Substrate and Fertilizer Choice Interact to Affect Growth of Conventionally and Organically Produced Calibrachoa ‘Cabaret Lavender'

Calibrachoa, a popular floriculture bedding plant, can be challenging to produce in a greenhouse environment because it requires moderately high fertility, and at high pH, it often exhibits iron deficiency. The objective of this experiment was to determine growth of Calibrachoa ‘Cabaret Lavender’ in response to a combination of four substrates and four fertilizer treatments derived from both organic and conventional sources.  Rooted Calibrachoa cuttings were transplanted into 4-inch pots containing either: Cornell mix with no starter nutrient charge; a commercial peat-based substrate with a conventional starter charge (LM-111, Lambert); a commercial peat based substrate with organic starter charge (Jeff’s Organic, Lambert); or a locally formulated unbranded compost- and peat-based substrate with an organic starter charge (Rochester, NY).  The plants received daily irrigation with one of four liquid fertilizer treatments: tap water only; 150 ppm N from a conventional complete fertilizer (21-5-20, J.R. Peters Inc.); 150 ppm N from an organic-based fish emulsion fertilizer (APG-em 5-1-1, Eco-Logic); or a kelp extract based root enhancer (SG, Eco-Logic) applied daily at a 1:100 rate in the irrigation water.  The experiment was designed as a 4 x 4 factorial experiment with 10 replicate plants for each of the sixteen treatment combinations.  Substrate pH and electrical conductivity (EC) were monitored weekly using the PourThru method during the five week experimental period.  The lowest average pH values were came treatments receiving either of the Lambert substrates along with either of the two 150 ppm N fertilizers (average pH 6.4).  The Cornell and Rochester substrates that received only tapwater had the greatest pH (pH 7.1–7.3).  EC varied from 0.4 to 3.4 dS/m depending on treatment.  Lowest EC values were with the Cornell substrate that received either tapwater or root-enhancer. The Rochester substrate that received either of the fertilizers had the highest EC values.  Plant growth (dry weight) was greatest for the treatment receiving Jeff’s organic substrate and the fish emulsion fertilizer.  The next largest group of plants received Jeff’s organic with 21-5-20 or Cornell substrate with either of the fertilizers.  The results suggest that high quality ornamental plants can be produced with only organic fertilizer sources.  When applied at an equal rate of nitrogen, fish emulsion fertilizer was at least as effective as 21-5-20 under our experimental conditions.  A substrate starter charge was not essential to produce high quality plants under conditions of constant liquid feed with 150 ppm N.