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Spinach is a nitrogen (N)-demanding crop characterized by a shallow root architecture. Especially in the first weeks after sowing, significant N uptake is limited to the uppermost few centimetres of the soil. However, base fertilization is usually based on the soil mineral N (Nmin) concentration in the upper 30 cm. Therefore, the objective of this study was to examine whether the soil sample depth for calculating the base N fertilization can be reduced to the 0-15 cm layer. In seven field trials, conducted during spring, summer and autumn seasons, either a low or high base fertilization dose was applied at sowing. Until top dressing, soil samples were frequently taken in the upper 0-15 and 15-30 cm layers to determine the average Nmin concentration in each layer. Top dressing was applied when the first true leaves had unfurled. With this fertilizer application, the total N supply was aligned between both treatments based on the Nmin concentration in the upper 30 cm of the soil. Aboveground fresh and dry masses were determined after reaching a fresh mass yield of 15-20 t ha‑1 and related to the mean Nmin concentration in the first 3 to 4 weeks of cultivation between sowing and top dressing. It was shown that the Nmin concentration in the upper 0-15 cm of the soil highly reflects the base fertilization rate. By contrast, the Nmin concentration in the 15-30 cm layer remained unaffected. However, the Nmin concentration of both top soil layers can affect fresh and dry mass yield at harvest. Therefore, the entire 0-30 cm soil layer should be considered when calculating the base N fertilization rate in field-grown spinach. Measurements revealed that spinach fresh and dry masses were increased until the N availability of between 54 and 59 kg ha‑1 (0-30 cm) was reached at the seedlings stage, respectively.
The mineralization of soil organic nitrogen (N) and crop residues can significantly contribute to the N supply of vegetable crops. However, short-term mineralization dynamics are difficult to predict. On the other hand, fast-growing crops like spinach are highly sensitive to N shortage. Therefore, in situ soil columns have been tested to estimate the actual N supply via mineralization in field-grown spinach. In ten fertilization trials covered soil columns (20 cm in diameter) were driven into the soil to a depth of 30 cm at the start of the cultivation. Eight columns were repeated in three blocks within a total trial area of 0.10 to 0.25 ha. Net N mineralization was derived by subtracting the soil mineral N concentration (Nmin) in the upper 30 cm before installation from the concentration inside the columns at harvest. For comparison, a balance sheet was calculated for spinach plots receiving no N fertilization (zero plots) as well as fertilized plots and used as a proxy for net N mineralization. In this approach the initial Nmin concentration in the upper 30 cm of the soil, the N supply via irrigation, and fertilization as well as the total aboveground N uptake by spinach and the Nmin residue were considered. By using soil columns, N mineralization was determined with a mean coefficient of variation of 18%. A higher spatial variability of up to 43% was observed when spinach was grown as a second crop. The average net N mineralization rate ranged between 2 kg ha‑1 week‑1 (0-30 cm) in winter-grown spinach and 3-7 kg ha‑1 week‑1 (0-30 cm) in the other seasons. Nitrogen mineralization measured by the soil columns was qualitatively confirmed with the data obtained by the balance sheet. Soil columns enable repeated samplings during the spinach cultivation. In this way, top dressing rates can be adjusted to the actual N supply.
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.