One potential advantage of aquaponic systems is reduced resource consumption compared to separate fish and plant rearing systems, but little has been published on the costs of providing nitrogen and phosphorus nutrients by fish in comparison to the purchase of inorganic fertilizers. The cost of providing nitrogen and phosphorus from six commercial fish feeds was compared to 9 commercial agricultural fertilizers. Waste production from fish was corrected for excretion of urea and the impact of feed wastage. For a typical aquaponic system without mineralization of fecal solids, the soluble nitrogen and phosphorus excretion ranged from 36.9 to 44.0 g/kg feed and 1.4 to 3.7 g/kg feed, respectively. The cost to provide 1 kg of nutrients from feeds ranged from $15-$29/kg for nitrogen and from $115 - $583/kg for phosphorus. Compared to the purchase of these elements from inorganic fertilizers, the feeds were 7-14 times more expensive for nitrogen and 17-88 times more expensive for phosphorus. The feed/fertilizer cost ratio ( ) required to replace the nitrogen and phosphorus in 1 kg of feed ranged from 2-4 times for monoammonium phosphate to 14-17 times for anhydrous ammonia and triple superphosphate.
Based on the specific conditions and assumptions in this work, the economic value of nutrients provided by fish in aquaponic systems has been greatly over-stated.