Comparing the Adequacy of Controlled-release and Water-soluble Fertilizers for Bedding Plant Production

Four complete water-soluble fertilizer (WSF) formulations including micro-nutrients applied at 200 mg.L-1 nitrogen (N) at each irrigation [Peters Excel (21N-2.2P-16.5K), Daniels (10N-1.8P-2.5K), Peters Professional (15N-1.3P-20.8K), and Jack's Professional (20N-1.3P-15.7K)] were compared with two controlled-release fertilizer (CRF) products (also containing micronutrients) substrate incorporated at transplant at a rate of 3000 g.m(-3) of substrate [Osmocote Plus (15N-4P-9.9K, 90 to 120 days longevity at 21 degrees C) and Osmocote Bloom (12N-3.1P-15K, 60 to 90 days longevity at 21 degrees C)] in the greenhouse production of four commonly produced bedding plant species with high alkalinity irrigation water (pH 7.1, 280 mg.L-1 CaCO3 equivalent). Species included Argyranthemum frutescens (L.) Sch. Bip. 'Madeira Cherry Red' and iron-inefficient Calibrachoa Cerv. hybrid 'Cabaret Pink Hot', Diascia barberae Hook. f. 'Wink Coral', and Sutera cordata Roth 'Abunda Giant White'. Additional treatments included a combination of 100 mg.L-1 Excel and 2100 g.m(-3) Osmocote Plus and an Osmocote Plus treatment irrigated with reduced alkalinity water (acidified to pH 6.3, 92 mg.L-1 CaCO3 equivalent). Bedding plants were evaluated at the end of a finish or market stage (3 or 5 weeks depending on species) for shoot dry mass (SDM) and root dry mass (RDM), tissue nutrient concentrations, and visual quality rating (0 to 4). At 3 weeks, there were no significant differences in SDM and RDM between fertilizer treatments for any of the four species. Shoot dry mass significantly increased at 5 weeks in the WSF and combination treatments over the three CRF only treatments for Argyranthemum and over the non-acidified Osmocote Plus treatment only for Calibrachoa. At finish, 3 weeks for Sutera and Diascia and 5 weeks for Argyranthemum and Calibrachoa, visual quality rating for all species was lowest when using Osmocote Plus with or without acidified irrigation water compared with the WSF treatments, except the Daniels treatment in Argyranthemum, which also resulted in a low visual quality rating. Leaf tissue N for all species and phosphorus (P) for all except Diascia were below the recommended range for bedding plant crops in the CRF treatments, which was reflected by the lower substrate electrical conductivity (EC) for the CRF alone and combination treatments. Leaf tissue N and P were related to visual quality rating for all species, leaf tissue potassium (K) for Argyranthemum and Calibrachoa only, and leaf tissue iron (Fe) for Diascia only.