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Organic pot-based production of basil (Ocimum basilicum L.) often has lower biomass yield than conventional cultivation. Previous investigations indicate that this growth impairment is related to high ammonium (NH4+) concentrations in the growing media released by the mineralization of organic nitrogen (N) fertilizers. However, as a result of this ammonification process substrate pH may also increase. Under neutral to alkaline conditions NH4+ is converted to ammonia (NH3), which is known to be phytotoxic even at low concentrations. Therefore, we investigated the impact of both ammonical N species on basil grown in a peat substrate. In total, three fertilization pot experiments were conducted in a greenhouse in order to compare the effect of different organic base dressings [250 and 750 mg N (L substrate)-1 mainly supplied by a liquid amino acid fertilizer (AAF)] and two initial substrate pH levels (5.5 and 6.5). In two treatments, 5% (v/v) mature compost was mixed into the peat 1 day and 12–days before the substrate was used for sowing, respectively. The aim of this procedure was to stimulate nitrification in this way to reduce ammonical N concentration. Ammonia concentration in the aerial plant surrounding environment was measured by using NH3 detector tubes in combination with an open-top chamber method. The results showed that the growth of basil (number of plants, fresh matter yield, plant height) was significantly inhibited in the second and third week of cultivation by rising NH3 and NH4+ exposure, as well as by a substrate pH ≥ 7.0. These adverse effects were reduced by lowering the organic base dressing rate and adjusting the initial substrate pH to 5.5. Furthermore, the addition of mature compost to peat in combination with a 12-day storage was proven to be effective for promoting nitrification in the organically fertilized substrate. As a result, plant growth was improved by both lower NH3 and NH4+ exposure as well as a faster supply of nitrate (NO3-) as an additional N source. Using this approach, it was possible to feed organically fertilized basil right from the seedling stage with a NO3--N/NH4+-N-balanced and later on providing a predominant NO3--N supply.

In open-field vegetable production, high quantities of soil mineral nitrogen (Nmin) and N-rich crop residues often remain in the field at harvest. After the harvest of crops in autumn, this N can lead to considerable nitrate (NO3−) losses during the subsequent winter leaching period. In four field trials, different tillage depths (3–4, 10, 30 cm) and dates (early autumn, late autumn, early spring) were investigated to reduce N losses after growing spinach in the autumn. In a further treatment, the nitrification inhibitor 3,4-Dimethylpyrazole phosphate (DMPP) was directly applied to the crop residues. Potential N losses were calculated by a balance sheet approach based on Nmin concentration (0–90 cm), measured N mineralization and N uptake by catch crops. By postponing the tillage date from early to late autumn or spring, resprouting spinach stubbles acted as a catch crop, reducing N losses by up to 61 kg ha−1. However, if the spinach biomass collapsed, the N losses increased by up to 33 kg ha−1 even without tillage. The application of DMPP as well as the tillage depth were less effective. Overall, postponing tillage to spring seems to be the most promising approach for reducing N losses during the off-season.

Im ökologischen Anbau von Topfbasilikum treten des Öfteren Wachstums- und Qualitätsbeeinträchtigungen auf. Diese machen sich bereits an den Jungpflanzen in Form chlorotischer und nekrotischer Keimblätter bemerkbar. Nachfolgend können Infektionen mit Schwächeparasiten wie Botrytis auftreten. Im Rahmen eines Düngungsversuches sollte geklärt werden, inwieweit diese Probleme im Zusammenhang mit der Anreicherung von Ammo­nium stehen, welches durch die Mineralisierung organischer Dünger in das Kultursubstrat freigesetzt wird. Versuchsfaktoren waren das Ammonium-N/Nitrat-N-Verhältnis (100/0; 50/50; 0/100) und die Stickstoffkonzentration in der Nährlösung (8, 12 und 16 mmol N/L). Ammonium wurde mittels des Nitrifikationshemmstoffes 3,4-Dimethylpyrazolphosphat (DMPP) stabilisiert. Zusätzlich war in den Versuch eine organische N-Düngevariante einbezogen, die neben einer Grunddüngung mit festen Düngern (Hornspäne und DCM ECO-MIX 4) eine flüssige Nachdüngung (Organic Plant Feed) beinhaltete. Die Kultur der Pflanzen erfolgte in einem Torfsubstrat, das zu Versuchsbeginn auf pH 6,5 eingestellt war. Mit Nitrat (NO3-) als alleiniger Stickstoffquelle zeigte Basilikum über den gesamten Kulturzeitraum ein vitales Wachstum. Ein reines Ammoniumangebot (NH4+) ging, unabhängig von der N-Stufe, mit einer geringeren Keimrate sowie mit verminderten Pflanzenhöhen- und Frischmassezuwächsen einher. Außerdem waren hier chlorotische Keimblätter und eine verringerte Turgeszenz des Sprosses zu beobachten. In der organischen N-Dünge­variante blieb das Pflanzenwachstum zunächst ebenfalls hinter dem mit NO3--Angebot zurück. Des Weiteren waren hier die Schadsymptome an den Keimblättern besonders stark ausgeprägt. Im Zuge der Ammonifika­tion der organischen N-Dünger kam es in den ersten Versuchswochen zu einer Anreicherung von bis zu 350 mg NH4+-N/L Substrat als alleiniger mineralischer Stickstoffform. Mit fortschreitender Nitrifikation setzte dann ein stimuliertes Pflanzenwachstum ein. Zu Versuchsende wiesen die organisch gedüngten Pflanzen den höchsten NO3--Gehalt im Spross auf. Der kompakteste Wuchs und die höchste Turgeszenz der Pflanzen konnten mit ausgeglichenem NH4+/NO3--Angebot erzielt werden.