Determination of ammonia exposure of potted herbs in organic cultivation

In organic production of pot-grown herbs the ammonium (NH₄⁺) concentration and pH in the growing media may temporarily increase to high levels after a base dressing of organic nitrogen (N) fertilizers, thus impairing plant growth. Under these conditions, ammonia (NH₃), which is phytotoxic even at low concentrations, can also be formed. To avoid crop damage, the exposure to both ammonical N sources should be distinguished in order to develop appropriate fertilization strategies. However, the determination of NH₃ in the aerial environment of plants is challenging in small-scale in situ experiments. In order to determine the NH₃ concentration in the canopy-atmosphere of pot-grown basil, a novel small-scale open-top chamber (OTC) approach was evaluated in a series of three fertilization trials at different organic fertilization rates and initial substrate pH values. Basil pots were placed into OTCs and the air above the substrate surface was sampled and passed through an NH₃ indicator tube. The detected NH₃ concentrations were compared to both NH₃ exposure determined via passive acid traps and to calculated concentrations based on known chemical equilibria. The latter approach took into account pH and NH₄⁺ concentrations in the growing media as well as air temperature at the time of NH₃ measurement. Results showed that measured NH₃ concentrations were closely correlated to values obtained from the two comparative approaches. The reproducibility of the NH₃ determination in air samples was sufficient at NH₃ concentrations above 0.2 ppm, as indicated by a coefficient of variation of ≤15%. At lower concentration levels, the variability of the readings substantially increased. However, plant growth (number of plants, fresh matter yield, and plant height) was adversely affected even at concentrations below 0.2 ppm NH₃, depending on the substrate NH₄⁺ concentration. Therefore, further improvements, in particular to the sensitivity of NH₃ detection, are required to increase the reproducibility of the method in the lower concentration range.