## 4 Discussion  
This study investigated the interactions between coir fiber and soil properties, including physical characteristics, mechanical performance, and crack resistance, in sandy and silty soils with varying fiber lengths and contents. The results demonstrate that fiber incor  
Figure 4: Two soil principal component analysis loading diagrams. Figure 5: Two soil 2D response surface plots. poration effectively enhances overall soil stability, optimizes soil structure, and mitigates soil cracking. From a mechanical perspective, fibers formed a three-dimensional network within the soil, strengthening interparticle cohesion and friction, thereby increasing soil cohesion and shear strength [22]. The optimal reinforcement was observed at fiber lengths in the range of 3-4 cm and a content in the range of 0.4-0.6%, where fibers were well-dispersed without excessive aggregation, significantly reducing lateral particle displacement during shear failure. This effect was more pronounced in sandy soil due to its larger porosity and weaker structural framework. However, sandy soil was also more sensitive to fiber content, as excessive or overly long fibers tended to accumulate locally, weakening reinforcement efficiency. Similar phenomena have been reported in studies on other natural fibers, such as straw and jute [23]. Regarding crack formation and propagation, soil cracking typically results from severe volume shrinkage due to water loss, leading to concentrated stress. Coir fiber, with its high tensile strength and elongation, acted as a constraint network within the soil, distributing localized tensile stress during moisture evaporation and preventing crack connectivity [24]. However, when fiber length or content was excessive, dispersion became less uniform, forming large aggregated structures. These fiber clusters created stress concentration zones at their peripheries, ultimately promoting cracking rather than preventing it [25]. Thus, the precise control of fiber length and content is essential to minimize soil cracking while maintaining adequate aeration and moisture retention. The addition of fibers also altered soil physical properties, increasing porosity while decreasing bulk density. Previous studies indicate that natural fibers not only increase soil pore volume, but also contribute organic matter upon degradation [26], benefiting plant growth and soil aggregation. In this study, fibers of 3-4 cm length with a moderate-to-high content resulted in the most significant porosity improvements, particularly enhancing soil aeration and permeability. However, capillary porosity showed a slight decline, suggesting that excessive fiber content disrupted continuous capillary pores. In hydroseeding applications, this presents a trade-off: improved aeration and faster infiltration prevent surface runoff, but excessive permeability may limit water retention, potentially affecting plant root development. Therefore, fiber composition should be optimized based on local rainfall conditions and vegetation requirements. Finally, the laboratory experiments inherently do not account for complex field conditions, including wet-dry cycles, rainfall erosion, and vegetation root reinforcement. Additionally, the long-term effects of fiber degradation on soil porosity and mechanical properties remain unexplored. Some studies suggest that fiber degradation can enhance soil organic matter while dynamically altering soil stability [27; 28]. Future research should include long-term field monitoring to examine fiber-root interactions and assess their comprehensive effects on soil stabilization and ecological restoration. In conclusion, coir fiber effectively improved soil properties in both sandy and silty soils. The appropriate fiber length and content combination balanced soil mechanical strength and crack resistance. To maximize effectiveness, fiber parameters should be tailored to specific soil conditions and engineering applications.