Implementation of construction works on weak (e.g., compressible, collapsible, expansive) soils such as peatlands often is limited by logistics of equipment and shortage of available and applicable materials. If preloading or floating roads on geogrid reinforcement or piled embankments cannot be implemented, then soil stabilization is needed. Sustainable soil stabilization in an environmentally friendly way is recommended instead of applying known conventional methods such as pure cementing or excavation and a single replacement of soils. Substitution of conventional material (cement) and primary raw material (lime) with secondary raw material (waste and byproducts from industries) corresponds to the Sustainable Development Goals set by the United Nations, preserves resources, saves energy, and reduces greenhouse gas emissions. Besides traditional material usage, soil stabilization is achievable through various secondary raw materials (listed according to their groups and subgroups): 1. thermally treated waste products: 1.1. ashes from agriculture production; 1.2. ashes from energy production; 1.3. ashes from various manufacturing; 1.4. ashes from waste processing; 1.5. high carbon content pyrolysis products; 2. untreated waste and new products made from secondary raw materials: 2.1. waste from municipal waste biological treatment and landfills; 2.2. waste from industries; 3. new products made from secondary raw materials: 3.1. composite materials. Efficient solutions in environmental engineering may eliminate excessive amounts of waste and support innovation in the circular economy for sustainable future.

Reusing organic waste as fertilizer is one method to reduce the use of mineral fertilizers and minimize waste disposal in landfills. Regulations have been enacted for the processing of organic waste and for recycling end products, but the humic content of organic fertilizers has been neglected. We studied seven composts with different organic input materials and technologies. Humic substances (HSs) were detected in all composts. The total organic carbon in the HSs constituted 8.7 ± 0.1% (SD)–27.0 ± 0.2% of the compost dry matter. Spectral differences between the studied samples in FTIR spectroscopy could be observed at 1700–1000 cm⁻¹, indicating differences in compost precursor material. The EEM peak, associated with humic acids (HAs), was high in composts containing animal by-products (e.g., fish waste, horse manure, and kitchen biowaste). Kitchen biowaste, also when processed by Hermetia illucens larvae and vermicompost, exhibited slower organic material transformation with low humic acid/fulvic acid ratios (<1.60). The results show the importance of source material origin and amendments, which influence the composting process and final products. Our study emphasizes the role of humic substances in the comprehensive evaluation of composts. To maximize the added value of composts, marketing strategies should consider determining the share of humic substances besides the content of organic matter and nutrients.

Composting is a sustainable method for recovering nutrients from various organic wastes, including food waste. Every input waste has different nutrient contents, in turn, suggesting that every compost has different fertilizer and/or soil improvement values. The phosphorus (P) concentration and relative distribution of P forms is related to the original organic material. The relative distribution of P forms determines how readily plants can absorb P from the compost-amended soil. The aim of this study was to investigate the content and relative share of P forms in composts made from fish waste, sewage sludge, green waste, and horse manure. Six forms of P (labile; bound to reducible metals; bound to non-reducible metals; bound to easily degradable organic material; and bound to calcium) were determined using sequential extraction method. The results indicated that fish waste compost had relatively high proportion of labile P, suggesting good biological availability. In comparison, sewage sludge compost contained the highest overall P concentration per dry weight unit, while labile P constituted only 6% of summary of P forms. The results indicate that the evaluation of composts as alternative P sources in agriculture should rely on the relative distribution of P forms in the compost in addition to the typically recognized value of the total P.

Pesticide residues in agricultural soils represent an environmental concern that requires special attention due to their potential ecological and public health risks. We analyzed 614 pesticides in 188 wheat fields across Europe subjected to both conventional and organic farming systems. At least one pesticide residue was detected in 141 soils. Seventy-eight pesticides or their metabolites were detected. The presence of pesticides was significantly higher in both number and concentration in conventional fileds (up to 0.98 mg kg^-1) compared to organically managed sites (up to 0.40 mg kg^-1). A total of 88 % of conventional fields and 63 % of organic fields contained two or more pesticides. Conversion from conventional to organic farming does not guarantee that soils will be pesticide-free in the short term. Fenbutatin oxide was the most frequently detected pesticide in both farming systems, followed by AMPA. Other substances, such as boscalid, epoxiconazole, diflufenican, tebuconazole, dinoterb, bixafen, and DEET, were found in ≥ 10 % of samples. Some Persistent Organic Pollutants, including dieldrin, endosulfan sulphate, and chlorpyrifos, were also detected. Ecological risks were higher in conventionally managed fields, with 46 % exhibiting high-risk levels, compared to just 1 % in organic fields. Epoxiconazole and boscalid were the substances with the highest risk levels.

Using kukersite oil shale ash for the stabilization of organic-rich soils has been extensively studied but without much success. The main reason for this is the retarding effect of humic acids present in organic-rich soils on the formation of the C–S–H (calcium–silicate–hydrate) phase, which effectively prevents the solidification of cementitious materials in peat soils. Based on the studies performed at the University of Tartu, we have developed a novel method for the in situ mass stabilization of peat soils using waste materials (calcareous fly ash and silica fume). In this manner, the perfect hardening of peat–ash mixtures (to each 1 m³ of soil, 150–400 kg ash, 25–50 L pozzolanic additives, and 2.5–5 kaNaOH were added) can be achieved. A test road embankment was constructed under extremely hard conditions in a peat quarry that is currently in use.

Construction of road embankments in peatlands commonly involves replacement of the peat with a fill-up soil of an adequate load-bearing capacity. This usually requires a lowering of the water level, turning a peatland from a carbon sink to a source of greenhouse gases. Thus, alternatives are sought that are less costly in both economic and ecological terms. Mass-stabilization technology can provide a cheap substitute for Portland cement. Calcareous ashes (waste materials), supplemented with pozzolanic and alkali additives to facilitate and accelerate the setting and hardening processes, are attractive alternatives to soil excavation or replacement techniques. Silica fume and waterglass were used as pozzolanic agents and KOH as a soil-alkalizing agent. X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analyses and stress–strain tests were performed for the hardened samples. Crystallization of alkali feldspars was observed in all test samples. Comparable hardening of peat soil was achieved for both ashes. It was shown that the ashes of Estonian kukersite (oil shale) from both pulverized firing and a circulating fluidized bed incineration process (produced in energy sector as quantitatively major solid waste in Estonia) can be used as binding agents for peat stabilization, even without the addition of Portland cement. Hardened peat soil samples behaved as a ductile material, and the cellulose fibers naturally present in peat gave the peat–ash composite plasticity, acting mechanically in the same way as the steel or glass fiber in ordinary reinforced concrete. The effect of peat fiber reinforcement was higher in cases of higher load and displacement of the composite, making the material usable in ecological constructions.