## 5 Conclusions  
In order to determine the potential of organic and mineral materials in mitigating the effects of diesel oil pollution on the trace element content of soil, a test was conducted using three different remediation additives. Specifically, the test materials used were compost, which is a source of organic matter with high sorption capacity; bentonite, which is a clay mineral with immobilising properties; and calcium oxide, which is an alkaline additive capable of stabilising soil pH and precipitating trace elements in less accessible forms.  
The content of trace elements in the soil was clearly correlated with the diesel oil pollution and the materials used to mitigate its impact. Diesel oil pollution had the greatest effect on cadmium, nickel, iron and cobalt levels. The contents of the first three elements increased by 50% (Cd), 42% (Ni) and 41% (Fe), while the concentration of the last element decreased by 53% (Co). Lower doses of diesel oil (2.5 and 5 cm\({}^{3}\) per kg of soil) increased the content of the other trace elements in the soil, particularly chromium, whereas higher doses decreased it. All the materials applied to the soil (compost, bentonite and calcium oxide) clearly reduced the content of nickel, chromium and iron. Additionally, the application of compost and calcium oxide reduced the accumulation of cobalt in the soil. Bentonite had the strongest reducing effect on the nickel and chromium content of the soil, while calcium oxide had the strongest reducing effect on iron and cobalt accumulation. Inverse relationships were noted for cadmium (mainly bentonite), lead (especially compost),copper (mainly compost), manganese (mainly bentonite) and zinc (only compost) content. Bentonite limited the accumulation of lead, zinc and manganese in the most contaminated object, while compost had the same effect on cadmium.  
Using various materials, particularly bentonite and compost, limits the content of certain trace elements in the soil. This positively impacts the effect of minor diesel oil pollution on soil properties and can result in improved plant biomass growth.  
Future research will assess the influence of different soil types (with varying organic matter content, pH and mineralogical composition) on observed trace element mobility phenomena. It will also be crucial to carry out experiments under field conditions to confirm the relationships obtained in a natural soil environment.  
Conceptualisation, M.W.; methodology, M.W.; validation, M.W.; formal analysis, M.W. and N.K.; investigation, N.K.; resources, M.W.; data curation, M.W.; writing--original draft preparation, M.W. and N.K.; writing--review and editing, M.W. and N.K.; visualisation, M.W. and N.K.; supervision, M.W.; project administration, M.W.; funding acquisition, M.W. All authors have read and agreed to the published version of the manuscript.  
The research was financed by the Department of Agricultural and Environmental Chemistry, Faculty of Agriculture and Forestry, University of Warmia and Mazury in Olsztyn (grant No. 30.610.004-110).  
Not applicable.  
The data are contained within the article.  
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.