## 3 Results and Discussion
### Ability to Release Selected Mineral Components into the Aqueous Phase
#### 3.2.2 Retention Properties
In all variants containing biochar (AW_1, AW_3, BW_1,  
Figure 5: Calcium cation content in aqueous solutions from the produced substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–d) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. Figure 6: Magnesium cation content in aqueous solutions from the produced substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–d) Identical superscripts denote no significant (\(p<0.05\)) differences between the experimental objects according to the post hoc Tukey HSD test. BW_3), a significantly higher content of K\({}^{+}\) in the solutions was observed, which can be explained by the fact that biochar itself constitutes a source of potassium. Moreover, its porous structure and cation sorption capacity may promote retention and gradual release of these ions [24]. Similarly, in the variants where only biopolymer coating was applied (AW_2, BW_2), elevated concentrations of K\({}^{+}\) ions were found compared to the control samples, although these values were noticeably lower than those in the biochar variants. This may suggest that the biopolymer does not act as a direct source of potassium, and its influence on the K\({}^{+}\) content in the solution may result from modifications to the physicochemical properties of the substrate (improving its structure or water retention), which in turn affects the solubility and mobility of mineral nutrients (Figure 7). The average content of phosphorus in the form of phosphate anions (PO4\({}^{3-}\)) in aqueous solutions obtained from substrates based on coconut fibre types A and B ranged from 2.16 to 9.01 mg dm\({}^{-3}\) and 3.36 to 7.61 mg dm\({}^{-3}\), respectively (Figure 8). These values indicate a varied availability of phosphorus depending on the applied substrate modification variant. Similarly to the observations regarding potassium ions (K\({}^{+}\)), the use of additives--including biochar and/or biopolymer--significantly influenced the increase in average concentrations  
Figure 8: Phosphate anion content in aqueous solutions from the produced substrates (mean \(\pm\) SD). Mean values \(\pm\) standard deviation. (a–c) Identical superscripts denote no significant (\(p<\) 0.05) differences between the experimental objects according to the post hoc Tukey HSD test. Figure 7: Potassium cation content in aqueous solutions from the produced substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–d) Identical superscripts denote no significant (\(p<\) 0.05) differences between the experimental objects according to the post hoc Tukey HSD test. of phosphate anions in the soil solution. The highest PO\({}_{4}\)[3]- content was recorded in variants AW_3 and BW_3, i.e., in substrates where both biochar and a biopolymer coating were applied simultaneously (Figure 8). The elevated levels of PO\({}_{4}\)[3]- in these variants may result from the synergistic effect of both additives. Although biochar itself is not a significant source of phosphorus, it can indirectly enhance its availability by altering the sorption and buffering properties of the substrate, as well as by limiting phosphorus fixation. Meanwhile, the biopolymer coating may reduce the leaching rate of nutrients and influence the stability of the microenvironment around the root zone, which could contribute to better phosphorus retention [25]. Compared to the control samples (AW_0, BW_0), all variants with additives showed an increase in PO\({}_{4}\)[3]- content, confirming the beneficial effect of substrate modifications on improving the availability of this nutrient. The results indicate the potential effectiveness of biochar and biopolymers in optimising phosphorus release and retention in horticultural substrates.