In this work, a biodegradable, amphoteric, and renewable hydrogel, called HEDTA, was used as a substrate for a controlled-release NPK fertilizer (nitrogen, phosphorus, and potassium) in a field study of Eucalyptus urograndis seedlings. In the field study, four treatments with different hydrogels and a NPK fertilizer were compared. The treatments were HEDTA without the NPK fertilizer (HEDTA), HEDTA with the NPK fertilizer (10−10−10) (HEDTA + NPK), commercial potassium polyacrylate hydrogel without the NPK fertilizer (PAP), and solid fertilizer NPK (7−31−13) (NPK). The influence of each of these treatments on seedling development was monitored by measurements of neck circumference (nc), mean diameter growth at breast height (DBH), and height (ht), where the HEDTA + NPK treatment showed a positive and increasing response in relation to nc, with greater performance when compared to HEDTA and PAP. The results regarding the DBH of the seedlings prove the theory that the HEDTA + NPK treatment results in the slow release of the fertilizer according to the plant's needs. HEDTA + NKP also showed a higher ht growth rate when compared to the commercial hydrogel. In addition, the amount of nutrients present in the leaves of the seedlings that received each of the four treatments was also evaluated. The results showed that eucalyptus seedlings developed faster with the use of HEDTA + NPK treatment, a factor attributed to the controlled release promoted by the treatment. Effectively, HEDTA has proven to be an excellent substrate for the slow release and retention of water in the soil, reducing fertilizer leaching, as well as being nontoxic, biodegradable in the soil, and environmentally friendly.