Evaluation of Nine Canadian Retail Consumer Potting Mixes for Growing Container Plants

Abstract

Commercially available Canadian retail potting mixes were evaluated for physical and chemical properties, as well as for plant performance of petunia (Petunia ×hybrida ‘Storm Pink’), tomato (Solanum lycopersicum ‘Better Bush’), and zonal geranium (Pelargonium ×hortorum ‘Savannah Red’) plants grown outdoors at the Vineland Research and Innovation Center in the Niagara Peninsula in Ontario, Canada. Chemical properties, but no physical properties, resulted in significant correlation with plant growth index, overall appearance, and yield (i.e., flower, fruit, or inflorescence number for petunia, tomato, and zonal geranium, respectively). The performance of all species was best when initial potting mix pH and electrical conductivity (EC) values were in the ranges of 5.20 to 6.17 and 2.76 to 4.33 mS·cm−1, respectively. The physical properties of the container capacity, total porosity, air space, and bulk density were acceptable for all plants in this study and ranged from 71% to 80%, 78% to 96%, 8% to 20%, and 0.08 to 0.22 g·cm−3, respectively. The minimum concentrations of the initial nitrate (NO3−), ammonium (NH4+), phosphorus (P), and potassium that were acceptable were 104.4, 61.3, 47.9, and 150.5 ppm for petunia and 96.8, 61.3, 51.7, and 143.3 ppm for tomato, respectively. The minimum concentration of NO3− that was acceptable was 66.1 ppm for zonal geranium. Overall appearance at 4, 8, and 10 weeks after transplanting was correlated with initial potting mix EC, NO3−, and calcium for all species, with pH, NH4+, and P for petunia, with P for tomato at all time points, and with P for zonal geranium after 10 weeks. Although it is difficult to discern how each nutrient impacts plant performance, this study indicated that it is essential to have a balanced and adequate supply of nutrients in consumer potting mixes. The ability of a potting mix to maintain an appropriate pH for the duration of the growing season may prevent nutrient deficiency symptoms, especially for pH-sensitive species like petunia. This study is the first to provide a benchmark of currently available retail potting mixes for Canadian consumers.