Optimizing nitrogen and water inputs for greenhouse vegetable production

Greenhouse vegetable production systems require high N and irrigation inputs. Commonly, these systems are associated with environmental problems caused by nitrate leaching. Given increasing societal pressure to reduce these problems, there is a requirement to optimally use N and water inputs. Optimal management practices will be influenced by the greenhouse technological level, the growing medium and the quality of the available water supply. Four broad categories of greenhouse vegetable production can be considered: (i) high technology (HT) in soil, (ii) HT in substrate, (iii) low to medium technology (LMT) in soil, and (iv) LMT in substrate. Examples of HT systems are glasshouses in The Netherlands and Belgium, and of LMT are plastic greenhouses in the Mediterranean Basin. Combined drip irrigation and fertigation are being increasingly used which provide a high technical capacity for precise N and irrigation management. Tools are required that enable growers to take advantage of this technical capacity. For optimal irrigation management, suitable tools are decision support systems (DSS) incorporating simulation models to estimate daily crop water requirements and soil/substrate moisture sensors. For optimal N management, suitable tools are DSS employing simulation models that estimate N fertilizer requirements based on an N balance approach combined with soil or soil solution testing. Crop/plant monitoring is potentially useful, but as yet further work is required. For both irrigation and N management, the recommended approach is combined prescriptive-corrective management, which can also be described as combining modelling and monitoring approaches whereby models are used to prescribe management plans and monitoring is used to identify when adjustments are required.