Soilless growing systems (hydroponics)
Many greenhouse crops, especially vegetable crops are grown ‘soilless’, either in a growing medium or in a variation of water culture. The popular media-based cultures use either an inert substrate such as rockwool/stonewool, pumice, sand, perlite, vermiculite, synthetic foam, or an organic media such as peat, coco fibre, sawdust, bark (or a mixture of various products). The growing medium is put in bags, tubes, slabs or troughs.
Water cultures or ‘true hydroponics’ use no growing medium, or only a very small block or pot that holds the young plant in the early stage. Nutrient film technique (NFT) and deep flow technique (DFT) are examples of water cultures, where the roots lay or hang in the nutrient solution that flows through a gully. NFT is popular for lettuce and occasionally used for other crops, DFT is rarely used anymore. Aeroponics is used here and there. It involves nutrient solution being sprayed onto the plant roots that hang in an enclosed space created from plastic. Aquaponics is trialled in various places and used on fairly small scale. It involves producing crops (e.g. tomatoes) and fish in one system.
Open en closed growing systems
Soilless systems can be open, closed or partly-closed. In an open system, the surplus of nutrient solution that is not taken up by the plants drains out ('runs to waste'). In a closed or ‘recirculating’ system, the surplus drains out and is collected and re-used. This requires rather precise topping up of water and nutrients. Closed systems use 20-40% less water and fertilisers, and pose less pressure on the environment than open systems. Water cultures are nearly always closed systems, and media-based cultures were often open. Obviously there is strong pressure towards closing the systems to prevent emission of nutrients to the environment. In practice, most systems are only partly-closed, because salt or other unwanted compounds accumulate in a recirculating nutrient solution. Either a portion of the nutrient solution is allowed to seep out continuously so that the nutrient solution is renewed gradually, or the nutrient solution is dumped and renewed periodically.