Horticulture Principles and Practices

13.8.4 AGGREGATE HYDROPONIC SYSTEMS
Under the substrate system, plant roots are surrounded by either inert or natural organic
material. These substrates provide only limited physical support. They may be placed in
bags, troughs, or trenches.
A variety of inert and organic materials are placed into containers to provide physical
supports for plants in culture. The common aggregate systems are as follows:
Bag Culture
Bag culture is the preferred method of greenhouse vegetable production by many
producers in the United States. It is relatively easy to establish and manage. It entails
growing plants in a soilless medium placed 2–5 gallon polyethylene bags that may be
laid flat or stood upright in the greenhouse. The former layout can handle 2–3 plants per
bag, while the upright bag is usually limited to one plant per beg. The media may be
peat, rockwool, peat/vermiculite, or mixtures of various soilless mix ingredients. Plants
may be grown in pots with slits at the bottom and pot inserted into the bag (Figure 13–4)
Bag-cultured plants are fertilized and watered through drip irrigation systems.
Vertical Towers
These are similar to bag culture. Because the bags are stood on their shorter end, support
wires or beams are usually needed to stabilize them in the greenhouse. A company, Verti-
Gro, markets square Styrofoam pots that are stackable and reusable. The media types are
variable as in the case of bag culture.
Trough Culture
Materials that may be used as substrates include sand, vermiculite, horticultural rockwool,
and perlite. The critical factor in the choice of material is that it be able to hold
moisture and yet drain adequately for good aeration. Systems that use these materials
are generally referred to as sand cultures (or trough cultures). They are not widely used
commercially since it is not easy to maintain a good balance for drainage, water retention,
and aeration (Figure 13–5).
Commonly used containers include those made of concrete for permanent troughs
or beds. Other materials such as timber coated with asphalt or fiberglass may be used.
The nutrient solution is applied by using perforated pipes with suitable jets in fixed
irrigation models or directly to the soil surface. Sand beds are often sterilized annually
to control soilborne diseases and pests.
13.8.5 ORGANIC SUBSTRATES
Plants may be grown in organic substrates in various arrangements. As previously discussed,
these substrates may also be used in bag or vertical tower culture.
FIGURE 13–4 Greenhouse production of tomato in rockwool
bags. The hot-water heating pipes located on the floors of the isles
also serve as tracks for carts used for plant maintenance and
harvesting. (Source: Dr. AJ Both, Bioresource Engineering, Department of Plant
Biology and Pathology, Rutgers University, New Brunswick, NJ 08901)
Greenhouse Production 447