## 2 Materials and Methods
### Study Area
#### 2.1.2 Bio-Based Fertiliser End User  
The agronomic and environmental performance of two BBFs were tested in a crop field of a potato farmer that specialises in using precision agriculture. This field experiment was carried at a field in Eersel (51\({}^{\circ}\)21\({}^{\prime}\)14.6\({}^{\prime}\) N, 5\({}^{\circ}\)19\({}^{\prime}\)58.0\({}^{\prime}\) E), 34 km from the location where the BBFs were produced. The crop rotation is based on one year of potato and three years of maize. The soil is typically referred to as a plaggic Anthrosol [15], which indicates that the topsoil is enriched by OM through the plaggic farming system that existed in the area before mineral fertiliser started to be used. The subsoil is poor in OM and consists of cover sand, which was deposited during the Weichselian glaciation. The elevation of the field  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline  & \begin{tabular}{c} **Dry** \\ **Matter** \\ **\%** \\ \end{tabular} & \begin{tabular}{c} **OM** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **N** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **NH\({}_{4}\)+-N** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **Organic N** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **Total C** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **TOC** \\ **Ratio** \\ \end{tabular} & \begin{tabular}{c} **C/N** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **P** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **K** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \begin{tabular}{c} **S** \\ **g kg\({}^{-1}\)** \\ \end{tabular} & \
\begin{tabular}{c} **pH** \\ \end{tabular} &
\begin{tabular}{c} **EC mS** \\ **cm\({}^{-1}\)** \\ \end{tabular} \\ \hline AS & 36 & N/A & 81.6 & 81.6 & N/A & 0.3 & 0.1 & N/A & 0.1 & N/A & 11.9 & 2.8 & 278 \\ KC & 12 & 69 & 6.4 & 2.5 & 3.9 & 41.4 & 27.8 & 6.5 & 4.9 & 20.7 & 3 & 7.4 & 59 \\ LFD & 3 & 9 & 4.5 & 3.7 & 0.8 & 9.2 & 2.8 & 2 & 0.7 & 6.9 & 0.1 & 8.0 & 44 \\ Man & & 3.8 & & & & & & 2.4 & 4.0 & & & \\ \hline \end{tabular}
\end{table}
Table 1: The chemical composition of ammonium sulphate (AS), potassium concentrate (KC), liquid fraction of the digestate (LFD), and pig slurry manure (Man).  
ranges from 25.4 m above sea level in the east to 26.2 m above sea level in the west. The weather during the growing season was extremely dry and warm (Appendix A) and the farmer did not have the capacity to apply irrigation.  
Soil of the topsoil layer (0-30 cm, loamy sand) was collected from this crop field in April 2020 and used for the experiments on the N release rate, the GHG emissions, and the nutrient fertiliser replacement value (NFRV). The soil characteristics are given in Table 2. Near infrared spectroscopy (NIRS) and CaCl\({}_{2}\) extraction, common practices of the laboratory Eurofins, were used for the soil analysis and the analysis was elaborated with a fertiliser recommended for consumption potato (Table 2). The soil of the potato field is rich in potassium (K) and therefore it is not recommended to test the NFRV of KC on this soil. A K-poor soil was used instead for NFRV determination (Wageningen, 51\({}^{\circ}\)59\({}^{\prime}\)14.8\({}^{\prime}\) N, 5\({}^{\circ}\)39\({}^{\prime}\)54.4\({}^{\prime\prime}\) E). The texture of the K-poor soil is also loamy sand (88% sand, 6% silt, and 2% clay), with a pH-H\({}_{2}\)O of 6, OM content of 3.6%, an initial total N and mineral N content of 3930 kg N ha\({}^{-1}\) and 45 kg N ha\({}^{-1}\), respectively (Table 2). For the analyses, the N or K application rate varied while other nutrients were kept at optimal condition by following the fertiliser recommendation for consumption potatoes.