Influence of the Application of Zn and Mn Obtained from Black Mass on Young Citrus Plant Growth

The reuse of industrial waste is essential to reduce environmental impact and move towards sustainable development through methods that do not depend on limited resources. To this end, a fertilizer was developed from recycled alkaline batteries, transformed into a useful product rich in zinc and manganese (black mass). The aim is to use industrial waste to create an environmentally safe fertilizer. An experiment was conducted on young citrus plants grafted onto Carrizo rootstock, grown in pots with coconut fiber under greenhouse conditions in Valencia (Spain) for one year (2023–2024). A total of 120 plants were arranged in randomized blocks with three replicates of 10 plants per treatment. Four nutrient solutions derived from the Hoagland formulation were evaluated: control solution without Zn or Mn (SoC), solution with Zn and Mn sulfates (SoH), solution with Zn and Mn sulfates extracted from black mass (BMS), and solution with Zn and Mn lignosulfonate derived from black mass (BMLS). Morphological, growth, physiological, and nutritional parameters were analyzed in March and October. While morphological traits showed no significant differences among treatments, some physiological (stomatal conductance, transpiration) and biochemical variables (chlorophyll, carotenoids, P, K, Mg, and S concentrations) differed significantly depending on the nutrient source. Nevertheless, all plants maintained healthy growth and nutrient levels within optimal ranges, and no signs of phytotoxicity or heavy metal accumulation were detected.