Compilation of Black Soldier Fly Frass Analyses

In the context of black soldier fly (BSF) rearing, often the residues from production — mainly faeces but also undigested substrate — are addressed as frass in a broader sense. As BSF production is expanding, the valorization of BSF frass as organic fertilizer is gaining importance. However, in contrast to established organic fertilizers, little is known on the properties and variation of this by-product, which is essential for assessing possible application purposes. To elaborate a first overview on this relatively new waste stream, BSF frass analyses from the literature and anonymized frass analyses enquired from BSF producers were compiled. For a possible agricultural use as a fertilizer, their nutrient patterns were assessed. With a balanced nitrogen:phosphorous pentoxide:potassium oxide (N:P2O5:K2O) ratio of 1:0.9:1.1, BSF frass can be characterized as a slightly alkaline (pH 7.5), nutrient-rich compound fertilizer, with the nutrient and especially the micronutrient contents showing high variation. The comparably high carbon-to-nitrogen ratio and low shares of ammonium nitrogen indicate a limited immediate nutrient release and point to possible applications of frass as a long-term fertilizer. The use of frass as an organic fertilizer as one element in a fertilization strategy is promising. By improving nutrient cycling back to the field, BSF frass can represent an important element of sustainable circular agriculture. However, more information on BSF feed and production systems needs to be combined with the nutrient analyses to better understand the variation in frass nutrient contents.