## 3 Results and Discussion  
### Physicochemical and Biological Characterisation of Soils  
The physicochemical characterisation of a soil provides important information on its structure and ability to supply nutrients to the plants and microorganisms present in it. The results of the physicochemical characterization are shown in Table 3. It should be noted that, except for the EC, WHC and K indicators, all of the other indicators showed significant differences between the analysed soils. Regarding the pH indicator, the soils were considered to be moderately acidic (Ac4 \(<\) Ac3), slightly acidic (Sa6 \(<\) Ac2 \(<\) Ir2 \(<\) Sa1) or neutral (Sa2 \(<\) Ir3 \(<\) Ir1 \(<\) Ac1 \(<\) Ac1). The tendency towards acidification for most of the soils can affect the availability of nutrients to plants and microorganisms due to interaction with aluminium (Al) [41], which in turn affects nutrient cycling. This acidification may be linked to excessive use of chemical fertilisers containing urea, as these fertilisers can release organic acids during their metabolism by microorganisms present in the soil [42]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Soil Quality** & **Very High** & **High** & **Moderate** & **Low** & **Very Low** \\ \hline Scale & 0.80–1.00 & 0.60–0.79 & 0.40–0.59 & 0.20–0.39 & 0.00–0.19 \\ Class & \(1\) & \(2\) & \(3\) & \(4\) & \(5\) \\ \hline \end{tabular}
\end{table} 
Table 2: Soil quality classification [40]. The EC indicator showed that high levels of salts were not detected in the solution of the evaluated soils. This could be due to the use of irrigation water with low salt concentrations, in contrast to the use of well water--characteristic of the region--that usually has a high concentrations of salts and thus leads to the accumulation of salts on the soil surface [43]. Similarly, the WHC indicator did not show significant differences between the analysed soils; where they were classified as low capacity (Ac2 < Ir3 < Ir2 < Ir4 < Ir1 < Ac1 < NaCl < Sa2) or moderate capacity (Sa6 < Ac3). A low WHC causes a water deficit for crop development and microorganism communities, leading to a decline in yield and nutrient cycling [44]. These low values may be related to tillage practices used in conventional agriculture and a low OM concentration or high Na concentrations in the soil, which can break down macro- and micro-aggregates in clay soils, decrease the amount of micropores and damage their structure [45; 46]. In the case of the OM-related indicators TOC and TN, the soils presented concentrations considered to be high and moderate, respectively. In general, this variability could be related to the possible application of organic amendments such as biosolids, biochar or manure, which could counteract the impact of conventional agricultural practices that lead to low C concentrations [47; 46]. As for N, it is possible that the use of chemical fertilisers has contributed to its moderate concentration in the soil, which in turn could lower the soil's pH [42]. The analysed soils presented significant differences in their capacity to provide nutrients, which is related to the indicators CEC, Na, K, Ca and Mg indicators. All of the soils had a low CEC, possibly due to the physical damage caused by conventional agricultural ploughing. Moreover, this could be attributed to the effect caused by their pH, causing a greater interaction between the negative charges of the clay with Al and increasing the susceptibility to the leaching of the other elements [41]. All of the soils showed high Na levels, which could result in the disintegration of aggregates and affect nutrient retention and water infiltration. The accumulation of Na could be due to irrigation with water containing a certain amount of salts, which accumulated over time. High Na levels could reduce the CEC capacity of the soil, a phenomenon that can be observed in the results obtained in the present study.