S13 - Session P1 - Field-grown spinach - can the soil sampling depth for calculation of the nitrogen base fertilization be reduced to 15 cm?

Authors: Christian Frerichs *, Diemo Daum

Spinach is a nitrogen(N)-demanding crop characterized by a shallow root architecture. Especially in the first weeks after sowing, root growth is limited to the uppermost centimeters of the soil. However, base fertilization is usually based on the soil mineral N (N min ) 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 in spring, summer, and autumn, 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 N min concentration in each layer. Top dressing was applied after the first true leaves became visible. With this fertilizer application the total N supply was aligned between both treatments based on the N min concentration in the upper 30 cm of the soil. Aboveground fresh and dry mass were determined after reaching a fresh mass yield of 15-20 t ha -1 and related to the mean N min concentration in the first weeks of cultivation between sowing and top dressing. It was shown that the N min concentration in the upper 0-15 cm of the soil highly reflects the base fertilization rate. Contrastingly, the N min concentration in the 15-30 cm layer remained unaffected. However, the N min concentration of both top soil layers can affect fresh and dry mass yield. 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 mass was increased until a N availability of between 54 and 59 kg ha -1 (0-30 cm) was reached, respectively.