# Anaerobic Digestate from Biogas Plants--Nuisance Waste or Valuable Product?
## 3 Results and Discussion
### Chemical Composition of Crops  
The application of digestates and digestate-based products affects the chemical composition of crops [33]. The lowest nitrogen content was found in plants growing on the control object (14.57 g N kg\({}^{-1}\) DM) (Table 5). Nitrogen content in plants growing on objects fertilized with LFs was more than 11% higher than on the control object. Higher nitrogen content was determined in plants fertilized with DGs and SFs than those fertilized with other forms of digestate. Nitrogen content in plants on these objects was 17% and 25% higher in comparison to nitrogen content in plants from the control object. Fertilization with digestate pellets increased nitrogen content in the plants to the lowest degree (by approximately 8% in comparison to control). Similar to our results on the nitrogen content, the phosphorus content in plants was lowest for the control (1.40 g P kg\({}^{-1}\) DM). The highest P content was determined in plants fertilized with pellets (PDG1, PDG2, PSF2, and PSF1). It was approximately twice as high as on the control object. P content in plants on objects DGs, LFs, and SFs was significantly higher (by approximately 33\(-\)50%) than on the control object. As expected, mean potassium content (20.31 g K kg\({}^{-1}\) DM) in maize was higher than that of nitrogen, phosphorus, calcium, and magnesium (16.60 g N, 2.33 g P, 2.13 g Ca, 0.94 g Mg kg\({}^{-1}\) DM, respectively). Potassium contents strongly depended on the form of the applied digestate (Table 5). The lowest potassium content was determined for plants growing in the soil fertilized with DG2 (14.75 g K kg\({}^{-1}\) DM). Potassium content in maize on objects fertilized with DGs, LFs, and SFs was significantly lower than that on the control object. Fertilization with PSFs and PDGs considerably increased potassium content in plants by approximately 22-31% in comparison to the control object. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Intercept** & **Slope** & \(p\)**-Value** & \(\mathbf{R^{2}}\) \\ \hline N-NH4 & **341.25** & **163.14** & **<0.0000** & **58.03** \\ C & 386.99 & 0.29 & \(<\)0.86 & 0.77 \\ P & 385.51 & 15.67 & \(<\)0.73 & 0.28 \\ K & 356.98 & 49.97 & \(<\)0.06 & 8.08 \\ Mg & 387.42 & 10.51 & \(<\)0.85 & 2.90 \\ Ca & 383.62 & 34.20 & \(<\)0.67 & 0.43 \\ Zn & 390.84 & \(-\)0.24 & \(<\)0.88 & 0.05 \\ Cu & 395.35 & \(-\)2.30 & \(<\)0.56 & 0.80 \\ Mn & 389.05 & 0.01 & \(<\)0.99 & - \\ \hline \hline \end{tabular} Boldface types indicate statistically significant correlations (\(p<0.05\)).  
\end{table} 
Table 6: Results of linear regression analyses describing the relationship between maize yields and nutrient dose in the pot experiment. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Fertilization \({}^{*}\)**} & **Yields** & **N** & **P** & **K** & **Mg** & **Ca** & **Zn** & **Cu** & **Mn** \\ \cline{2-9}  & **g FM pot\({}^{-1}\)** & & & **g kg\({}^{-1}\) DM** & & & & **mg kg\({}^{-1}\) DM** \\ \hline DG 1 & 410.00 d  & 16.70 d  & 2.03 b  & 19.10 d  & 0.74 a  & 1.86 b  & 15.26 a  & 1.96  
The magnesium content in plants varied from 0.73 to 1.22 g Mg kg\({}^{-1}\) DM. Only fertilization with DG2, LF1, and SF2 significantly increased Mg content in maize. On the remaining experimental objects, Mg content in maize was approximate to that on the control object. The calcium content in plants varied from 1.40 to 3.12 g Ca kg\({}^{-1}\) DM. Only on objects fertilized with LFs, was significantly higher Ca content in maize in comparison to the control object determined. The use of digestate and digestate-based products for fertilization raises concerns about the deterioration of the quality of biomass, especially in the context of the content of heavy metals (HMs) [45; 46]. Among the analyzed HMs, Mn amounts in plants were highest. The average content of manganese in plants was higher than zinc and copper content (52.80 mg Mn, 17.34 mg Zn, and 2.51 mg Cu kg\({}^{-1}\) DM, respectively).