3-, K, and Ca+Mg; nutrient x nutrient interactions of N form × K, N form × Ca+Mg, PO₄^3- × Ca+Mg, and Ca+Mg × S, and time x nutrient interactions with N form, PO₄^3-, K, and S.  The model allows us to calculate that at 0, 2.8, and 5.6 mmol NH₄⁺; 0.138, 0.69, and 1.38 mmol PO₄^3-; and 0.728, 2.081, and 3.538 mmol K the calculated pH levels were at 45 days 6.01, 5.25, and 4.95; 5.07, 5.25, and 5.22; and 5.42, 5.27, and 5.18, respectively.   Of all the nutrients used in the experiment, N form had the greatest impact on substrate pH; higher concentrations of NH₄^+­­ decreased substrate pH, while higher concentrations of NO₃^- increased substrate pH.