EVALUATION OF THE CURRENT BELGIAN N FERTILISATION ADVICE SYSTEM FOR BUTTERHEAD LETTUCE GROWN UNDER GLASS

N fertilization advice systems for lettuce are designed to meet EU regulations to control nitrate concentrations in edible plant parts. This study evaluates current fertiliser practice on the uptake and loss of N for butterhead lettuce. Field trials were carried out in 8 greenhouses in the province of West-Flanders, Belgium. Results show that under normal lettuce growth, N uptake varied between 90 and 120 kg N ha^-1 and the NO₃ -limit was never exceeded. Although product-quality was assured, discrepancies between decrease of N_min, expected residual soil N_min at harvest and measured residual soil N_min were observed. As N uptake mainly occurs from the 0–30 cm layer (rooting depth), a residual N_min of 60–80 kg N ha^-1 (0–30 cm) and low N_min values in the 30–120 cm layer were expected. However, during winter there was no decrease in the soil mineral N and N_min values (0–30 cm) up to 300 kg N ha^-1 were measured. During spring and early summer, the decrease in the upper soil layer greatly exceeded maximum N uptakes, sometimes leaving less than 60 kg N ha^-1 in the upper layer, possibly because of gaseous N₂O and N₂ losses. In addition, high N_min values (30–120 cm), of up to 900 kg N ha^-1, were measured.

To avoid on the one hand high residual soil N_min at harvest and possible high leaching losses and on the other N deficiency and possible gaseous losses, measures were evaluated to minimise the difference between the available nitrogen in the soil profile and the N need of the crop. An alternative mineral N balance method is proposed, where in addition to uptake and residual N_min, N mineralisation and N losses will be considered. This method should result in an improved use of N fertilisers with minimal impact on the environment.