In layman's terms, hydroponics is the science of growing plants without soil-- although the plants may or may not be suspended in a solid medium such as gravel, or expanded clay balls.
Soil retains minerals and nutrients, which "feed" flora, as we all know. Plant roots can't absorb dirt, however; when water passes through soil, it dissolves and collects some of the nutrient particles embedded. This "food" solution is absorbable as a liquid. As you can see, the soil itself is not an integral part of a plant's feeding cycle-- it is simply a stabilizer for the roots, and a convenient filter.
Why eliminate the soil?
Plants breathe air, just like humans. School children are taught a simple lesson: plants take in carbon dioxide, and release oxygen. The entire plant-- not just leafy material-- contributes to this process.
If not properly maintained, soil can retain too much moisture, effectively suffocating ("drowning") a plant's root system. Alternatively, if the soil doesn't contain enough moisture, the plant will be unable to absorb the nutrients it needs to survive.
The roots of a hydroponic plant have constant access to both air and water, and it can be much easier to maintain that balance since the roots are typically visible.
The average plant needs at least five things to survive. Air, water, nutrients, minerals, and light. So long as you can provide these things in plenty, your plants should stay healthy.
Growing your own food can be a rewarding experience. It's a good way to save money on pesticide-free produce, and you'll know it wasn't shipped from a third-world serf farm supporting bad business. If your hydroponic system is indoors, you can grow food during the off-season in winter, too.
That being said, there may be more efficient systems out there for the home grower. I created this instructable to inform, more than anything.
After all, if anything's worth doing, it's worth doing right. Gotta do your research, kids.
Step 1: Substrate 101
Even simple rock can alter the PH of your system. When checking your PH balance, be sure to check it after it has circulated through your substrate.
In the moisture-rich conditions hydroponics typically provide, substrate can be generally classified into the following categories: sandy, granular, and pebbled.
Sandy environments consist of particles between .06 (fine) and 2mm (coarse) in diameter. Even coarse sand retains a considerable amount of water (except in comparison to soil), and is not generally considered appropriate for use in a hydroponic system. If you use a pump, for example, the small particle size may lead to clogging. However, it is cheap and readily available, and, when wet, is heavy enough to provide a reasonable anchor for plant roots.
There is some absorbable nutrient in sand. Typically speaking, the nutrients latent in sand culture vary widely on the substrate's color and origin. Most sand contains a large quantity of shell fragments, and thus has a high calcium content.
Black sand usually has a high magnetite content originating from volcanic rock, known for its fertility. Orange or yellow sand might be an indicator of a high iron content.
White sand tends to be very high in silica, which helps build healthy cell walls in plantlife. Diahydro, for example, is made from diatoms, a type of algae.
Sand is semi-reusable. Sterilizing it between uses can be messy. (Sand can be sterilized by boiling it in water for extended periods of time.)
Granular particles range between 2 and 4mm. This may consist of gravel, or plant mulch.
Stone gravel makes a heavy, non-biodegradable anchor for plant roots, and is highly recommended for use in hydroponic systems. Stone gravel contains very little latent plant nutrition, just like sand. There are several grades of gravel readily available to choose from.
Creek rock and Pea Gravel consist of round, shiny stones. The smooth shape of these stones allows for great aeration and root growth, although the drainage may be excessive.
Crushed rock is typically made by crushing large chunks of limestone or dolomite into smaller pieces. Crushed rock has sharper edges than creek rock, and tends to interlock better. This tighter knit makes for higher water retention, although limestone tends to weigh less. Limestone is a strong alkali. Check your PH, and balance accordingly.
Stone-based substrate is highly re-useable. It is considerably less messy than sand to boil for sterilization.
If weight is not a concern (ie: the plants you grow are not expected to reach considerable heights) you might consider using a plant mulch, such as peat mulch, cedar shavings, or coir (coconut peat). Mulches retain a high quantity of water, but also breathe very well. Mind you, they are also highly degradable, which can lead to clogged pumps, and wood shavings often contain aromatic oils which can inhibit plant growth. Mould and algae growth poses a higher risk when mulches are involved, but pose one considerable advantage over rocky substrate: they can be composted and replaced with fresh material. It does not need to be stored. I would n't suggest re-using 'em, anyway. This is especially convenient if you use hydroponic systems exclusively to start seeds, or grow during the off-season.
Pebbled substrate measures between 4 and 64mm.
Stone pebbles have the basic characteristics of creek rock. They are typically smooth, often shiny, and the gaps between the stones make for low water retention and high aeration. The shinier the stone, the worse the water retention will be. A matte or pockmarked surface indicates a porous stone, which will stay damper, longer, whilst still providing excellent aeration. Pebbles-- especially the porous variety-- can explode when heated for sterilization.
A common alternative to these substrates is mineral (rock) wool. You've probably seen it used as insulation in housing. Rock wool contains fiberglass, and it can be absorbed into the body by inhalation-- irritating eyes, skin, and lungs. It needs to be treated before it is a tolerable substrate for plant growth. Altogether, I don't recommend its use.
As I've said, you should boil your substrate between uses to sterilize it. Bacteria love warm, wet environments and will probably thrive in a hydroponic system.
Just a heads-up, here... algae loves wet and warm (and lukewarm... and cold) systems, too, and it can look unsightly. If you care about appearances, boiling your substrate between uses will discourage blossoming, but if you use grey (recycled from previous use) water you'll be fighting a losing battle.