SUSTAINED LOW SOIL MINERAL N CONTENT (SUSON) AS A NOVEL FERTILIZATION STRATEGY FOR COMMERCIAL PRODUCTION OF GLASSHOUSE BUTTERHEAD LETTUCE

With the aim of reducing nitrate emission to the environment while maintaining plant productivity and quality level, a new fertilization strategy, named SUSON, was studied. Instead of standard basic dressing at planting up to some disputable target level, SUSON strategy suggests to base fertilization on frequent soil analysis during the cultivation period, trying to withhold soil nutrient content from dropping under a fixed threshold level. This novel fertilization strategy was tested for nitrogen (N) fertilization in head lettuce during two years at four glasshouse holdings in Flanders (Belgium), having different soil types. At each holding, in eight succeeding lettuce rotations, two SUSON modifications (SL and SH, resp. having low and high threshold level) were compared with standard fertilization strategy (St). Threshold levels resp. target level were 60–90 (SL), 90–120 (SH) and 180–210 (St) kg/ha N for resp. winter-summer period. For SUSON treatments, soil nitrogen content at planting was adjusted to their respective threshold level. Frequency of soil analysis was once and twice per two weeks, resp. for winter and summer period. When soil nitrogen content was beneath the threshold level, adjustment was done up to a level of 50% above the threshold level, applying fertigation with calcium nitrate. In most cases the SUSON fertilization strategy allowed, even at the lowest threshold level, to produce normal heavy lettuce crops. Significant (but moderate) weight reduction and more compact heads occurred after periods with excellent weather conditions, causing fast N uptake. Especially in such conditions timely soil analysis is essential for SUSON implementation. Another problem was leaf necrosis occurring after fertigation with high fertilizer concentration. To avoid the need of high supplementary fertilizer addition it is suggested to supply anyhow, in the second half of the cultivation period, some small basic quantity of nitrogen at every irrigation turn.