Statistical Model for Describing Macronutrient Impacts on Container Substrate pH Over Time

• Published in: HortScience Vol. 49; no. 2; pp. 207 - 214
• Main Authors: Barnes, Jared, Nelson, Paul, Whipker, Brian E., Dickey, David A., Hesterberg, Dean, Shi, Wei
• Format: Journal Article
• Language: English
• Published: 01.02.2014
• ISSN: 0018-5345; 2327-9834
• DOI: 10.21273/HORTSCI.49.2.207

Although many factors that influence substrate pH have been quantified, the effect from fertilizers continues to be elusive. A multifactorial experiment was conducted to test macronutrient effects using a rarely used statistical method known as the central composite design. Five nutrient factors, including nitrogen (N) carrier ratio (NH 4 + vs. NO 3 – ) and concentrations of phosphorus (P) (as H 2 PO 4 – ), potassium (K), combined calcium (Ca) and magnesium (Mg), and sulfur (S), were varied at five levels each encompassing the proportionate range of these nutrients in commercial greenhouse fertilizers. Although a typical factorial experiment would have resulted in 5 5 = 3125 treatments, the central composite design reduced the number to 30 fertilizer treatments. An experiment was conducted twice in which ‘Evolution White’ mealy-cup sage ( Salvia farinacea Benth.) 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 ≈5.6 to 5.8. Harvests occurred after 3 and 6 weeks of growth. A statistical model described substrate pH over time with significant effects including four main effects of N carrier ratio, P, K, and combined Ca and Mg; three squared terms of N carrier ratio, P, and K; and seven interaction effects. The resulting model was used to calculate substrate pH levels between 25 and 45 days after planting, and it showed that N carrier had the greatest impact on substrate pH.