## 3 Results and Discussion  
### Physical Parameters of Modified Horticultural Substrates  
Coconut fibre is a natural material with proven suitability as a horticultural substrate for plant cultivation. However, the main drawback of such a substrate is its limited durability. During the cultivation process, coconut fibre loses volume relative to its original state, which is a result of its degradation [15]. This phenomenon restricts the transport of nutrients delivered to plants via the retention system, primarily due to a loss of retention capacity. As a consequence, these substrates have a limited lifespan, typically lasting up to two growing seasons in container cultivation. The defined lifespan of the substrate directly affects the profitability of production.  
One approach to extending the durability of coconut substrates is surface modification of the fibres. However, this process involves the introduction of additional components, which should be of natural origin and exhibit a high degree of biodegradability. Such a method was described by [PERSON] et al. [12], who demonstrated that such coatings can be successfully applied in horticultural production. To date, studies have confirmed no negative impact of surface modification on plant growth and development, as assessed by physiological parameters, as well as on the quality of raspberry fruits (vitamin C content, total polyphenol content, and total antioxidant capacity). However, the studies were conducted using only one type of coconut fibre and were limited to selected substrate parameters.  
The modified substrates produced in this study were assessed in terms of selected physical properties, changes in which reflect the degree of modification of the coconut fibre surface. The addition of pelletised biochar reduced the porosity of the substrate; however, the impact varied depending on the type of coconut fibre used [16]. Coconut fibre of variant A with the addition of biochar showed a 21% reduction in porosity (AW_1), whereas variant B showed only an 11% reduction (BW_1). This decrease was attributed to the type of coconut fibre used, as the biochar addition was identical in both substrate variants, amounting to 10% by weight.  
Surface coating of the coconut fibres with biopolymer, as expected, reduced the specific surface area of the fibres by 12.7% (AW_2) and 24% (BW_2), respectively. As reported by [PERSON] et al. [17], coating porous fibres with polymer layers generally reduces their specific surface area due to the surface smoothing process. This trend was observed in the present study for the tested coconut fibre types following biopolymer coating. The smaller reduction observed for fibres with higher initial porosity indicates a lower degree of surface modification, which may result in increased susceptibility to substrate degradation during use [18].  
The combination of the proposed modifiers for coconut-fibre-based horticultural substrates--namely the biopolymer coating and the addition of pelletised biochar--resulted in increased porosity of the tested substrates. The change in porosity directly influenced the bulk density of the substrates. The trend in bulk density was inversely proportional to that of porosity (Table 1). Materials with a low specific surface area, and consequently low porosity, are characterised by higher bulk density, which is related to a tendency toward fibre agglomeration [19].  
However, the compaction susceptibility of the substrate depends solely on the type of coconut fibre used. The biopolymer coating process and the addition of pelletised biochar had virtually no effect on this parameter (Table 1).