The Montreal Protocol has established a phase-out date of 2005 for developed countries and 2015 for developing countries for the use of methyl bromide, thereby forcing growers to look for alternative methods to control soil-borne pathogens in greenhouses. The soilless cultivation on inorganic substrates is one of the most efficient alternatives to soil fumigation since it provides a pathogen-free root environment at planting, while promoting earlier and higher yields. Furthermore, the cultivation on substrates offers the possibility to recycle the fertigation effluents, thereby contributing to a reduction of environmental pollution originating from leaching of fertilizer residues. Nevertheless, the availability of good quality irrigation water is a prerequisite for a complete recycling of the fertigation effluents (closed-cycle cultivations). If the nutrient solution effluents are recycled, the drainage percentage is not restricted by environmental concerns and hence the irrigation frequency may be considerably higher than that resulting in leaching fractions typically recommended for open cultivation systems. A high irrigation frequency may improve crop performance due to a higher availability of nutrients, specifically P and Mn, while maintaining constantly higher moisture levels in the root zone. As a consequence of the latter, the hydraulic conductivity, and thus also the water availability, are maintained for longer times at high levels. The only precaution regarding the application of a frequent irrigation schedule is the possible creation of excessive moisture conditions in the root zone that might reduce oxygen availability. Nevertheless, this problem may be tackled by selecting growing media with optimal physical characteristics in combination with proper placement of the media in hydroponic installations. Both root aeration and water availability in the rhizosphere strongly depend on the physical properties of the substrates, which in turn are conditioned by the shape and size of their constituent particles. However, the actual container capacity of a containerized substrate, and thus the air filled porosity and the water holding capacity, depends also on container height. Indeed, the container used to accommodate the growing medium, which determines the 3-dimensional distribution of the substrate volume, may considerably influence plant performance. Moisture retention curves described by model equations may enable an accurate estimation of the anticipated water and air holding capacity of any particular containerized substrate. Overall, growing media dominated by fine particles perform better when placed in pots or tall and narrow channels, while the cultivation on shallow bags or channels requires the use of coarser substrate grades.
