## 4 Discussion
The positive effects of in situ stabilisation of contaminated soils using calcium compounds were also confirmed by [PERSON] et al. [92] and [PERSON] et al. [93]. As a source of dissolved organic carbon, compost increases the organic matter content of the soil, thereby improving its sorption capacity [94] and increasing the retention of TEs [95]. In our own research, applying compost reduced the accumulation of Cd, Co, Ni, Cr and Fe in the soil. Similar results were obtained by [PERSON] et al. [96], who noted a reduction in the mobility and bioavailability of Cd, Pb, and Ni when 10% green waste compost was added to industrial soil from a copper smelter site. The researchers observed an increase in the proportion of TE fractions associated with organic matter, compared to their exchangeable forms. Humus substances, which constitute a significant proportion of the organic matter in compost, can reduce the solubility of TEs by forming stable complexes with them [97]. Additionally, the surface of the compost is rich in functional groups such as carboxyl and phenolic groups, which provide sites for adsorption and electrostatic interactions with TE cations [98]. [PERSON] et al. [99] conducted a study assessing the effect of poultry manure and rice husk compost applications on the immobilisation and biotoxicity of Cd in contaminated soils. In the most contaminated series (50 mg Cd per kg), adding compost at a rate of 120 g per kg reduced the amount of soluble/exchangeable Cd by 95.7%. Under the same conditions, the authors also observed significant increases in the fraction of Cd bound to organic matter (7.8-fold) and inorganic sludge (1.5-fold). Introducing compost into soil increases its pH, promoting the precipitation of trace element hydroxides and carbonates [100], and stimulating soil microbial activity. This can result in the transformation of TEs into geochemically more stable forms [101]. [PERSON] et al. [102] and [PERSON] et al. [103] also demonstrated the effectiveness of compost in reducing the availability of TEs in contaminated soils. Further advantages of using compost for soil remediation include increased soil fertility and biological activity in the soil, long-term protection and a sustainable, closed-loop approach [104]. The immobilisation efficiency of soil additives depends on soil properties, including initial pH and cation exchange capacity (CEC). Soils with a high CEC have a higher retention capacity for trace element cations [105] and better stabilisation of introduced sorbents [106]. Soil pH determines the surface loading of soil colloids and sorbents [107]. In an alkaline environment (pH \(>\) 7), negatively charged surfaces predominate, increasing the soil's retention capacity for cationic compounds [108] and leading to increased adsorption of trace elements onto soil colloids. Soil pH controls the solubility of trace element combinations and affects the availability of organic ligands [109]. At a pH level that is neutral to slightly alkaline, the carboxylic and phenolic groups in humic substances become deprotonated and negatively charged. This allows for the efficient formation of complexes with trace element cations through electrostatic forces [110]. Although complexation with organic matter provides long-term immobilisation efficiency of TEs over a wider pH range, it is sensitive to changes in redox potential [40]. Elements immobilised by precipitation are most stable under alkaline conditions but can be remobilised when the pH decreases [111]. Compared to biological alternatives (e.g., bioremediation and phytoremediation), the in situ stabilisation method for contaminated soils produces faster results, enabling earlier site development [37]. It is also characterised by its independence from climatic and seasonal conditions, its applicability to a wide range of soil types and contaminants, predictable results, and an absence of secondary waste generation [112].