Evaluation of Insect Farming Residue (Frass) as a Phosphate Fertilizer Within the Context of the Circular Economy

Phosphorus (P) stock scarcity is driving the need to develop alternatives to mineral fertilizers. The growing production of insects for high-protein feed results in significant amounts of residues (frass), which can be used as fertilizers. However, its efficiency as such a basic indicator for promoting the recycling of these residues has been rarely assessed. This work aimed to evaluate the efficiency of frass as a P fertilizer. To this end a study was conducted involving P fractionation of frass from two different species (TM: Tenebrio molitor and BSF: black soldier fly or Hermetia illucens) together with vermicompost and a 48-day pot experiment with lettuce (Lactuca sativa). In both frasses, water-soluble P and organic P accounted for more than 30% and 50% of total P, respectively. These P fractions explained the short- and long-term effects of frasses as P fertilizer, which showed a higher P use efficiency than mineral phosphate and vermicompost, with mineral fertilizer replacement values (MFRVs) of 150 and 180% for BSF and TM frass, respectively. Additionally, frass increased P bioavailability in soils more than superphosphate and boosted C and P cycling, thereby enhancing the soil P availability to plants. Therefore, frasses can be effective alternatives to mineral P fertilizers which also contribute to the enhancement of soil health indicators.