Influence of nitrogen form and concentration on yield and quality of pot grown basil

In the organic production of pot grown basil, yield depressions and quality impairments are often observed. During the early development stage, cotyledons become chlorotic and necrotic. Subsequently, fungal diseases such as botrytis occur. One possible reason for this problem could be the high concentration of ammonium in the growing media released by the mineralization of organic fertilizers. Therefore, a fertilization trial was carried out to investigate the effect of ammonium (NH₄⁺) on basil in comparison to nitrate (NO₃^-). The experiment included different NH₄⁺-N/NO₃^--N ratios (100/0, 50/50 and 0/100) and nitrogen (N) concentrations in the nutrient solution (8, 12 and 16 mmol N L^‑1). Plants were cultivated in a peat substrate and fertilized with a nutrient solution which, in addition to the different N sources, contained equal concentrations of a base fertilizer as well as the nitrification inhibitor DMPP. Furthermore, an organic fertilization treatment was realized. Basil fertilized solely with NH₄⁺ showed a diminished growth in comparison to well-developed plants receiving NO₃^- as N source. Germination rate, plant height and fresh matter yield were significantly reduced by NH₄⁺ nutrition. Similar results occur in the organic treatment where the NH₄⁺ concentration rose up to 350 mg NH₄⁺-N L^‑1 substrate at the beginning of the cultivation period. Along with a reduction in biomass production, chlorotic cotyledons were observed. These effects might have been caused by NH₄⁺. When N mineralization declined and NH₄⁺ was largely converted to NO₃^-, plants exhibited improved growth. Within the mineral N treatments, rising NO₃^- concentration and NO₃^--N/NH₄⁺-N ratio promoted plant height and reduced plant compactness due to an increased internode elongation. At the end of the experiment, the NO₃^- content in basil shoots was highest in the organic treatment and lowest with NH₄⁺ as the sole N source. The best herb quality in terms of plant compactness, turgidity and healthiness of cotyledons was observed when basil was fertilized with ammonium nitrate.