ASHS 2015 Annual Conference

Changes in substrate pH during crop production are closely tied with the relationship between uptake of cations and anions.   We utilized a central composite design to investigate how macronutrient applications influenced substrate pH and total shoot accumulation trends in a crop and the effect of a species and crop duration different from Salvia grown for 45 days from a previous study.  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.  ‘Pittsburgh Purple’ chrysanthemum (Chrysanthemum ×morifolium Ramat.) was grown in 14-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.  Harvests occurred after 35 and 65 days of growth.  A statistical model explained 98.1% of the substrate pH data, with statistically significant effects including four main effects of N carrier ratio, P, K, and S; four squared terms of N carrier ratio, P, K, and Ca+Mg; and nine interaction effects.  The resulting model was used to calculate substrate pH levels between 35 and 65 days after planting, and N carrier had the greatest impact on substrate pH.  Calculated pH ranges for the two N carrier extremes were 2.04 (6.31–4.27) and 2.00 (6.17–4.17) for harvest 1 and 2, respectively.  Changes in substrate pH were then compared with trends observed from models generated to describe uptake for P, K, Ca, and Mg.  Decreases in substrate pH for increasing concentrations of applied K, Ca, and Mg matched with increasing accumulation with these three elements.  For P, increasing accumulation was observed as P concentrations in the nutrient solution increased; however, substrate pH initially rose and then decreased at higher P concentrations possibly due to protons generated from the formation of precipitates.  We also observed different effects on substrate pH with Chrysanthemum from the previous experiment with Salvia farinacea, indicating a species effect exists.