Another distinction between types of systems is whether it is closed (recirculating) or open. All NFT systems recirculate nutrient solution. In traditional NFT systems, the nutrient solution is usually “topped off” with fresh water as it is used up by plant transpiration, then discarded after 2 weeks and made up fresh to correct any nutrient imbalances that may have developed. Great care must be taken to keep recirculating systems free of nematodes and other root pathogens. In an open (non-recirculating) system, irrigation is applied in approximately 20% excess and plants draw from a reservoir in the individual bag or slab. Water does not move from bag to bag or slab to slab and does not return to a central tank for reuse so pathogen spread is less of a problem.
The main problem with open systems is there may be significant discharge of nutrients into the environment in areas of intensive production. Thus, there has been increasing interest over the last decade in systems, which recycle nutrients, with provision for disinfection of the water, and/or replenishment of nutrients before reuse. Because of the problem of counter-ions and salts not being taken up by plants, monitoring EC levels is not a good indication of whether adequate nutrients are available. In many large recirculating systems, concentrations of individual nutrients are monitored, and replenished when they drop below prescribed levels. These systems become quite complex because nutrients cannot be added as the pure element, but must be added as salts, or combinations of nutrients, making it difficult to maintain specific nutrient, pH, and EC levels. Nevertheless, provisions for reuse or at least re-capture of greenhouse runoff should be designed into new greenhouses as they may well be required in the near future.