## 3 Results
### Statistical Analysis  
Among the copper forms in the sandy soils, the most significant correlations were found for fractions III and IV mainly with the organic carbon content (r = 0.802 at \(p<0.01\) and r = \(-\)0.788 at \(p<0.05\), respectively), the total content (r = 0.823 and r = \(-\)0.865 at \(p<0.01\)), and the bioavailable form (r = 0.865 and r = \(-\)0.878 at \(p<0.01\)) (Table 2). In the case of the residual fraction, there was a significant correlation with pH measured in water (r = \(-\)0.668 at \(p<0.005\)). The most mobile fractions did not show any significant correlations, except from the correlation of fraction II with the silt fraction content (r = 0.936 at \(p<0.001\)). Weaker statistically significant relationships were found between the bioavailable copper form and properties that may influence its mobility: pH reaction (r = 0.858 and 0.865 at \(p<0.01\)) and organic carbon content (r = 0.734 at \(p<0.05\)). Moreover, this form was characterized by a very high correlation coefficient (r = 0.936 at \(p<0.001\)) with the total Cu content. Noteworthy, no significant correlations were found for the indicators of the mobile forms. In turn, the C\({}_{\mathrm{a}}\)/C\({}_{\mathrm{t}}\) and BF indices were positively correlated with parameters of the bioavailability of the elements (r = 0.772 and 0.715 and 0.668 at \(p<0.05\)) and the organic matter content (r = 0.681 and r = 0.788 at \(p<0.05\)). Furthermore, the correlation analysis revealed a significant correlation between the BF index and the content of total and bioavailable Cu (r = 0.865 and r = 0.878 at \(p<0.01\)) (Table 2). In the silty soils, there were statistically significant positive correlations of the bioavailable Cu form with the C\({}_{\mathrm{f}}\) (r = 0.714 at \(p<0.05\)), C\({}_{\mathrm{a}}\)/C\({}_{\mathrm{t}}\) (r = 0.755 at \(p<0.05\)), and Igeo (r = 0.912 at \(p<0.001\)) indices and with the organic carbon content (r = 0.721 at \(p<0.05\)). Although there were no significant relationships between the total and bioavailable copper content, the C\({}_{\mathrm{a}}\)/C\({}_{\mathrm{t}}\) ratio was negatively correlated with the content of the silt fraction (r = 0.774 at \(p<0.05\)) (Table 3). The results of the PCA analysis of the parameters of the Cu content in the soils are presented in Figure 4 and Table 4. The particle size distribution was the most important variable discriminating the content of copper. A majority of points marked with lowercase Arabic letters (si, sa) were located on the opposite sides of the first component axis, which explains over 40.18% of the variance. Unfortunately, there were no regularities in relation to the mode of land use (do, al, ga, ag), as the results showed high heterogeneity. Except for BF, all indices described in the present study were negatively correlated with the basic physicochemical properties (pH or silt content) and with the content of the most bioavailable Cu fractions I and II or the total copper content. A negative correlation was also noted between the BF index indicating the proportion of mobile forms and the residual fraction content. Importantly, there was a positive correlation between the organic carbon content and the content of fraction III. Fractions I and II correlated positively with the parameters of the soil conditions, i.e., pH and the silt fraction content, as shown mainly by