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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.
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.
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.
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.
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.
Background
Spinach is a nitrogen (N) demanding crop with a weekly N uptake of up to 60 kg ha–1. Consequently, a high N supply is required, which can temporarily lead to high quantities of nitrate (NO3–) being at risk of leaching.
Aims
The objective of this study was to develop a N fertilization approach to reduce the risk of NO3– leaching in field-grown spinach production without adversely affecting crop yield and quality at an early and late harvest stage.
Methods
Ten fertilization trials were conducted to compare different base fertilization rates and splits of top dressings. For top dressings, granulated fertilizers or foliar sprays were used. In a further treatment, N supply was reduced by withholding the second top dressing of 50–70 kg ha−1.
Results
Nitrate concentration at risk of leaching was considerably reduced by decreasing the base fertilizer rate as well as by splitting the top dressing. However, at an early harvest stage, total aboveground dry mass was reduced by, on average, 6% by these measures across all seasons. In contrast, at a later harvest stage, spinach was less affected by the fertilizer schedule. Urea foliar sprays proved to be insufficient in promoting plant growth and caused leaf necrosis. A reduced N supply led to impaired plant growth and yellowish leaves in both spring and winter.
Conclusions
Base N fertilization of spinach is only required in spring, but not in other seasons. Despite slight yield reduction, the top dressing should be split to reduce the risk of NO3− leaching after an early harvest.
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.