Validation of Rustica bio-based fertilisers from fruit and vegetable waste: upscaling from laboratory to field

Introduction
Production of novel bio-based fertilisers from fruit and vegetable waste can reduce the dependence on mineral fertiliser production and mining, and decrease the nutrient losses in the form of agricultural residues. Moreover, applying such bio-based organic fertilisers into the field can increase soil and plant health. In the Rustica project, 5 distinctive technologies producing fertiliser building blocks from organic residues are considered: carboxylic acid platform (CAP), microbial biomass production, electrodialysis, insect breeding yielding insect biomass and frass, and biochar production. Starting from laboratory production over small-scale testing towards pilot production, blending and field trials, the project aims to develop environmentally sustainable and marketable end-products.
Methodology
First of all, biochar, insect biomass and frass, and microbial biomass and NPK solution from CAP solution, all produced at laboratory/small scale from various fruit and vegetable wastes, were characterized by analyses of chemical and biological parameters (nutrients and carbon contents, stability, etcetera) and incubation trials after addition to the soil. Based on this information, blends composed of these building blocks (with compost as a potential additional one) were designed to match fertiliser and soil quality requirements for several soils, crops and regions (BE, FR, IT, ES and Colombia). Effects of addition of the prototype blends to soil were assessed in incubation trials (28 days). During and after two consecutive series of incubation trials, the blend composition was adapted based on the obtained results. After that, building blocks for the bio-based fertilizers were produced on pilot scale. After mixing to the desired blend composition, the bio-based fertilisers are validated in field trials in BE, FR, IT, ES and Colombia.
Results and discussion
Building blocks characterization showed that microbial and insect biomass produced from the fruit and vegetable wastes were very rich in nitrogen (6-12% N), with a high and easily degradable organic matter (OM) content, interesting to boost the soil microbial life. Both biomasses could significantly increase N availability and activity of microbial biomass when added to soil as highlighted by the incubation experiment. Insect frass had a large electrical conductivity and high K content. The N content and boosting effect on microbial activity was for insect frass somewhat smaller than for the microbial and insect biomass. High pH (up to 10.5), low EC, low TN (0.8-1.7%) contents and high contents of stable OM were observed for the biochars, with no significant increase in N availability and microbial activity in the incubation experiment. When biochar was mixed with N delivering building blocks, the N delivering capacity of the blend was decreased. At the NERM conference, the results of an additional 16 weeks incubation trial of soil mixed with microbial biomass, with and without biochar, will be presented. It will be verified if biochar can release the immobilized N later on. The NPK solution showed a rather high salt, high K and low N and P content. Feedstocks (particular fruit and vegetable waste) influenced partly the building block characteristics, especially for insect frass.
The blends, adapted after building blocks characterization and produced at pilot scale, are validated in field trials. From June 2023 to February 2024 a trial with leek was run in Flanders (Belgium). Preliminary results show that two Rustica bio-based fertilisers give similar to higher leek quality and less N soil leaching compared to conventional practices (mineral fertilizers).
Conclusion
The widely varying characteristics of the different building blocks produced from fruit and vegetable waste allow for the design of distinctive fertiliser blends that are adapted to soil and crop requirements. Compositions of the blends were finetuned according to the outcomes of the incubation trials and further tested in field trials. In parallel, a multi-actor approach supported by market, lifecycle and legal analyses will foster the implementation potential of these technologies.