Picture 1: Tomatoes in a hydroponic system in a climate controlled greenhouse can increase the yield significantly. USDA-accredited certification agencies have certified hydroponic operations as organic. Since hydroponics employs a soil-less system, produce from this system will not be certified as organic by most certification bodies. Moving from soil to growing mediums in the 1960s is however one of the fundamental reasons for obtaining higher yields in many greenhouse crops. In a soil-less system many soil-borne diseases can be prevented or easily treated. Soilless systems also allows for the accurate application of water and nutrients, optimising the growth and yield of crops. A Picture 2: Root development in rockwool; a commonly used growing medium in soilless production systems. high productivity is essential for justifying the high cost of implementing and running a greenhouse hydroponic operation. Should a growing medium then be used that contains sufficient organic material to support microorganisms other factors might influence the productivity of the system. First of all the physical and chemical stability of the growing medium; in an inert medium such as rockwool or perlite there is no change in the composition and structure of the medium throughout the crops growth period. This makes it easier to optimise water and fertilizer application as well as managing other pa- Figure 2. From Shinohara et al (2016). Optimum dose of organic fertilizer. The optimum concentration of organic fertilizer required to promote nitrification in the presence of 5 g/L of bark compost inoculum was determined. Fish-based soluble fertilizer at 0.5, 2.5, or 5 g/L was added daily for seven days from the start of the experiment.