Irrigation
Large amounts of water are needed for greenhouse production. For greenhouses in the Netherlands[35], water consumption is estimated at 0.5-0.9 m3 per m-2 greenhouse area per year. It seems likely that in lower latitude greenhouses, water requirements would be higher. More than 90% of this water is ‘lost’ through transpiration, which serves to cool the plant and to transport various materials from the roots to the leaves and fruit. Before committing to building a greenhouse, it is important to ensure that both water availability and water quality is high. Electrical conductivity should be below 0.5 mS cm-1, pH from 5.4 to 6.3 and alkalinity below 2 meq L-1. This topic is discussed further in the chapter on watering and fertilization.
Frequency of irrigation varies with the rooting volume and water-holding capacity available. In rockwool slabs, rooting volume is very restricted, and slabs may be watered up to 30 times daily. Watering frequency in perlite systems is usually less than in rockwool systems, but more frequently than in peat bag systems, which are watered 3-4 times daily. In soil systems, daily irrigation is most common.
In rockwool production, to ensure sufficient water and fertilizer supply to all plants in the greenhouse and to prevent increasing solute concentrations in the substrate, about 20% extra nutrient solution is usually supplied, which drains out into the soil, or in some cases is recycled. There is increasing interest in having systems capable of recapturing this leachate, since at some point, discharge may be prohibited or require a permit. In the Netherlands, an increasing amount of the greenhouse vegetable acreage uses recirculating systems to reduce fertilizer discharge to the environment.[36]