IMPACT OF WINTER LETTUCE CROP MANAGEMENT ON NITRATE POLLUTION AND QUALITY : DIAGNOSIS AND MODELLING APPROACH

A major challenge for creating sustainable greenhouse winter lettuce cropping systems is to produce high quality lettuces of the required weight and nitrate content, while being kind to the environment (i.e. limiting N pollution). The aim of the research was to design crop management programs that achieve this objective by adopting the dual approach of integrating (a) both experiments and an agronomic diagnosis, to achieve an understanding of the main factors determining the behaviour of the cropping system and how the farmer manages it; and (b) simulation modelling to test the effects of crop management on product quality and N pollution. The diagnosis and experiments showed that the farming techniques used under plastic create widely different growing conditions in different parts of the greenhouse. This makes it harder to control production and increases the likelihood of N pollution and lower product quality.

A model of the cropping system's behaviour was built to integrate this structured heterogeneity of the greenhouse environment, the parameters set, and the model validated against independent data. Simulation results show that the system is highly sensitive to input quantities of water and nitrogen, and the model has been used to explore different ways of managing irrigation and nitrogen applications (including the balance-sheet method, management according to the state of the system, and fertigation). The model could also be used to test decision rules for managing organic matter and greenhouse climates. This would require further research to extend its scope. In the long run, it could be incorporated in a decision aid for technical management of lettuce crops.