Studies for establishing organic hydroponics have been conducted for long time. Kennedy Space Center had studied organic hydroponics for crop production in space [2].
It was necessary to develop the method to generate nitrate from organic fertilizer via ammonification and nitrification, because most of crops are nitrate-phylic but not ammonium-philic.
It is easy to generate ammonium from organic fertilizer by saprophytic microorganisms. However it was difficult to degrade organic fertilizer to nitrate efficiently
because the growth of nitrifying bacteria, such as the obligate chemolithoautotrophs Nitrosomonas spp. and Nitrospira spp., is particularly inhibited by the presence of organic compounds (Jensen 1950; Quastel and Scholefield 1951; Rittenberg 1969; Smith and Hoare 1977; Krummel and Harms 1982; Takahashi et al. 1992; Stutte 1996; Xu et al. 2000; Tomiyama et al. 2001).