Soil Remediation and Improvement Through the Application of By-Product Amendments

Soils are fundamental to terrestrial ecosystems, human health, food production, and global environmental stability [1,2,3,4]. However, the combined threats of soil contamination and degradation—alongside growing pressures from industrial and agricultural waste production—require urgent and integrated responses [5,6,7]. This Special Issue of Environments, entitled “Soil Remediation and Improvement through the Application of By-Product Amendments”, responds to this challenge by exploring how various waste-derived materials can be repurposed as soil amendments for both remediation and fertility enhancement [8,9,10,11].

The loss of productive soil due to contamination (e.g., from mining or industrial activities) and the simultaneous overproduction of organic and inorganic by-products represent two critical and interconnected environmental issues [12,13]. The aim of this Special Issue is to connect both problems by highlighting sustainable solutions based on the circular economy. The articles published here present innovative uses of by-products such as biochar, hydrochar, compost, digestate, ashes, and other organic materials for the rehabilitation of degraded soils (e.g., mine soils, brownfields) and the enhancement of soil quality in agricultural settings.

Among the contributions, Sinclair et al. (contribution 1) examine the composting of spent coffee grounds (SCGs) and its impact on sunflower germination and early development. Their study demonstrates that composted SCGs, when properly aged, can improve water retention and reduce phytotoxicity, supporting their reuse as soil amendments. Similarly, Huslina et al. (contribution 2) assess the phytoremediation of arsenic-contaminated mine waste using native Australian plant species supported by biosolid-derived biochar. Their results underscore the importance of selecting suitable amendment–plant combinations to optimize both biomass production and contaminant uptake.

Regarding organic waste valorization, Licitra et al. (contribution 3) provide an insightful case study combining the composting and vermicomposting of organic municipal waste in Sicily. By integrating Eisenia fetida earthworms, they achieved significant reductions in the carbon-to-nitrogen ratio and heavy metal content, demonstrating the potential of this approach for nutrient recycling and soil health restoration. De Carolis et al. (contribution 4) further show that digestate derived from livestock waste enhances lettuce biomass and photosynthetic performance, even under low copper stress, pointing toward the efficacy of anaerobic digestion by-products in sustainable horticulture.

The Issue also includes studies highlighting the effects of by-product amendments on soil microbial communities and nutrient cycling. Deinert and Schmalenberger (contribution 5) demonstrate that ash-derived fertilizers stimulate microbial phosphorus mobilization in the rhizosphere of Lolium perenne, offering an alternative to conventional superphosphate fertilization. Works like this, due to their effectiveness, have been carried out by researchers such as Schiemenz et al. [14] and Doni et al. (contribution 6), who examine the combined use of zeolite and winery waste compost in vineyards, showing improvements in nutrient retention, carbon sequestration, and enzymatic soil activity—especially with a 10% zeolite blend.

In the area of sustainable crop production, Chanda et al. (contribution 7) present a compelling case for cyanobacteria biofertilizers in okra cultivation. Their experiments highlight comparable yield performance between cyanobacteria and synthetic fertilizers, while also promoting iron uptake and reducing chlorosis. Similarly, Carril et al. (contribution 8) evaluate the application of wood distillate (pyroligneous acid) to three leguminous crops, showing significant improvements in yield and mineral composition, particularly for lentils.

Finally, Sabia et al. (contribution 9) present a broader perspective by calculating the carbon footprint of livestock feed production using by-product-based concentrates. Their findings reveal that incorporating agricultural by-products can reduce carbon emissions by up to 37%, supporting the idea that soil amendment strategies can also contribute to climate mitigation goals when considered across the food system [15].

Collectively, the articles in this Special Issue represent valuable contributions to the literature on soil remediation, organic waste valorization, and sustainable land management [16,17,18]. They highlight that the use of by-products is not only environmentally sound but also agronomically effective, especially when paired with biotechnological and ecological strategies such as phytoremediation, vermicomposting, and microbial activation [19,20,21].

Beyond highlighting case studies within diverse agroecological and industrial contexts, the articles in this Special Issue collectively underscore a paradigm shift toward integrated and circular approaches to soil remediation and improvement through the application of by-product amendments. The novelty of individual contributions lies in their interdisciplinary exploration of how waste-derived materials—such as biochar, compost, digestate, ashes, pyroligneous acid, and vermicompost—can be strategically applied to degraded or nutrient-poor soils. These studies go beyond laboratory assessments to offer field-applicable evidence on improving soil structure, microbial function, plant productivity, and contaminant immobilization. Importantly, many contributions adopt a systems perspective that connects soil recovery to broader goals such as waste valorization, climate change mitigation, and sustainable agriculture. Future research should build on these findings by refining amendment combinations for site-specific challenges, monitoring long-term ecological outcomes, and scaling up successful practices within circular bioeconomy frameworks. Overall, this Special Issue posits leveraging by-product-based soil amendments as crucial to the transition toward more resilient and regenerative land use systems.

We thank all authors for their high-quality contributions and the reviewers for their rigorous assessments. It is our hope that this collection inspires further interdisciplinary research and practical applications at the intersection of waste management, soil science, and sustainable agriculture. The adoption of circular economy principles in soil restoration practices is not only timely—it is essential.