## 2 Materials and Methods
### Soil Treatments  
Surface samples (0-20 cm) of two loamy soils were collected from n. Greece, one acid and one alkaline and calcareous. Soil samples were air-dried, ground to pass a 10-mm sieve \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Material** & **pH** & **EC** & **CEC** & **K** & **Na** & **Ca** & **Mg** \\ \hline  & **(1:10)** & **dS m\({}^{-1}\)** & **cmolc kg\({}^{-1}\)** & & **mg kg\({}^{-1}\)** & \\ \hline Bentonite & 7.7 \(\pm\) 0.0 & 1.48 \(\pm\) 0.07 & 71.2 \(\pm\) 4.1 & 1122 \(\pm\) 1 & 4237 \(\pm\) 103 & 8895 \(\pm\) 1522 & 4454 \(\pm\) 327 \\ Vermiculite & 9.0 \(\pm\) 0.2 & 0.10 \(\pm\) 0.00 & 90.2 \(\pm\) 0.4 & 456 \(\pm\) 41 & 1352 \(\pm\) 793 & 945 \(\pm\) 413 & 17,119 \(\pm\) 400 \\ Biochar & 9.2 \(\pm\) 0.1 & 0.82 \(\pm\) 0.04 & 9.3 \(\pm\) 0.4 & 844 \(\pm\) 51 & 1832 \(\pm\) 13 & 8714 \(\pm\) 1187 & 902 \(\pm\) 52 \\ Sludge & 7.3 \(\pm\) 0.1 & 1.20 \(\pm\) 0.02 & & 948 \(\pm\) 38 & 4494 \(\pm\) 210 & 7366 \(\pm\) 1736 & 5470 \(\pm\) 1216 \\ \hline  & **Fe** & **Mn** & **Cu** & **Zn** & **Ni** & **Cr** & **Pb** \\ \cline{2-6}  & & & & **mg kg\({}^{-1}\)** & & & \\ \hline Bentonite & 22,519 \(\pm\) 2201 & 386 \(\pm\) 23 & 20.6 \(\pm\) 0.3 & 92.5 \(\pm\) 5.0 & 11.2 \(\pm\) 1.6 & 6.1 \(\pm\) 0.6 & 14.2 \(\pm\) 1.9 \\ Vermiculite & 25,918 \(\pm\) 3724 & 363 \(\pm\) 47 & 42.5 \(\pm\) 6.0 & 32.3 \(\pm\) 1.7 & 9.7 \(\pm\) 1.0 & ND & ND \\ Biochar & 229 \(\pm\) 59 & 7.9 \(\pm\) 0.9 & 6.0 \(\pm\) 0.7 & 24.8 \(\pm\) 3.9 & ND & ND & 10.6 \(\pm\) 0.2 \\ Sludge & 20,588 \(\pm\) 2134 & 204 \(\pm\) 107 & 232 \(\pm\) 25 & 863 \(\pm\) 50 & 38.6 \(\pm\) 2.3 & 44.3 \(\pm\) 0.4 & 70.6 \(\pm\) 14.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Certain chemical properties and total composition (expressed on dry weight basis) of sewage sludge and the materials used for its stabilization. CEC: cation exchange capacity; EC: electrical conductivity; SAR: sodium adsorption ratio; ND: not detectable. and used for the pot experiment. Then, the treated and untreated (air-dried) sludge mixtures were added at a 2% rate (\(\approx\)80 Mg ha\({}^{-1}\)) to the two soils. Additional treatments were inorganic fertilization, i.e., N-P-K (IF) and P-K (IFX) corresponding to 100 mg N kg\({}^{-1}\) as NH\({}_{4}\)NO\({}_{3}\), 60 mg P and 75 mg K kg\({}^{-1}\) as KH\({}_{2}\)PO\({}_{4}\), and no organic or inorganic fertilization (control). Specifically, eight treatments were added to each soil, in three replications: acid (Ac) soil with (i) bentonite-treated sludge (AcB), (ii) vermiculite-treated sludge (AcV), (iii) biochar-treated sludge (AcBc), (iv) limed sludge (AcL), (v) untreated (air-dried) sludge (AcD), (vi) N-P-K inorganic fertilization (AcLF), (vii) P-K inorganic fertilization (AcIFX) and (viii) control (AcC); alkaline (Al) soil with (i) bentonite-treated sludge (AlB), (ii) vermiculite-treated sludge (AlV), (iii) biochar-treated sludge (AlBc), (iv) limed sludge (AIL), (v) untreated (air-dried) sludge (AID), (vi) N-P-K inorganic fertilization (AlIFX) and (viii) control (AIC). After equilibration for \(\approx\)20 days, employing wetting with deionized water and mixing, a quantity of each air-dried soil treatment was passed through a 2-mm sieve for laboratory analysis and the rest material was used for the pot experiment. Soil treatments were analyzed for the following properties: pH in a 1:2 (\(w/v\)) suspension with water, CaCO\({}_{3}\) with a calcimeter, organic carbon (OC) by the wet oxidation method [24], Kjeldahl-N [25], cation exchange capacity (CEC) [26] and electrical conductivity of the saturation extract (EC\({}_{\text{se}}\)) along with water-soluble Na, Ca and Mg. In addition, soil-available NH\({}_{4}\)-N and NO\({}_{3}\)-N were extracted with 1 M KCl, P with 0.5 M NaHCO\({}_{3}\), K, Ca and Mg with 1 M CH\({}_{3}\)COONH\({}_{4}\), B with the hot water method and Cu, Zn, Fe and Mn with 0.005 M DTPA [27]. Moreover, total Cu, Zn, Ni, Cr, Cd, Pd and Hg were determined after digestion with aqua regia [28].