Hydroponics is the act of growing plants without the use of soil. When growing plants in soil, water serves to make nutrients in the soil soluble and provides oxygenation for the plant's roots. In Hydroponics, however, the nutrients are instead made available directly in the water. In the most abstract sense, a hydroponics system requires a nutrient rich water supply, a means of oxygenating the water supply, and a means to transport the water to the plants. Since nutrients can be expensive, typical hydroponics systems are recirculating – the runoff from transporting water to the plants returns to the original water source. In the following, we discuss the main parameters that should be considered in a hydroponics system. This description is not intended to be complete, but rather gives an overview of the parameters that are most likely to affect a hydroponics system.
Water chemistry refers to the makeup of a water source. In hydroponics, we chiefly care about three factors: pH, concentration of Macro-nutrients, and oxygenation. pH is important due to nutrient lockout. The various nutrients within a water source are more easily absorbed by a plant's roots based on the pH of the water source – too basic or too acidic, and the plant will not be able to absorb the nutrients from the water, even if they are present. The desired pH range will vary depending on the plants in the hydroponics system, but typical values are between 5.5 and 6.5.
Macro-nutrients refer to Nitrogen, Phosphorus, and Potassium (fertilizers typically list these as relative concentrations: 10-10-10 means the nutrients are balanced) – plants use these three in greater concentrations than other nutrients like Iron and Magnesium, which are called Micro-nutrients. The particular concentrations your system will need depends heavily on the plants you plan on growing. Leafy vegetables like lettuce and cucumbers need a higher concentration of Nitrogen, whereas others (like tomatoes) require a more balanced mixture. In any case, hydroponics systems use special nutrient mixes designed for hydroponics – regular fertilizers are not water soluble enough for use in hydroponics, and will cause buildups that can be difficult to get rid of. As the plants use up the nutrients in the water, more fertilizer will need to be added. The common way of detecting when it is time to add more is with an Electrical Conductivity (EC) Meter. These meters measure how conductive a solution is. This is important for hydroponics because we typically use distilled water (or de-ionized water) and add nutrient solution. Since the nutrient solution is the only source of ions, EC is a reasonable measure of nutrient concentration. EC is not a direct measure of nutrient concentration, however, and if other sources of ions make their way into the water source, then EC will not be a very accurate measure of nutrient concentration.
Finally, oxygenation is a critical factor in the water source. If the water has low dissolved oxygen, the roots of a plant can 'drown'. In container gardening, this manifests as over watering, and the symptoms are very similar to under watering. Since hydroponics systems lack soil to retain water, the major concern is under oxygenated water. Luckily, we don't need to worry about over oxygenating the water. For the majority of setups, a small air pump with an air stone is sufficient – check the manufacturer's suggested gallon rating when choosing an air pump. Oxygen can also be introduced when the runoff returns to the water source if there is a waterfall (or the water free falls through air).
Watering cycles do not seem to be well researched, but the common wisdom states a 15-on-45-off cycle. This can be done all day, or the cycle can be reduced at night. However, certain hydroponics setups require constant water flow – Nutrient Film Technique (NFT) is an example of constant water flow. For more traditional systems, the cycling allows the water to drain completely from the roots of the plant – allowing the roots to come into contact with air directly – before the next cycle begins. This can also help prevent mold buildup on the roots. In any setup, drainage is very important. It is not typically an issue with drip-line or NFT systems, since the water can immediately drain back to the reservoir, but ebb-and-flow systems must typically employ a siphon or external pump so that the grow-bed can fill up before draining completely.
Finally, lighting is important for photosynthesis. For outdoor hydroponics systems, the available lighting is usually sufficient. However, indoor systems almost always lack enough sunlight for proper plant growth. Furthermore, indoor lights typically do not emit the proper wavelengths that plants use for photosynthesis. There are many commercially available lights that do produce the light plants need, and some of the newer lights use LEDs, which produce much less heat than older bulbs. The bulb should be placed directly over the plants, but care should be taken that the bulb is not too close so that any heat produced will not affect the plants. Common lighting cycles seem to match the sun cycle – about 12 on, 12 off. While it is very important that the plants receive enough light, it is also important that the reservoir gets no light – algae can grow in the reservoir when introduced to light. While algae is not directly harmful to the plants (though algae buildup on roots can be detrimental), the biggest impact is to the nutrient concentration. Since algae use nutrients just like your plants do, if algae begins to grow in your reservoir, you will need to add nutrients more often to feed your plants. Since this can become expensive, it is vital that as little light as possible is permitted to enter the reservoir.
There are many more parameters that can be monitored in a hydroponics system, and here we presented the most obvious ones. If the suggestions described above are followed, there is a strong chance your system will work well. Tweaking additional parameters (such as humidity) may optimize plant growth, but is not necessary for creating a strong, productive hydroponics system.