Application of soilless culture technologies in the modern greenhouse industry – A review

Soilless culture systems (SCS) are increasingly adopted as a major technological component in the modern greenhouse industry. The core advantage of soilless culture, frequently referenced to as "hydroponics", is the independence of the crop from the soil which, as a natural medium, is heterogeneous, accommodates pathogens, tends to degrade in monoculture systems, and may be infertile, saline or sodic. The cultivation on horticultural growing media (GM) such as rockwool, perlite, and coconut is worldwide the most frequently used SCS for production of fruit vegetables and cut flowers. Water culture systems such as floating hydroponics, Nutrient Film Technique and aeroponics are mainly used for production of leafy vegetables. Modern, fully automated fertigation heads are used for the preparation and timely supply of nutrient solution (NS), which serves both the nutrition and irrigation of the plants. In soilless culture, the NS that drains out of the root zone can be easily collected and recycled, thereby considerably increasing the water use efficiency and minimizing environmental impacts arising from fertilizer residues. The spread of pathogens via the recycled effluents is a challenge that can be encountered by introducing a suitable system for their disinfection before reusing, based mainly on UV radiation, slow sand or membrane filtration, or a chemical treatment (mainly O 3 , H 2 O 2 or chlorination). In SCS, the NS composition has to be adapted to the composition of the water used for its preparation, the plant species and even the cultivar, the growth stage, the season of the year and the current climatic conditions, and this is a challenge that can be encountered by using modern information and computer technologies. Last but not least, the frequency of irrigation in GM-grown crops is high due to the limited volume of rooting medium per plant and has to be efficiently controlled. Suitable automation technologies are mostly based on real-time measurement of parameters related either to the greenhouse microclimate (e.g., solar radiation, vapor pressure deficit, air temperature) or to the GM water status (water tension or content).