In Europe, Canada, and in large greenhouse complexes in the US, 95% of greenhouse tomatoes are grown on inert, artificial substrates. Sometimes these systems are referred to as ‘hydroponic’, but generally the term ‘hydroponic’ refers to systems such as Nutrient Film Technique (NFT) in which no solid substrate is used and plants are grown in water-filled troughs (Fig. 4). Systems utilizing solid substrates other than soil are more properly termed ‘soilless culture’. Yields are comparable in rockwool, perlite, and NFT tomato production systems[12].
Rockwool is probably the most common substrate for soilless culture, but perlite is also widely used and growers are always experimenting with new substrates. Rockwool is produced from basaltic rock, and is usually provided as plastic-wrapped 15 x 7.5 x 91 cm long slabs of spun wool. Initially the rockwool has a high pH. Up to 6 plants can be placed in each slab [13] with a drip irrigation tube by each, but normally only 2-3 plants are placed in each slab. These slabs (Fig. 5) are generally placed on Styrofoam boards, which insulate the plant from the greenhouse floor (assuming the floor is not heated).
Perlite is a volcanic glass formed when lava cools very rapidly, trapping small quantities of water. The glass is crushed and heated, vaporizing the trapped gas, which expands the material into foam-like pellets. Initial pH of perlite is near neutral. Typically 0.03 m3 of medium to coarse perlite is sealed into opaque, white-on-black polyethylene bags treated with enough ultraviolet inhibitors to last two years. Bags are typically 1.1 m long by 0.2 wide and contain 3 plants. Drainage slits are cut about 25 mm from the bottom of the bag to provide a shallow reservoir for nutrient solution. As plants transpire, water rises from the reservoir through capillary action, maintaining a constant moisture profile as long as the reservoir is maintained. Reportedly, perlite culture requires less frequent watering and less leaching and runoff than rockwool systems [14].
Both rockwool and perlite have: excellent aeration and water-holding capacity; are sterile and lightweight when dry; are easily installed and cleaned-up; and both types of media can be unwrapped, steam sterilized, rebagged and used for a second crop and even more in some cases. Successful media re-use without sterilization has also been reported. Yields in Florida were comparable for new rockwool (2 brands), 1 and 2-year old rockwool, upright peat bags, and layflat peat bags [15].
Costs for prefilled rockwool and perlite bags are high, however, and in smaller ranges, especially in the southeastern US, peat-based substrates are more common. Commercial potting mixes are often mixed with vermiculite, perlite, coconut coir, pine bark, or locally available composts, such as rice or peanut hulls. Growers can also buy perlite in bulk and fill their own bags to reduce costs. Relatively few growers still grow directly in the soil, mainly because of the cost of steam sterilization and lack of control over the root-zone environment.