## 3 Results  
### Trace Elements in Soil  
The pollution of soil with diesel oil has been demonstrated to induce substantial alterations in the composition of trace elements within the soil post-oat harvest (Figures 2 and 3). In the initial series of experiments, in which no neutralising substances were employed, the presence of diesel oil pollution up to a dose of 10 cm\({}^{3}\) per kilogramme of soil resulted in a 50% increase in cadmium, a 42% increase in nickel, a 41% decrease in iron and a 53% decrease in cobalt. In the case of the other trace elements, lower doses of diesel oil resulted in an increase in their content in the soil. Consequently, the lowest dose of this petroleum substance (2.5 cm\({}^{3}\) per kg of soil) resulted in a 32% increase in the content of lead and 12% increase in that of chromium in the soil. In contrast, the average dose of diesel oil (5 cm\({}^{3}\) per kg of soil) resulted in a 16% increase in copper, 19% increase in manganese and 25% increase in zinc in the soil. The increase in some trace elements in the soil under the influence of small doses of diesel oil is undoubtedly due to the fact that they can act as organic solvents. This allows them to partially decompose organic matter and release previously bound trace elements (e.g., Cr) into the soil solution, increasing their availability. Lower diesel oil rates increased trace element content as a result of lower soil pH, redox potential and reduced availability of organic ligands, which may have led to increased mobilisation of trace elements associated with less stable fractions [69]. The application of higher doses of diesel oil had a reducing effect on the content of these elements in the soil, especially chromium, whose concentration in the soil with the highest diesel oil dose was 21% lower than in the control site (without this petroleum substance). Higher levels of diesel oil pollution inhibit the growth of soil microorganisms and consequently hinder the biogeochemical processes responsible for element mobilisation. They also restrict the availability of oxygen and water, which can lead to the immobilisation of some elements or their transformation into insoluble forms and adsorption of metals by oil particles, reducing their availability. Higher diesel oil doses increase soil acidification, which may favour the reduction of Cr(VI) to Cr(III). This is a less mobile cationic molecule due to its low solubility, high adsorption and complexation [69]. Figure 1: Soil analysis methods. Figure 2: Influence of different materials on the Cd, Pb, Cr, Ni, Zn and Cu content of soil contaminated with diesel oil, in mg per kg (averages \(\pm\) standard deviations). The application of all substances to the soil resulted in a limiting effect on nickel, chromium and iron. Furthermore, the application of compost and calcium oxide to the soil also resulted in a reducing effect on cobalt (Figures 2-4). However, it is noteworthy that bentonite exhibited the most pronounced effect on nickel and chromium, while calcium oxide demonstrated the strongest impact on iron and cobalt, reducing their concentration in the soil by 42%, 53%, 12% and 31%, respectively. Conversely, the levels of cadmium, lead, copper and manganese exhibited an increase following the introduction of the materials into the soil. For cadmium and manganese, bentonite exhibited the most significant impact, while lead and copper demonstrated sensitivity to the effects of compost. It is evident that compost had an analogous effect on the zinc content of the soil. However, it should be noted that compost contributed to the reduction in cadmium, while bentonite contributed to the reduction in lead, zinc and manganese in the object with the highest diesel oil pollution (10 cm\({}^{3}\) per kg of soil). Changes in the content of trace elements in the soil were probably associated with the beneficial effects of compost, bentonite and especially calcium oxide on other soil properties, in particular related to the neutralisation of its acidity [60].