Black Soldier Fly Frass as a Natural fertilizer: properties and prospects
Overview of black soldier fly frass, its nutritional values, properties as an organic fertilizer, and market prospects
Introduction
Farming of black soldier fly has been expanding in recent years and so is the availability of BSF frass. The frass is a promising natural fertilizer that can offer an instrument to a circular agricultural production and effective plant nutrition. However, it is a relatively new product with only limited data on its efficiency in terms of plant growth.
The aim of this article is to give an overview of BSF frass as an organic fertilizer, to highlight its properties which can increase or limit its use in agriculture. It summarizes the findings of the fresh study published in Journal of Soil Science & Plant Nutrition, and opens a discussion on commercial perspectives of BSF frass.
The above mentioned study collected 30 nutrient analyses of BSF frass from producers as well as from the literature to assess BSF frass as a natural fertilizer. It analyzes the physical properties and nutritional values of BSF frass and compares it to other organic fertilizers and manures.
Key findings
BSF frass from different producers showed different values of micronutrients and macronutrients. Such variation is in general common for organic fertilizers. In case of BSF frass, it points to several practical implications.
First, the end properties of BSF-based fertilizer may depend strongly on the feeding substrate and production methods (rearing, separation mechanisms, post-processing). Secondly, it is highly recommended to rely on a careful nutrient analysis of any given BSF frass product when utilizing it for fine-tuned plant fertilization.
Finally, BSF frass provides a slower release of nutrients. On the one hand, it brings long-term mineralization effects and prolonged plant nutrition, if BSF frass is applied over a longer period of time. On the other hand, for short-term results like higher yields, it is recommended to supplement frass with a NPK fertilizer.
As regards the most critical characteristics of a fertilizer, the study revealed the following findings:
1. Dry matter and organic matter
BSF frass generally has high contents of dry matter (DM) and organic matter (OM) – 69% and 86% on average respectively.
2. Micronutrients
In general, BSF frass offers high levels of the key micronutrients. The content of iron is comparable or higher than that in other organic fertilizers based on manures, reaching up to 5000 mg kg−1 DM. The average levels of other micronutrients like copper, manganese and zinc are also comparable or higher as compared to selected manures and commercial organic fertilizers. For example, the maximum values of Cu in BSF frass reach 263 mg kg−1 DM, for Zn – 386 mg kg−1 DM.
3. pH
The average pH of BSF frass is slightly alkaline averaging at 7.46. Former studies showed that BSF larvae actively alter the substrate pH towards an alkaline environment. The pH values of frass mainly depend on feed properties and larval density.
4. N-P-K
BSF frass offers a highly balanced nutrient profile with a nearly even ratio of primary nutrients – nitrogen (N), phosphorus (P) and potassium (K). The NPK ratio is estimated at 1:0.9:1.1, which is not common amongst farm manures.
The contents of phosphorus (P) are estimated at 12.4 g kg−1 DM, the contents of potassium (K) at 29.3 g kg−1 DM.
The total nitrogen (Nt) content in BSF frass is high, estimated at 32.2 g kg−1 DM. Compared to most types of farm manure and composts, BSF frass offers much higher total nitrogen contents in fresh matter. This is caused by generally high levels of dry matter of the frass and makes lower application rates for the same amount of nutrients possible.
The carbon-to-nitrogen ratio of 14.71 is comparable to solid manure or biowaste compost. Combined with low values of ammonium (NH4+-N) at 15.78% of the Nt in frass, a high C:N ratio indicates that BSF frass ensures a more gradual nutrient release.
On the one hand, it brings long-term mineralization effects and prolonged plant nutrition, if BSF frass is applied over a longer period of time. Additionally, it can facilitate the building up of soil organic matters, immobilize soil mineral nitrogen and eventually prevent nitrate leaching. On the other hand, for short-term results like higher yields, it is recommended to supplement frass with a NPK fertilizer.
Conclusion
BSF frass is promising organic fertilizer. Due to generally high contents of dry matter it provides high levels of macro and micronutrients. It offers a slightly alkaline pH and a very balanced profile of the primary nutrients – nitrogen, phosphorus and potassium. Finally, thanks to a high carbon-to-nitrogen ratio, it ensures a slower release of nutrients and prevents nitrate leaching.
