3.3. Evaluation of the Soil Quality Index (SQI)
The eigenvalues of the first four components were all ≥ 1 and explained 84.87% of the total variance (Table 3). In the first component (PC-1), the indicators with the highest weight were aeration porosity (AP), saturated hydraulic conductivity (Ksat), zinc content (Zn) and exchangeable acidity (EA). AP and Ksat showed a high correlation (r>0.80) (Table 4), therefore, due to the higher load and ease of measurement, AP was selected as an indicator in PC-1. In the second main component (PC-2), TOC and pH had the highest loads and showed a high degree of significant association (P≤0.01), with r = -0.79 (Table 4), thus, TOC was selected in PC-2. In the third main component (PC-3), total porosity (TP) and soil bulk density (BD) had the highest loads and also showed a significant correlation, with r = -0.63 (p<0.05). However, since total porosity did not show significant differences in Tukey’s mean comparison test and considering that bulk density is an integrative variable and influences different important functions that define soil quality, it was selected (Blanco-Canqui & Ruis, 2018). Finally, in PC-4, leaf litter (LL) production was selected because it showed the highest load.
Table 3.
Table 4.
Based on this, the minimum data set selected to calculate SQI were Zn, AP, TOC, BD and Leaf litter. With the value of the weights or loads based on the PCA (Table 5), the SQI was determined using the following equation Eq (3):