Nitrogen fertilization strategies to reduce the risk of nitrate leaching in open field cultivation of spinach (Spinacia oleracea L.)

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.