Tropentag, September 10 - 12, 2025, Bonn "Reconciling land system changes with planetary health"

Combined effects of vapour pressure deficit and nitrate to ammonium ratio on the growth of chenopodium quinoa in hydroponics

¹University of Hohenheim, Inst. of Agric. Sci. in the Tropics (Hans-Ruthenberg-Institute), Germany
²University of Hohenheim, Inst. of Agric. Sci. in the Tropics (Hans-Ruthenberg-Institute), Germany
³University of Hohenheim, Inst. of Agric. Sci. in the Tropics (Hans-Ruthenberg-Institute), Germany

Abstract

The scarcity of freshwater resources necessitates the exploitation of alternative water sources and the adoption of more efficient farming methods. Hydroponics combined with treated wastewater offers a promising solution for food production, yet high ammonium (NH₄⁺) levels in wastewater can hinder plant growth in these systems. To investigate the influence of ammonium to nitrate (NO₃^-) ratios on quinoa (Chenopodium quinoa Willd.) growth under contrasting vapour pressure deficits (VPDs), we cultivated the variety Titicaca in a deep-water flow system for 30 days at a low VPD of 1.2 kPa and a high VPD of 4.0 kPa. Nutrient solutions of four NO₃^-N:NH₄-N ratios (100:0, 85:15, 65:35, and 50:50) were used. The total nitrogen concentration was maintained at approximately 3.0 mM by adding stock solutions every third day. Simultaneously, the pH was adjusted to 5.3–6.3 using HCl or NaOH, as needed. During the experimental period, samples of the nutrient solutions were taken daily for analysis and plants were harvested every sixth day for biomass partitioning.
At high VPD, the general assumption that a low NH₄⁺ concentration promotes plant growth in hydroponic systems was confirmed. A 85:15 NO₃^-:NH₄⁺ ratio significantly enhanced shoot dry biomass (70.2 ± 4.1 g) compared to the other high VPD treatments. Conversely, under low VPD, the pure NO₃^- treatment yielded the highest shoot biomass (63.0 ± 4.6 g). Nutrient uptake analysis revealed a consistent preference for NO₃^- across all treatments and VPD conditions, with NO₃^- comprising approximately 65% of total nitrogen uptake even at the 50:50 NO₃^-:NH₄⁺ ratio. Notably, total nitrogen uptake was generally higher under high VPD, except for the pure NO₃^- treatment. These findings demonstrate that VPD significantly influences the optimal NO₃^-:NH₄⁺ ratio for maximising quinoa biomass in hydroponic systems, offering valuable insights for optimising treated wastewater utilisation in agriculture.

Keywords: Controlled environment farming, hydroponics, vpd, wastewater

Contact Address: Sebastian Heintze, University of Hohenheim, Inst. of Agric. Sci. in the Tropics (Hans-Ruthenberg-Institute), Garbenstr. 13, 70599 Stuttgart, Germany, e-mail: sebastian.heintzeuni-hohenheim.de