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With a growing world population and increasing demand for sustainable farming practices, farmers and scientists are looking for alternatives to traditional chemical fertilisers. Among these alternatives, organic fertilisers such as black soldier fly frass (BSF) and poultry droppings are emerging as promising solutions. A recent study compares these two types of fertiliser, revealing significant differences in their chemical properties, nutrient content and potential impact on the environment. This article, based on a study by Oxford University last January, highlights the specific benefits of BSF frass, positioned as the organic fertiliser of choice for sustainable agriculture.
A principal component analysis (PCA) shows that the source and production conditions of fertilisers considerably influence their elemental composition. Samples of BSF frass (IF-C and IF-L) show similar but distinct compositions from those of poultry droppings (PL-AR and PL-NC). The results show that BSF frass has physico-chemical properties and nutritional profiles that clearly distinguish it from poultry droppings. This differentiation is essential for determining the specific application and benefits of each type of fertiliser in different agricultural contexts.
pH and Electrical Conductivity(EC) :
- The pH of BSF frass is generally lower than that of poultry droppings, indicating a slightly more acidic nature.
- Electrical conductivity, an indicator of salinity and the presence of soluble nutrients, is highest in IF-C and lowest in IF-L and PL-NC. High EC can influence the solubility of nutrients and their availability to plants.
Total Carbon (C) and Nitrogen(N) :
- Carbon(C) and nitrogen (N) contents are significantly higher in BSF frass than in poultry droppings.
Total C : IF-C > IF-L > PL-NC > PL-AR
Total N : IF-C > IF-L > PL-NC > PL-AR
- A high C and N content indicates a potential to improve soil structure and provide essential nutrients for plant growth over a prolonged period.
Phosphorus (P) :
- Phosphorus (P) content is higher in poultry droppings, particularly PL-AR, but comparable between IF-L and PL-NC.
- Soluble reactive phosphorus (SRP) is highest in IF-L, which may be beneficial for the rapid release of phosphorus into the soil.
Ammonium (NH4-N) and Nitrate(NO3-N) :
- NH4-N is higher in poultry droppings, while NO3-N is higher in BSF frass. Nitrate is immediately available to plants, while ammonium can be transformed into nitrate in the soil.
Analyses reveal significant differences in macronutrient and micronutrient concentrations between the two types of fertiliser:
Potassium(K), Calcium (Ca) and Sulphur (S) :
- Poultry droppings have higher concentrations of these elements, which can be advantageous for soils deficient in these nutrients.
Magnesium (Mg) :
- BSF frass has a higher concentration of magnesium, essential for photosynthesis and overall plant health.
Potentially Toxic Elements (PTE) : BSF frass contains lower levels of cadmium (Cd) and chromium (Cr), and no detections of arsenic (As) or lead (Pb), reducing the risk of toxicity to plants and soils.
At an application rate of 5 Mgha-1 :
Nitrogen (N) :
- The highest contribution comes from IF-C, indicating a potential for vigorous plantgrowth.
Phosphorus (P) :
- The highest contribution comes from IF-L, essential for root development and flowering.
Potassium (K) :
- The highest contribution comes from PL, crucial for water regulation and disease resistance.
Poultry droppings contain significantly higher levels of microelements and heavy metals (e.g. copper(Cu), iron (Fe), manganese (Mn), zinc (Zn), sodium (Na), aluminium (Al)) than BSF frass. These elements, although necessary in small quantities, can become toxic in high concentrations.
Microbiological analyses reveal notable differences in the microbial load between the two types of fertiliser. BSF frass shows higher counts of aerobic bacteria, Enterobacteriaceae, total coliforms and E. coli compared with poultry droppings. However, pathogens are less present in poultry droppings, with some such as Enterobacteriaceae not detected in PL-AR.
BSF frass shows significant potential as an organic fertiliser, offering several distinct advantages:
Nutritional Content:
- Higher total carbon and nitrogen content than poultry droppings, providing a superior nutrient supply for sustained plant growth.
Environmental Safety:
- Lower levels of potentially toxic elements, making it a safer option for the environment.
Agronomic Efficiency:
- Similar or higher nitrate (NO3-N) content, crucial for rapid availability of plant nutrients. However, the higher microbialload in BSF frass, including potential pathogens, requires careful management and further research to ensure safe agricultural use. Heat treatment practices are required to reduce the presence of pathogens prior to application.
Agricultural practices:
- BSF frass could reduce dependence on synthetic fertilisers, promoting sustainable agriculture. Its application could improve soil health and increase crop yields.
Environmental impact:
- The lower heavy metal content and the possibility of using food waste are in line with the principles of the circular economy, reducing the ecological footprint of agriculture.
Food security:
- In addition to its use as a fertiliser, BSF's frass production offers an alternative source of protein and a nutrient-rich fertiliser, meeting the future challenges of food demand.
For professionals in the insectindustry, these results open up new prospects :
Product diversification :
- Developing frass-based products for different types of crops and soils, highlighting their specific advantages.
Certification and Quality Standards :
- Working on quality standards for BSF frass, ensuring optimum safety and efficacy for end-users.
Research and Development :
- Encourage research into frass processing techniques to reduce pathogen loads and optimise fertilising properties.
This study highlights the need for further research into the safe use of BSF frass, particularly with regard to microbial contamination and long-term effects on soil health. In short, organic fertilisers such as BSF frass offer significant advantages and promising prospects for sustainable agriculture. With proper management and research, they could play a crucial role in promoting farming practices that are more respectful of the environment and human health. For insect industry professionals, this represents an opportunity to add value to the by-products of insect production, while contributing to greener, more sustainable agriculture.
Read the full article here : Insect frass composition and potential use as an organic fertilizer in circular economies | Journal of Economic Entomology | Oxford Academic (oup.com)
