Does a Commercial Organic Fertilizer with Hydrogel or Biochar Guarantee the Quality of Eucalyptus Seedlings?

Our objective was to evaluate the effect of a commercial organic fertilizer and substrate conditioners on the production of Eucalyptus benthamii seedlings. Two experiments were conducted with different doses of organic fertilizer (0, 5, 10, 15, 20, and 25 kg m−3) and levels of hydrogel (0 and 3 kg m−3) and biochar (0 and 30%). In each experiment, plots were divided into two subplots, with one subplot receiving topdressing with mineral fertilizers. At the end of each experimental period, quality, root morphology, and physiological characteristics of the seedlings were assessed. When only the organic fertilizer was applied with substrate conditioners, seedlings exhibited limited growth, averaging 5.02 cm in height and 0.81 mm in stem diameter. Topdressing fertilization combined with higher organic fertilizer doses (20–25 kg m−3) enhanced key traits, such as height (up to 24.15 cm) and stem diameter (up to 2.39 mm). Hydrogel and biochar often reduced seedling quality and root development. Some interactions between factors affected certain root variables, but physiological characteristics remained largely unaffected. Overall, even when combined with a substrate conditioner, the commercial organic fertilizer is insufficient to produce high-quality seedlings. Neither hydrogel nor biochar is recommended under our experimental conditions. However, the commercial organic fertilizer shows potential when used with mineral fertilizers and further should be conducted to explore this possibility.