| Root substrate pH is a variable in horticultural substrates that requires acute attention. However, the effects of macronutrients on substrate pH are not well quantified. Therefore, we developed a novel protocol to quantify the effects from macronutrient influences on substrate pH. We investigated the combined physiological and chemical effects of macronutrient applications on substrate pH by using a rarely used statistical design known as the central composite design where we varied five nutrients at five levels. Experiments with Salvia and Chrysanthemum revealed that the main effect of nitrogen carrier ratio provided the most predictability. The models for Salvia and Chrysanthemum were different, explaining 86.7% and 98.1% of the data, respectively. Chrysanthemum had a larger impact on substrate pH than Salvia . After 45 days of growth, the calculated Salvia substrate pH range between high NO3- and high NH 4+ levels was 1.06 (6.01--4.95) and was less than the Chrysanthemum calculated range of 2.04 (6.31--4.27) after 35 days of growth. Because N was the species that had the greatest impact on substrate pH, we conducted an experiment where we evaluated four concentrations of nitrogen at five ammonium : nitrate ratios. The ammonium : nitrate ratio had a more pronounced effect on substrate pH than the nitrogen concentration.;Additional experiments evaluated the chemical effect of fertilizers on substrate pH. A comparison between planted and fallow substrates supported that an effect separate from the physiological plant effect existed from application of fertilizers. Acidification of fallow substrates increased as levels of ammonium and other cations rose. This result led us to apply the central composite design method to fallow substrate. A decrease in substrate pH was observed, and the model explained 71.3% of variation in the data. We attributed the acidification to nitrification. In another experiment leaching fractions of 0% and 20% were evaluated to assess differing contributions to substrate pH; however, a difference was not observed. This result indicated that the leached portion of fertilizer did not affect substrate pH.;An additional experiment investigated the pH buffering capacity of substrate. Typically, peat substrates are evaluated for their lime requirements by using incubations, but we evaluated parameters in a faster, more accurate, and less labor intensive method using automatic titrators. We showed that the optimum rate of titration was 2.00 mL acid/base addition per minute.;Another experiment investigated compost that was added to substrate to help buffer changes in pH. Compost was heavier than peat moss, potentially negatively impacting shipping capacity; however, it increased wettability of the mix, and the 15% compost substrate could be dried to 30% H2O and still have the same wettability as the industry normal of 50% H2O. |
