Abstract
Watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai `Crimson Sweet'] was seeded in a commercial peat medium in multicell containers. Five NO3: NH4 ratios and five levels of supplemental Ca were combined factorially in a 100N-31P-265K mg·liter-1 pretransplant nutrition regime. The medium was amended with CaCO3 in 1989; the medium was not amended in 1990. Dry-matter accumulation had decreased with increasing NH4-N 3 weeks after seeding both years. In 1989, increasing NH4-N also had decreased seedling growth by the last sampling date due to decreased NH+4 uptake. Shoot N concentration was higher in 1990 than 1989, but N uptake was similar. On the first sampling date in both years, increasing the Ca concentration decreased seedling growth and increased medium electrical conductivity (EC). In 1990, increasing the Ca concentration raised shoot N concentration but did not increase seedling growth or N uptake. Either N form at 100 mg N/liter and Ca at 4 to 8 mmol·liter-1 were optimal for watermelon transplant production, but Ca at >8 mmol·liter-1 suppressed transplant growth.
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