Temperature Effects on Tomato Growth and Nutrient Release in Substrates Amended with Organic and Conventional Fertilizers

Fertility management of organic vegetable transplants can be challenging. Difficulties include small container volumes with limited nutrient holding capacity and slow nutrient release which depends on biological processes to convert organically bound nutrients into plant available forms. Conditions that promote microbial activity include warm temperatures, a well-aerated root-zone and a balanced pH. The objective of this work was to evaluate the interaction between temperature and fertilizer on plant growth and nutrient release rate for several organic and conventional fertilizers. A peat perlite mix with limestone but no additional fertility was used as the base substrate for the trial. Several granular organic fertilizer treatments were used: Sustane 8-4-4, Worm Power vermicompost 1.5-0.7-1.5, Verdanta EcoVita 7-5-10, and Perdue microSTART60 Plus 7-2-2. Conventional fertilizers included Osmocote Bloom 12-7-18 controlled release fertilizer (CRF) and Jack’s 21-5-20 constant liquid fertilizer (CLF) applied at 100 ppm N. The granular fertilizers and CRF were incorporated into the substrate at the rate of 400 mg L^-1 N. Four week old seedlings of tomato (Solanum lycopersicum) ‘Celebrity’  were transplanted into 10 cm containers in growth chambers with average daily temperatures of 10, 15, or 20 °C. Fluorescent lights provided 300 µmol m^-2 s^-1 for 16 h daily. The CLF treatment was applied at each irrigation event, whereas no added fertilizer was given to the granular organic and Osmocote treatments. After 6 weeks the plants were destructively harvested for plant quality measurements and tissue nutrient analysis. At 10 and 15 °C root dry weight (DW) did not differ based on fertilizer treatment. At 20 °C, root DW with vermicompost was smaller than the CLF treatment. At 10 °C shoot DW was very low and did not differ based on fertilizer treatment. At 15 °C shoot DW of vermicompost plants was significantly smaller than CLF and CRF, but the other organic fertilizers performed comparably to conventional fertilizers. At 20 °C shoot DW was greatest for CLF, least for vermicompost, and medium-sized for the other organic fertilizers and CRF. Our results indicate that vermicompost might be best used as a complementary fertilizer source for tomato; whereas the other organic granular fertilizers can be used as a substitute for CRF at 20 °C and a substitute for WSF and CRF at 15 °C.  Leachate analysis is being used to determine nutrient leaching patterns. This will be combined with tissue and substrate analysis to develop a nitrogen budget for each fertilizer.