Irrigation management of European greenhouse vegetable crops

In Europe, most greenhouse vegetable crops are grown in soil. Where soilless systems are used, apart from in The Netherlands, Belgium and France, they are mostly free-draining systems, in which nutrient solutions are not recirculated. Both soil-grown and free-draining soilless systems commonly have large nitrate (NO3-) leaching loss. Irrigation is a major contributing factor to NO3- leaching loss. Irrigation management of greenhouse vegetable crops needs to be improved to reduce the appreciable N loss to subterranean water and surface water bodies. This article reviews the state-of-the-art, of methods and tools that are available, or are being developed, to optimise irrigation management of both soil- and soilless-grown vegetable crops in greenhouses. Adaptions for greenhouse conditions and cropping cycles of the FAO56 approach to calculate crop water requirements are reviewed. Attention is paid to (i) the developments and suitability of various adaptations of the Penman-Monteith FAO56 equation and simpler equations to calculate reference crop evapotranspiration (ETO), and (ii) equations to calculate crop coefficient (K-c) values under greenhouse conditions, in which cropping cycles may differ appreciably from those of outdoor crops. The various classes of soil/substrate moisture sensors that have been used in greenhouse crops are reviewed, regarding their general suitability and practical use. Their use in both soil and substrate is considered, as are the effects of salinity and the use of some sensors to measure the salinity of the growing media. The use of various plant sensors with vegetable crops under greenhouse conditions is reviewed. The results of a survey that assessed, for greenhouse vegetable crops in Europe, the use of different (a) types of irrigation systems, and (b) irrigation management methods, for both soil and soilless production are presented. Following the revision of this information, recommendations are made regarding the scientific and practical value of the available tools and technologies to aid growers to optimise irrigation management of vegetable crops grown in greenhouses.