Hydroponic plants can be grown with their roots in any of a variety of different aggregates. They are likely to be suitable, as long as they are inert (free of nutrients or chemical properties that might affect the plants); and have the capacity to deliver air and water as required to the roots.
Often the least expensive and most readily available materials may be either sand or gravel.
Sand
Typically, sand culture systems include the grow bed or trough, media, and possibly a solution tank.
The key element in these systems is obviously the sand itself. It is often salty, coming from beaches and so must be leached of excess salt before being introduced to grow beds. River sands can also be used. Sands need to have fine silty particles removed as these can cause puddling which makes the sand impervious to water. Washing away fine particles will enable the sand to drain freely.
Sand culture grow beds can be constructed similarly to those used in aggregate culture. That is, they can have U or V-shaped bottoms containing a drainage pipe. The bases of these channels can be dug into the greenhouse floor and raised timber sides used. The interior is lined with PVC or rubber sheeting.
The grow beds are constructed on a slight slope and the drain pipes feed into a larger drain at one end, from where the nutrient solution is disposed of. The plants are fed from drip irrigation nozzles or perforated pipes on the surface of the sand.
Another type of system involves grading the greenhouse floor to a gentle slope, then installing two layers 6mm thick polythene sheeting across the entire greenhouse floor. Drainage pipes are then laid at 1.2-1.8m intervals so that they slope downwards towards a collection drain at the end. Next, a layer of between 30-40cm sand is added. Once again micro-irrigation is used on the surface to drip feed the plants.
In recirculating sand culture systems the nutrient solution is returned to the reservoir tank rather than drained to waste.
In sand culture as with aggregate culture, the media must be sterilised for re-use after each crop is harvested. This is more costly and time-consuming since bleach (which can be used in gravel culture) is not an option. Chemical or steam sterilisation must be performed.
Sand culture lends itself well to low growing crops or root crops like cabbages, radishes, lettuce, turnips, eggplant, and peppers. Plants which require support are also grown by training them as cordons and attaching them to overhead wires e.g. tomatoes, beans, and cucumbers. Many herbs have also been successfully grown in sand culture including basil, mint, chives, and sage.
Gravel Aggregates
Aggregate culture is the more usual method adopted by commercial hydroponics enterprises. There are more components involved, though these will vary according to the type of system chosen.
The type of aggregate used will depend upon local materials, or otherwise the availability of materials. Whatever the choice of aggregate, it must not be too soft that they break down (can clog pipes and interfere with nutrients and pH), it also need to be freely draining, but be capable of holding water in the pores between them. Limestone, sandstone, and so forth should be avoided for these reasons. For subirrigation type systems, gravel works well. In drip fed systems pea gravel and other smaller particle-sized aggregates are better since solution is able to spread laterally towards roots more easily.
One of the drawbacks with using aggregate as a growing medium is that it has to be replaced after some time. This is because of root build-up in the media, and it is usually too costly to remove all roots and debris. Sterilising the gravel after the harvesting of each crop is also necessary to limit the spread of pathogens. However, due to the inevitable build-up of roots in the media sterilising becomes less efficient.
Most gravel culture utilises subirrigation methods. The ebb and flow method is a widely adopted example of a subirrigation system which can be used with gravel culture, as well as other media. The grow beds are filled with gravel and positioned over the nutrient reservoir. A submersible pump in the reservoir is connected to a timer, or a moisture sensor in the grow beds, which triggers the pump to push water onto the grow bed. Solution should cover the plant roots up to an inch or two from the surface. The timer then switches the pump off and the nutrient solution drains out of the grow bed back into the reservoir. As the solution drains away the roots are oxygenated once again. Coarser aggregates require more frequent irrigations than finer grade ones. This system has relatively low maintenance requirements. The nutrient solution is changed every 4-6 weeks.
In other systems the nutrient solution is supplied to the top of gravel beds via micro-irrigation pipes. The nutrient solution passes down through the beds and the roots of plants where it is collected in perforated drain pipes which return it to the nutrient reservoir.
PVC pipes, fittings, pumps and tanks (or PVC lined tanks) are preferred in these systems because metals are easily corroded when they are in constant contact with nutrient solutions and some release significant toxins. The grow beds may be timber and PVC lined, or concrete depending upon financial restrictions. Nutrient tanks are often constructed of concrete and lined with bitumen, and must be large enough to contain enough water to flood the beds. A float valve can be included in the tank to help maintain the water level.
In drip feed or trickle irrigation systems piping is either inserted directly into each pot using emitters, or perforated pipes may be used. The nutrient solution is applied from above the media and runs downwards and laterally to the roots. The nutrient solution is supplied to the feed pipes either directly from the reservoir tank or via injectors.
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