Marketable yield quantity and quality of leek and residual soil mineral nitrogen in relation to nitrogen fertilization

In Flanders threshold values of soil mineral nitrogen (Nmin) in the soil profile (0-90 cm) in autumn are used in legislation to judge fertilization practices and evaluate the nitrate (NO3-) leaching risk during winter. The average Nmin measured in leek fields are high compared to most arable crops. The objective of this study was to investigate if high Nmin values at harvest can be decreased without reducing yield quantity and quality. We re-analyzed Flemish leek field experiments with various nitrogen (N) fertilizer application rates (2001-2016). We examined marketable yield quantity and quality and residual soil mineral N (RSMN) to rooting depth (0-60 cm) at harvest from July till April the next year. Considering the N supply (= applied effective N + Nmin (0-60 cm) at planting), the marketable yield was on average higher in July till December than in January till April. The maximum marketable yield was obtained at ≈150-200 kg N supply ha‑1. Generally RSMN values measured in July till December were higher than those in January till April. This can be explained by higher N mineralization rates in the first case and possibly some NO3- leaching in the latter case. There was no significant difference in scores of abrasion hardness nor leaf color or uniformity at ≈150-200 kg N supply ha‑1 compared to higher N supplies. Taking into account a mean Nmin value (0-60 cm) at planting of 69±45 kg N ha‑1 (average±standard deviation), effective N application rates in the order of 80-130 kg N ha‑1 are maximum, which allows Nmin values below 50 and 70 kg N ha‑1 in the 0-60 cm and 0-90 cm layer, respectively. These results indicate that N fertilization advices of leek can be reduced without risking a decreased marketable yield quantity and quality but minimizing NO3- leaching during winter.