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 regulati ons to control nitrate concentrations in edible plant parts. This study eva luates current fertiliser practice on the uptake and loss of N for butterhe ad 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 decre ase 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 the re was no decrease in the soil mineral N and N-min values (0-30 cm) up to 3 00 kg N ha(-1) were measured. During spring and early summer, the decrease in the upper soil layer greatly exceeded maximum N uptakes, sometimes leavi ng less than 60 kg N ha(-1) in the upper layer, possibly because of gaseous N2O and N-2 losses. In addition, high N-min values (30-120 cm), of up to 9 00 kg N ha(-1), were measured. To avoid on the one hand high residual soil N-min at harvest and possible h igh leaching losses and on the other N deficiency and possible gaseous loss es, measures were evaluated to minimise the difference between the availabl e nitrogen in the soil profile and the N need of the crop. An alternative m ineral N balance method is proposed, where in addition to uptake and residu al N-min, N mineralisation and N losses will be considered. This method sho uld result in an improved use of N fertilisers with minimal impact on the e nvironment.