Applying nutrient dynamics to adjust the nutrient-water balance in hydroponic crops. A case study with open hydroponic tomato crops from Barcelona

Background: Global food production systems generate impacts such as eutrophication, caused by nutrient run-off from agricultural exploitations and exacerbated by climate change. Hydroponic cultivation methods are common in Mediterranean areas, although there is a gap in the literature with regard to their study. This study aims to fill that gap, by assessing nutrient flows in hydroponic crops. Results: The results showed that most of the nutrients were drained with the leachates (51% on average), a figure which could be lower, if the nutrient supply were adjusted to the needs of the plants or if (closed hydroponic) nutrient recirculation were implemented, without compromising the nutrient uptake of the plant. Moreover, the study revealed that a significant quantity of nutrients was retained in the substrate (perlite) during the crop, reaching average values of 5% of incoming calcium, 6% of nitrogen, and 7% of phosphorus. In the case of phosphorus and calcium, a regression model is presented for the estimation of their retention in hydroponic crops. Conclusions: Although further studies will be needed to confirm the above trends, the study makes a significant contribution to understanding the metabolism of nutrients in hydroponic crops and to finer adjustments of the nutrient balance.