Agri Kultuur September / September 2016 - Page 12

rameters such as the O 2 and temperature in the rootzone. Using an organic medium that is prone to decomposition will mean that there is no consistency during the growth cycle (Burnett et al 2016). As the organic material is broken down both the physical and chemical characteristics, including the pH will be affected. The rootzone pH as we know has a huge effect on the availability of nutrients to be taken up by plants. Another important implication relates to the fertilisers being used. In traditional hydroponic production systems, highly soluble inorganic fertilizers are dissolved in water and then applied to crops at regular intervals. This is ideal to maintain the concentration around the roots at maximum for fast uptake. Nutrient solutions are also designed to promote uptake of all nutrients, limiting antagonism between specific nutrients and promote either vegetative or reproductive growth. When tomato plants for instance require a higher potassium concentration to improve the colour and flavour of the fruit, it is easily done by adjusting the nutrient recipe. Under current regulations, the use of these fertilizers are however prohibited and only nonchemically derived nutrients such as fishmanure, seaweed, bone meal etc. can be used. The nutrients in organic fertilizers are not in a form available to be taken up by plant roots, and few microorganisms are present in hydroponic solutions to mineralize the organic compounds into inorganic nutrients. Organic fertilizer therefore needs to be microbially pre-processed before incorporation into hydroponic solutions. The conversion of ammonium to nitrate is especially crucial since ammonium can easily become toxic to crops in a soilless system. The pre-processing is usually conducted in two or more reaction tanks to provide ammonification and nitrification of organic nitrogen compounds. However it was shown recently that adding a fish-based soluble organic fertilizer directly into a hydroponic system yield good results when an inoculum is also added to the system. Bark compost, Picture 3: Cucumber plants in a hydroponic system using untreated organic fertiliser (front) and conventional nutrient so). field soil, nursery soil, and even sea water can be used as inoculum to promote ammonification and nitrification and thereby increase the N nutrition of crops in a hydroponic system (Shinohara et al 2011). A lot of progress has thus been made with regards to the fertilization of organic hydroponic crops increasing the productivity and also the adoption of this method. Although the basic principles of organic agriculture did not take into consideration growing crops in soilless systems, this production method can definitely be classified as sustainable. References Burnett SE, Mattson NS, Williams KA. 2016. Substrates and fertilizers for organic container production of herbs, vegetables, and herbaceous ornamental plants grown in greenhouses in the United States.Scientia Horticulturae. Kelly C. and Metelerkamp L. 2015. Smallholder farmers and organic agriculture in South Africa. A literature review commissioned by the Southern Africa Food Lab and funded by GIZ. SI Projects, Sustainability Institute, Stellenbosch University, Stellenbosch. Shinohara M, Aoyama C, Fujiwara K. 2011. Microbial mineralization of organic nitrogen into nitrate to allow the use of organic fertilizer in hydroponics. Soil Sci. Plant Nutr. 57:190–203.