## 2 Materials and Methods  
### Field Portable X-ray Fluorescence (pXRF) Analysis  
The analyzer Niton XL3t 950s GOLDD+ XRF (Thermo Scientific Inc., Billerica, MA, USA), with its laboratory stand, was used throughout the study. More detailed information about the analyzer can be found on the manufacturer's website [25]. This analyzer was equipped with an X-ray tube with an Ag anode, operated at 50 keV, 200 uA, and 2 W, with a geometrically optimized large Si drift detector (GOLDD). The analyzer can measure elements from Mg to U. Detailed information on the fundamentals of the technique can be found in References [5,6]. The analyzer can operate in two factory calibration modes called mining (m) mode and soils (s) mode. Hereafter, the letters (m) and (s) are used after the chemical symbol of each element to indicate the measurement mode in which the readings were recorded. The mining mode is a fundamental parameter calibration and it is recommended for high concentrations and quantification [5]. In the mining mode, readings were obtained for the following elements (the number of samples of the Manure and Refuse Sample Exchange Programme (MARSEP) with results above the limit of detection (LOD) are in parenthesis): Mo (11), Zr (29), Sr (30), Rb (30), Pb (22), As (10), Zn (30), Cu (26), Fe (30), Mn (20), Cr (24), V (22), Ti (29), Ca (30), K (29), S (30), Ba (22), Sn (11), Ag (9), Bal (30), Nb (11), Al (22), P (30), Si (30), Cl (29), Mg (9). The mining method gives a parameter called Bal (30), which is the difference from 100% of the sum of all measured elements. This parameter is likely related to the organic matter content of the amendment. The elements U (0), Th (0), Au (0), Se (0), Ni (0), Co (1), W (0), Sb (0), Cd (4), Pd (1), and Bi (1) were also measured, although few readings were obtained since they were below the LODs, and consequently, these were not included in the following statistical procedures. The mining mode is recommended for element concentrations greater than 10 g kg\({}^{-1}\).  
The soil mode is based on Compton normalization and is often used for scanning and detection for soil metallic elements [4] at low concentrations (<1%). In this mode, readings were obtained for the following elements: Mo (19), Zr (29), Sr (30), Rb (30), Pb (24), As (15), Zn (30), Cu (28), Ni (12), Fe (30), Mn (26), Cr (17), V (14), Ti (29), Sc (28), Ca (30), K (30), S (30), and Ba (18). Furthermore, U (5), Th (5), Au (0), Se (0), Co (0), Hg (0), W(0), Cs (5), Te (0), Sb (1), Sn (6), Cd (0), Ag (4), and Pd (0) were included in soil mode, although they were below the LODs in almost all samples and they were not used in the following.  
Dried and finely ground samples were measured by filling an XRF container (model SC-4331, 26 mm internal diameter, 24 mm height, Premier Lab Supply Inc., Port St. Lucie, FL, USA) capped with a 4 um propylene film (model 240255, 63 mm diameter, Premier Lab Supply Inc., Port St. Lucie, FL, USA). The container was placed in the window of the laboratory stand, the analyzer was attached to this stand, each sample was scanned in triplicate in each of the mining and soil modes, and the average elemental readings for each mode were used thereafter. The analysis time for each scan was 90 s for the soil mode and 120 s for the mining mode.