The majority of greenhouse crops are grown using
artificial substrates in hydroponic systems. These substrates
are preferred to soil-based media for economic
reasons, and because of the improved control over water,
aeration, nutrition and root distribution. Traditionally,
these systems were developed as drain-to-waste or open
systems, in which excess nutrient solutions were allowed
to drain to the soil and groundwater. Crops grown under
these conditions generally are irrigated to excess (up to
40% of the nutrient solution dosed per day may be in
excess of crop requirements) to balance the variation in
transpiration and nutrient demands of the individual
plants and the variation within the system in supplying
the plants with nutrient solution [52, 81]. However,
hydroponic growers and governments have come to recognize
that for environmental reasons, the excess nutrient
solutions should not be allowed to drain to waste, but
should be collected and re-used within closed or recirculating
nutrient systems. As well as enabling good environmental
stewardship, these closed systems can reward
the grower with savings in water and fertilizer costs.