Tomato Seedling Performance in Five Commercial Organic Growing Media with and without Supplemental Fertilizer

Commercially available growing media permitted for use in certified organic production differ substantially in quantity and availability of essential macronutrients. Nutrient availability from fertilizers applied during production will depend on chemical, biological, and physical characteristics of both media and fertilizers; therefore, best practices for fertilizing seedlings are likely to differ. This study evaluated tomato ( Solanum lycopersicum L.) emergence, seedling growth, and posttransplant performance when grown in five organic growing media with and without supplemental fertilizer during transplant production in three diverse locations. A 5N–0.88P–3.32K–3Ca (5–2–4) dry fertilizer was tested in all media, and a liquid 3N–0.88P–2.49K (3–2–3) fertilizer was tested in two media. Fertilizer improved growth of seedlings in two media that did not contain compost, but not consistently in compost media. Incorporation of dry fertilizer before seeding inhibited emergence, especially in compost media with high electrical conductivity (EC). In noncompost media, liquid fertilizer improved growth more when application began early. Both fertilizers increased media EC and altered pH, with larger changes in noncompost media. Without fertilizer, nitrogen (N) and potassium (K) were deficient in plant tissue after 4 to 5 weeks. Fertilizer increased N and K concentrations, especially in noncompost media, but only applications of liquid fertilizer near the end of transplant production consistently prevented N deficiency. Four weeks after transplanting to the field, plants fertilized as seedlings were larger than unfertilized plants, especially when grown in noncompost media, and they had more flower clusters and set fruit earlier. At one location where yield was measured, early yield of seedlings grown in noncompost media was greater when they were fertilized. They also produced more tomatoes, but the average fruit size was smaller. In compost media, fertilizer had inconsistent effects on flowering, fruit set, and yield. Based on this work, for media without compost and low in nutrients, fertilization is recommended beginning early in seedling production. For compost media with high initial EC, delaying fertilizer application for approximately 3 weeks is recommended. More research is needed to identify best management practices related to fertilizer rates, application methods, and timing. Development of methods to assess the need for fertilization that can be implemented on farms will be valuable to improving organic transplant production.