## 3 Results and Discussion
### Ability to Release Selected Mineral Components into the Aqueous Phase
#### 3.2.2 Retention Properties  
The water retention capacity (MWHC) of the substrates was significantly influenced by both the type of coconut fibre used and the nature of the additives incorporated to modify their physical and chemical properties (Figure 3). Comparing the two types of coconut fibres analysed, fibre B exhibited approximately twice the MWHC values compared to fibre A. This suggests that fibre B has superior water retention properties, possibly due to its higher porosity or distinct structural characteristics. Figure 1: pH of aqueous solutions from the produced substrates (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard error. (a–b) Identical superscripts denote no significant (\(p\) < 0.05) differences between the experimental objects according to the post hoc Tukey HSD test. Figure 2: EC of aqueous solutions from the produced substrates (mean \(\pm\) SD). Mean values \(\pm\) standard error. (a–b) Identical superscripts denote no significant (\(p\) < 0.05) differences between the experimental objects according to the post hoc Tukey HSD test. For substrates based on fibre A, the addition of biochar (AW_1) negatively affected the substrate's water retention capacity, reducing MWHC compared to the control (AW_0). A similar trend was observed in the fibre-B-based substrates, where the inclusion of biochar (BW_1) led to a decrease in average MWHC values. This phenomenon can be attributed to the high porosity of biochar, which, while increasing the specific surface area of the substrate, may simultaneously limit capillary water retention within its structure. The application of a biopolymer coating (AW_2 and BW_2) had a varied impact on MWHC depending on the type of fibre. In the case of fibre A (AW_2), MWHC values were slightly lower than for the control, possibly due to pore sealing and reduced wettability. Conversely, in variant BW_2, the use of the biopolymer significantly increased MWHC, achieving the highest level compared to all other variants. This indicates a synergistic interaction between the structure of fibre B and the biopolymer coating, enhancing its ability to bind water. The most significant improvement in water retention for fibre A was observed when biochar was combined with the biopolymer (AW_3), where MWHC was substantially higher than in other variants. The synergistic effect of both additives likely led to favourable changes in pore structure and the fibre's capacity to retain water molecules. In contrast, for fibre B in variant BW_3, despite the presence of both additives, MWHC decreased compared to the control, possibly indicating a compensatory effect or an unfavourable interaction between the additives within the specific structure of fibre B. Changes in the water retention capacity of the analysed substrates are presented in Figure 4. In the control sample made from coconut fibre A (AW_0), an average and relatively unstable ability to retain water was observed, with a distinct decline noted in the 1 st and 7 th weeks of the experiment. The addition of biochar in variants AW_1 and AW_3 led to an improvement in initial water retention. However, no unequivocal positive effect of biochar on the maintenance of the substrate's retention capacity over time was observed, as evidenced by a significant decrease in the amount of absorbed water, especially in the 7 th week (60.91% for AW_1 and 67.58% for AW_3). The highest retention capacity among the AW variants was achieved in substrate AW_2 (with biopolymer) in the 4 th week (91.21%) and 3 rd week (88.79%), indicating that the biopolymer may significantly enhance the water properties of the substrate, particularly during the initial phase of plant growth. Figure 3: Maximum water holding capacity (MWHC) of the substrates (kg kg\({}^{-1}\)) (mean \(\pm\) SD). NOTE: Mean values \(\pm\) standard deviation. (a–e) Identical superscripts denote no significant (\(p\) < 0.05) differences between the experimental objects according to the post hoc Tukey HSD test.