## 2 Materials and Methods
### Samples  
The training set of samples consisted of organic amendments (composts, sewage sludges, and organic fertilizers) provided by the MARSEP program, which is part of the Wageningen Evaluating Programmes for Analytical Laboratories (WEPAL) [26]. Samples corresponding to the 1 and 3 quarterly periods of 2011 to 2019 (30 different samples) were included. The samples were of the following types: compost (14 samples), sewage sludge (11), and organic fertilizer (5). The samples came from different countries: Belgium (5), Netherlands (3), Switzerland (18), Philippines (1), and the United Kingdom (2). Samples were dried and finely crushed powders. These dry test samples have been shown to be stable over a number of years when stored at room temperature. The descriptive statistics (mean, minimum,and maximum values) for the aqua-regia-extractable contents of the selected elements in the three types of organic amendments are shown in Table 1.  
The reference material SdAR-M2, which corresponded to a metal-rich sediment produced by the US Geological Survey [27], was used to assess the accuracy and stability of the pXRF instrument. For the verification check to be acceptable, it was considered that the measured value for each target analyte in these samples should be within \(\pm\)20% relative difference (RD) of the true value [28]. The results for the readings taken seven times during the course of the work are shown in Table 2. As indicated by the RD values, the precision was very good for Pb(s), Ca(m), and Fe(m), and good for the rest of the elements considered, except for Ti(s) and Al(m), which were estimated by default, and S(s) and P(m), which were estimated in excess. The difference between the maximum and minimum values (range) showed great stability of the instrument during the study for Cu(s), Mn(s), Pb(s), Zn(s), Zr(s), Ca(s), Fe(m), and Si(m). The readings of K(s), Ti(s), Al(m), and P(m) varied by more than 10% with a maximum range of 20% for S(s). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Compost**} & \multicolumn{3}{c}{**Sewage Sludge**} & \multicolumn{3}{c}{**Organic Fertilizer**} \\ \hline
**Elem 1** & **Unit** & **Min 2** & **Max 2** & **Mean** & **Min 2** & **Max 2** & **Mean** & **Min 2** & **Max 2** & **Mean** \\ \hline LOI 3 & \% & 9.39 & 90.0 & 40.5 & 34.6 & 61.6 & 52.8 & 56.6 & 81.1 & 70.6 \\ Fe & g kg\({}^{-1}\) & 0.99 & 33.9 & 12.2 & 12.0 & 105 & 50.1 & 0.73 & 2.7 & 1.69 \\ Ca & g kg\({}^{-1}\) & 21.7 & 320 & 81.8 & 31.6 & 59.4 & 47.9 & 26.8 & 50.8 & 36.5 \\ K & g kg\({}^{-1}\) & 0.75 & 22.0 & 8.35 & 1.33 & 14.8 & 3.27 & 18.6 & 101 & 51.8 \\ P & g kg\({}^{-1}\) & 0.67 & 28.2 & 6.20 & 3.48 & 34.1 & 28.0 & 6.14 & 33.6 & 22.3 \\ S & g kg\({}^{-1}\) & 1.53 & 22.0 & 4.09 & 2.16 & 14.8 & 9.10 & 5.43 & 55.6 & 28.8 \\ Zn & mg kg\({}^{-1}\) & 34.9 & 1295 & 242 & 193 & 1305 & 859 & 65.0 & 186 & 127 \\ Pb & mg kg\({}^{-1}\) & 3.1 & 180 & 41.9 & 49.0 & 290 & 139 & 1.40 & 6.94 & 3.38 \\ Ba & mg kg\({}^{-1}\) & 8.7 & 668 & 130 & 106 & 524 & 408 & 10.0 & 167 & 46.9 \\ As & mg kg\({}^{-1}\) & 0.85 & 8.92 & 4.70 & 2.77 & 13.9 & 7.85 & 0.17 & 4.35 & 1.79 \\ Cd & mg kg\({}^{-1}\) & 0.18 & 2.07 & 0.50 & 0.47 & 2.66 & 1.53 & 0.05 & 0.24 & 0.13 \\ Cr & mg kg\({}^{-1}\) & 3.23 & 111 & 58.7 & 31.7 & 226.5 & 75.6 & 3.58 & 280 & 62.1 \\ Hg & \(\mu\)g kg\({}^{-1}\) & 29 & 1410 & 211 & 174 & 2871 & 1443 & 15 & 26 & 19 \\ Ni & mg kg\({}^{-1}\) & 1.63 & 80.5 & 26.3 & 22.0 & 134.1 & 45.9 & 1.23 & 19.3 & 7.46 \\ Se & \(\mu\)g kg\({}^{-1}\) & 210 & 3562 & 765 & 640 & 2134 & 1687 & 420 & 582 & 484 \\ \hline \hline \end{tabular} \({}^{1}\) Element. 2 Minimum and maximum values. 3 LOI: Loss on ignition.  
\end{table} 
Table 1: Ranges of aqua-regia-extractable contents of selected elements of the different amendment types.