Perlite is a volcanic rock that has been superheated into very lightweight expanded glass pebbles. It is used loose or in plastic sleeves immersed in the water. It is also used in potting soil mixes to decrease soil density. Perlite has similar properties and uses to vermiculite but generally holds more air and less water. If not contained, it can float if flood and drain feeding is used.

Like perlite, vermiculite is another mineral that has been superheated until it has expanded into light pebbles. Vermiculite holds more water than perlite and has a natural "wicking" property that can draw water and nutrients in a passive hydroponic system. If too much water and not enough air surrounds the plants roots, it's possible to gradually lower the medium's water-retention capability by mixing in increasing quantities of perlite.

Rockwool is probably the most widely used medium in Hydroponics. Made from basalt rock it is heat-treated at high temperatures then spun back together like candy floss. It comes in lots of different forms including cubes, blocks, slabs and granulated or flock. When this medium is dry, care needs to be taken so as not to inhale any particles - inhaling such particles may carry a health risk. Rockwool will cause a higher pH level. You must adjust the pH level of the nutrient solution to counteract this. A pH level of 5.5-6.5 should suffice to create a suitable pH.

Plant physiology researchers discovered in the 1800s that plants absorb essential mineral nutrients as inorganic ions in water. In natural conditions, soil acts as a mineral nutrient reservoir but the soil itself is not essential to plant growth. When the mineral nutrients in the soil dissolve in water, plant roots are able to absorb them. When the required mineral nutrients are introduced into a plant's water supply artificially, soil is no longer required for the plant to thrive. Almost any terrestrial plant will grow with hydroponics, but some will do better than others. It is also very easy to do; the activity is often undertaken by very young children with such plants as watercress. Hydroponics is also a standard technique in biology research and teaching and a popular hobby.

Hydroponics basically means working water ("hydro" means "water" and "ponos" means "labor"). Many different civilizations have utilized hydroponic growing techniques throughout history. As noted in Hydroponic Food Production (Fifth Edition, Woodbridge Press, 1997, page 23) by Howard M. Resh: "The hanging gardens of Babylon, the floating gardens of the Aztecs of Mexico and those of the Chinese are examples of 'Hydroponic' culture. Egyptian hieroglyphic records dating back several hundred years B.C. describe the growing of plants in water." Hydroponics is hardly a new method of growing plants. However, giant strides have been made over the years in this innovative area of agriculture.

Throughout the last century, scientists and horticulturists experimented with different methods of hydroponics. One of the potential applications of hydroponics that drove research was for growing fresh produce in nonarable areas of the world. It is a simple fact that some people cannot grow in the soil in their area (if there is even any soil at all). This application of hydroponics was tested during World War II. Troops stationed on nonarable islands in the Pacific were supplied with fresh produce grown in locally established hydroponic systems. Later in the century, hydroponics was integrated into the space program. As NASA considered the practicalities of locating a society on another plant or the Earth's moon, hydroponics easily fit into their sustainability plans. This research is ongoing.

Choosing the right system--whether you decide to build it or buy it--can make or break your hydroponic gardening experience.

Carefully consider your available space, lighting, budget, and time constraints before purchasing any equipment or settling on a unit to build yourself. Also think about what you want to grow, whether you may want to expand, and recurring costs.

Hydroponic systems can be "active" or "passive." They also can be "media-based" or "water culture." Active systems rely on a pump to flow nutrient around the plant's roots and to provide aeration. Passive systems work without a pump. A wicking material draws nutrients up to the roots or the root tips are suspended in a stationary solution with the main portion of the rootball hanging in the air.

Media-based systems, such as ebb-and-flow (flood-and-drain), top-feed (drip), or bottom-feed systems rely on a growing medium to support the plants and hold nutrient solution around their roots. Most operate on timers, alternately wetting the medium to wash out salts and replenish nutrients and then draining so the plants can draw in atmospheric oxygen.

Water culture systems usually operate without media. Sometimes rockwool cubes or small amounts of gravel are used because plants like tomatoes and cucumbers get top heavy when they start to bear fruit and need help to stand upright. You can also use plastic flaps, foam rings, fiber cups, or plastic collars for plant support. Some growers tie plants to a trellis.