## 2 Materials and Methods
### Analysis  
#### 2.2.1 Digestate  
The digestate used in this work was obtained at a biogas plant from a mixture of pig slurry and cereal straw during 30 days of retention time in mesophilic reactors with 3000 m\({}^{3}\) of capacity. Then, the digestate slurry was pressed to separate the solid and the liquid phases. In this work, we used only the solid fraction of the digestate. The solid fraction of the digestate was characterised for moisture content by the gravimetric method; pH was determined with a glass electrode (organic amendment to water ratio of 1:5, CRISON micropH 2002, Barcelona, Spain) and electrical conductivity \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Treatments** & \multicolumn{2}{c}{**Lettuce**} & \multicolumn{3}{c}{**Kale**} \\ \hline \multirow{3}{*}{**Before Planting**} & **1 st** & **2 nd** & **Before** & **1 st** & **2 nd** \\  & **Top-Dressing** & **Top-Dressing** & **Planting** & **Top-Dressing** & **Top-Dressing** \\ \hline \multirow{3}{*}{**December 2020**} & **February** & **March** & **April** & **October** & **December 2020** & **February 2021** \\  & **2020** & **2020** & **2020** & **2020** & **December 2020** & **February 2021** \\ \hline \hline Control & 0 & 0 & 0 & 0 & 0 & 0 \\ Ni85 & Ni35 & Ni25 & Ni25 & Ni35 & Ni25 & Ni25 \\  & CAN & CAN & CN & AS & AS & CAN \\ DG-N85 & DG-85 & 0 & 0 & DG-85 & 0 & 0 \\ DG-N170 & DG-170 & 0 & 0 & DG-170 & 0 & 0 \\ DG-N170+85 & DG-170+85 & 0 & 0 & DG-170+85 & 0 & 0 \\ DG-N170+170 & DG-170+170 & 0 & 0 & DG-170+170 & 0 & 0 \\ DG-N85+Ni60 & DG-85+Ni35 & Ni25 & 0 & DG-85+Ni35 & Ni25 & 0 \\  & DG+CAN & CAN & & DG+AS & AS & \\ DG-N170+Ni60 & DG-170+Ni35 & Ni25 & 0 & DG-170+Ni35 & Ni25 & 0 \\  & DG+CAN & CAN & & DG+AS & AS & \\ DG-N170+Ni25 & DG-170 & Ni25 & 0 & DG-170 & Ni25 & 0 \\  & CAN & & & AS & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Treatments and fertilisation done during the field experiments with lettuce and kale. with a conductivimeter in a 1:10 suspension (SF:water ratio). Organic matter was quantified by losing weight after ignition at 550 \({}^{\circ}\)C for 16 h (Controller B 180, Naberthem, Lilienthal, Germany). The Kjeldahl procedure evaluated total N (TKN) and organic (NO). Total P (PT) was determined after ignition at 550 \({}^{\circ}\)C for 3 h and extracted with H\({}_{2}\)SO\({}_{4}\) 0.5 M, inorganic P (Pi) was extracted with H\({}_{2}\)SO\({}_{4}\) 0.5 M and then quantified by molecular absorption spectrophotometry (spectrophotometer Thermo Electron Corporation evolution 300 LC, Waltham, MA, USA). Organic P (Po) was calculated as the difference between PT and Pi. All the P in the extracts were centrifugated at 3000 rpm, and P in the supernatant was quantified by the method of [PERSON] [19]. Total cations such as Fe, Mn, Cu, Pb, Cd and Cr were also extracted by the aqua regia digestion and quantified by atomic absorption spectrophotometry. Total K, Ca, Mg and Na were quantified by flame emission spectrophotometry (K and Na) and atomic absorption spectrophotometry (Ca and Mg) (Thermo Scientific Series iCE 3000, Waltham, MA, USA), from the hydrochloric acid solution of the ashes obtained by the organic amendment's ignition at 550 \({}^{\circ}\)C.  
The chemical composition of the solid digestates used in the lettuce (DGL) and the kale (DGK) experiments are shown in Table 2.  
The standards of some European countries [20; 21; 22] specify for products to be considered as soil amendments (those that have a beneficial effect on soil OM) an OM content between 20 and 90% on a DM basis; they should have a dry matter content higher than 30-50% [22; 23] and a C/N ratio higher than eight [23]. So, overall the digestates meet these standards except for the dry matter content slightly below the minimum threshold level (29 and 25%). Additionally, the electrical conductivity (EC of 1.5 and 1.3 dS m\({}^{-1}\)) of the digestate was within the normal range to prevent soil salinity, considering that 3 dS m\({}^{-1}\) is the threshold value [24].