Hydroponics

Hydroponics is a method of growing plants using mineral nutrient solutions, in water, without soil. Plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as perlite, gravel, mineral wool, expanded clay or coconut husk. Hydroponic reservoirs are typically now built of plastic, but other materials have been used including concrete, glass, metal, vegetable solids, and wood. The containers should exclude light to prevent algae growth in the nutrient solution.

While hydroponics can be much more productive for crop yields, only certain species of plants can survive for so long in water before they become waterlogged. Alternatives to hydroponics are Soil Bed Reactors and Aeroponics. However, for combining with aquaculture, hydroponics is most effective.

Types

The two main types of hydroponics are solution culture and medium culture.

Solution Culture

Solution culture does not use a solid medium for the roots, just the nutrient solution.

Medium Culture

The medium culture method has a solid medium for the roots, such as sand, gravel, or rockwool.There are two main variations for each medium, sub-irrigation and top irrigation. The solid support medium serves the dual purposes of providing structure for plant roots to grow in and surface area allowing proliferation of aerobic nitrifying bacteria, which are essential in aquaponics for converting nitrogen in the effluent to forms suited to the plants’ nutrient uptake.

Delivery

Plant roots need air in addition to water and nutrients. There are three schools of thought on delivery: continuous flow, reciprocal, or passive.

Continuous flow

In continuous-flow solution culture, the nutrient solution constantly flows past the roots. Typically a very shallow stream of nutrient solution is re-circulated past the bare roots of plants in a watertight channels. In an ideal system, the depth of the recirculating stream should be very shallow. This ensures that the thick root mat, which develops in the bottom of the channel, has an upper surface, which, although moist, is in the air such that an abundant supply of oxygen is provided to the roots of the plants. A properly designed system is based on using the right channel slope, the right flow rate, and the right channel length. A downside of this technique is that it has very little buffering against interruptions in the flow, e.g., power outages.

Reciprocal

In general, ebb-and-flow cycles in grow-beds should be about 15–20 minutes, regardless of the size or volume of the grow-bed. In its simplest form, there is a tray above a reservoir of nutrient solution. The tray is filled with growing medium. At regular intervals, a simple timer causes a pump to fill the upper tray with nutrient solution, after which the solution drains back down into the reservoir. This keeps the medium regularly flushed with both nutrients and air. The timer regulation can be achieved electronically by the the activity of water pumps, or by mechanical means, most often with a bell siphon.

Passive

Passive subirrigation requires an inert porous medium to transport water and nutrients to the roots by capillary action. Water and fertilizer are held in a reservoir and conducted to the roots as necessary, reducing labor and providing a constant supply of water to the roots. In the simplest method, the pot sits in a shallow solution of fertilizer and water or on a capillary mat saturated with nutrient solution. Since routine maintenance is much simplified, passive hydroponics can reduce the labor required to maintain a large collection of plants.

The various hydroponic media available contain more air space than more traditional potting mixes, delivering increased oxygen to the roots. Allowing air to reach the roots is particularly important in preventing root rot. Passive hydroponics also may add additional ambient humidity through evaporation. It is important in passive subirrigation to wash out the system from time to time to remove salt accumulation. Plants that require drying between waterings or a dry dormant period may fail to thrive under the constant moisture of passive hydroponics.

Many media are available for passive hydroponics, but the most common are expanded clay pellets, coconut husk chips, perlite, vermiculite, diatomite, charcoal, and rockwool. These are frequently used in combination.