Agri Kultuur January / January 2018 - Page 14

levels to plummet below the minimum. Any decomposing solid waste present in the canal would exacerbate this problem, further diminishing DO. Thus, aeration is required. The simplest method is to place several small air stones in the canals. The air stones should release about 4 litres of air per minute, and be arranged every 2–4 m 2 of canal area. In addition, Venturi siphons can be added to the water inflow pipes to aerate the water as it enters the fish tank and on the back-pressure line in the biofilter. Do not add any fish into the canals that could eat the plant roots, e.g. herbivorous fish such as tilapia and carp. However, some small carnivorous fish species, such as guppies, mollies, or mosquito fish, can be used successfully to manage mosquito larvae, which can become a huge nuisance to workers and neighbours in some areas. Planting in a DWC Unit: As mentioned previously, this method involves suspending plants in polystyrene sheets, with their roots hanging down into the water. The polystyrene sheets should have a certain number of holes drilled to fit the net cups (or sponge cubes) used for supporting each plant. The amount and location of the holes is dictated by the vegetable type and the distance desired between the plants, where smaller plants can be spaced more closely. The standard number is normally approximately 25 holes for every 1 m 2 . of the raft. In Large-scale units, one should clean the rafts with water to remove dirt and plant residues and immediately repositioned them with seedlings in the canals to avoid any stress to the nitrifying bacteria. Guide to construct the DWC Aquaponics system: Layout of tunnel, greenhouse, room or outside area for the system: It is important to plan your Aquaponics system according to space available, enough light as well as access to electricity and water. Wind and insects will damage the plants and ideally, one should try to close the system off. For this article, the plan is to construct the system indoors. In this case, sun would not be a problem for this demonstration unit will make use of red and blue LED lighting to assist with the photosynthesis of the plants. So, in the planning phase, it is important to consider room for openings (doors and hallways) as well as sufficient access to the various parts of the system i.e. draining of waste, feeding and general maintenance and inspection. Seedlings can be started in a dedicated plant nursery in soil blocks or a soil–less medium. Once these seedlings are large enough to handle, they can be transferred into net cups and planted into the DWC unit. The remaining space in the net cup should be filled with hydroponic media, such as volcanic gravel, Rockwool or LECA, to support the seedling. It is also possible to simply plant a seed straight into the net cups on top of the media. This method is sometimes recommended if vegetable seeds are accessible because it avoids the transplant shock during replanting. When harvesting, be sure to remove the whole plant, including roots and dead leaves, from the canal. After harvest, the rafts should be cleaned but not left to dry to avoid killing the nitrifying bacteria on the submerged surface AgriKultuur |AgriCulture 14