## 3 Results and Discussion
### Ability to Release Selected Mineral Components into the Aqueous Phase
#### 3.2.2 Retention Properties
In contrast, for substrates based on coconut fibre B, a varied dynamic of water retention changes was also noted. In the control (BW_0), water retention was highest in the first weeks of the experiment, with the peak value in the 2 nd week (89.67%). The addition of biochar in variants BW_1 and BW_3 did not unequivocally improve retention parameters. Variant BW_2, coated with biopolymer, achieved the greatest water retention capacity in the 3 rd week of the experiment (99.77%). The obtained results indicate that the applied biopolymer coatings have a significantly positive effect on the retention capacities of the substrates, especially during the most intensive period of plant growth (weeks 3-5). Conversely, despite biochar being a material with good sorption properties, it did not demonstrate a clearly positive impact on the retention parameters of the substrate in the conducted studies. Changes in the water holding capacity of the substrate after the addition of biochar were described in the study by [PERSON] et al. [20]. 2.3 Content of Inorganic Ions (Ca\({}^{2+}\), Mg\({}^{2+}\), K\({}^{+}\), PO\({}_{4}\)\({}^{3-}\)) in the Produced Substrates (Leaching Tests)  
The produced substrates were also analysed for the content of major nutrients in ionic forms available to plants. The analyses concerned the content of Ca, Mg, K, and P in forms accessible to plants. The highest average content of Ca\({}^{2+}\) extracted with water was found in the control variants for both types of coconut fibre, A and B (Figure 5). After the application of biochar and biopolymer, the average Ca\({}^{2+}\) content decreased, which was observed for both analysed coconut fibre variants. This relationship may be due to the fact that the applied additives promote greater sorption of this element, meaning it is not easily leached from the substrate, which is characterised by high permeability, thereby resulting in increased retention and availability for the developing plant roots. A pertinent study by [PERSON] et al. [22] investigated the effects of biochar amendments on the sorption and leaching of nitrate, ammonium, and phosphate in sandy soils. The researchers found that certain biochars significantly reduced the leaching of these nutrients, indicating enhanced sorption and retention within the soil matrix. This effect was particularly notable in sandy soils, which are characterised by high permeability and typically low nutrient retention [23]. Figure 4: Water retention dynamics of the produced substrates (%). In the case of Mg\({}^{2+}\) ions, a similar trend was observed; however, this applied exclusively to substrates produced from coconut fibre A. Conversely, the opposite relationship was noted for substrates made from coconut fibre B. In this variant, the average content of Mg\({}^{2+}\) ions was lowest in the BW_0 control variant and increased noticeably following the application of the tested additives, reaching the highest values in variant BW_3 (Figure 6). Unlike the observed trends for calcium (Ca\({}^{2+}\)) and magnesium (Mg\({}^{2+}\)) cations, the behaviour of potassium (K\({}^{+}\)) ions in the tested substrates showed different tendencies. Potassium is an element with high mobility and very good solubility in water, which makes it highly susceptible to leaching from the rhizosphere, particularly under conditions of intensive irrigation. For this reason, its availability to plants can vary significantly depending on the substrate composition and the presence of additives with sorption properties. In this study, the lowest content of K\({}^{+}\) ions in aqueous solutions was recorded in the control samples without modifying additives (AW_0 and BW_0), confirming the limited ability of coconut fibre, as a base material, to effectively retain and release potassium. The use of additives such as biochar and biopolymer significantly influenced the dynamics of K\({}^{+}\) ion release into the soil solution.