Open hydroponic systems, or bag culture systems, are the most widely used hydroponic systems in Africa. They are used for the production of crops such as tomatoes, peppers, melons, brinjals, roses etc. Any crop that can be trellised or that grows at least 1 m high is suitable for bag culture.
These systems are relatively cheap to install and manage, but have a higher maintenance cost than closed hydroponic systems. A major disadvantage is that the nutrient solution is not re-circulated. In Europe these systems are banned so most of the open systems are converted to closed systems. It is obvious that there is considerable waste in terms of the effluent but with careful management of the timing of the irrigations system, this can be minimized.
Characteristics of open hydroponic systems are:
- Each plant is in a separate bag
- Each plant receives its own water and nutrients
- Each water application has a complete nutrient balance
- Water is NOT re-circulated
- The risk of spreading disease throughout the system, through irrigation water, is low.
- Bags are at least 10 L in size and have holes in the sides or at the bottom.
- The most common growth media used is sawdust, wood shavings, sand or combinations of the three.
Plants are planted in a 10 – 15 L bag as illustrated below. These bags are cheap and are filled with the growing medium such as pine shavings, pine chips, sand or choir. There are many other growing mediums that can be used, it just depends on what is cheap and inert.
The bags have holes either in the side or underneath. Some growers prefer having the holes on the side and others underneath. There are pro’s and con’s to both these systems. It is generally assumed that if the holes are placed in the side of the bag, a small volume of water that can serve as a reservoir during periods where water is limited or when the water requirements exceed application. The problem is that the reservoir loses the oxygen very quickly which renders it useless for root development. Having holes in the bottom of the bag allows for at least 30 mm for extra root development. Although yields are not affected significantly.
It is important to place the dripper end piece close the stem of the small seedling to ensure minimum seedling mortality. Placing the end piece to far will prevent the water reaching the roots of the seedling. As the plant grows the end piece can be moved further away from the stem. The end piece should never wet the stem continuously since the moisture can increase the risk of Botrytis developing on the stem.