## 4 Discussion
The contents of Ca\({}_{10}\)-P were higher than those of Ca\({}_{2}\)-P and Ca\({}_{8}\)-P (Figure 2), as the latter are easily absorbed and utilized by plants [34; 56; 57], whereas unstable phosphorus forms (such as Ca\({}_{2}\)-P) are transformed into stable phosphorus forms (such as Ca\({}_{10}\)-P) during inorganic phosphorus transformation [58; 59]. The smaller the _silica-alumina ratio_ and the _silica-sesquioxide ratio_, the higher the weathering degree of the soil [19]. Generally speaking, clay soil with a low _silica-sesquioxide ratio_ has a stronger ability to adsorb and withhold phosphorus than sandy soil [60]. In this study, the _silica-alumina ratio_ and _silica-sesquioxide ratio_ of bulk soil sample A were 5.43 and 4.66, respectively, and those of bulk soil sample B were 5.59 and 4.80, respectively, indicating that the weathering intensity in the study area was weak, resulting in a lower ability of soil to adsorb phosphorus (Figure 5). Through a comprehensive analysis of mineral components (Table 5) on the basis of XRD patterns (Figure 3) and FT-IR spectra (Figure 4), the contents of quartz, muscovite, calcite, etc., could be estimated as a function of their phase characteristics and infrared characteristics. The bands near 3697 cm\({}^{-1}\) and 646 cm\({}^{-1}\) (Figure 4) were consistent with estimative minerals data (Table 5), confirming that there was no kaolinite in coarse sand and no feldspars (albite and orthoclase) in clay. The main components of clay and silt were kaolinite and muscovite (Table 5), respectively. The main mineral components of clay and silt were similar, but the adsorption capacity of clay with respect to phosphorus depended on the specific surface area of these minerals [61]. As clay particles are smaller than silt particles, their specific surface area is larger [62], providing more sites for the adsorption of phosphorus. Accordingly, the adsorption capacity of clay was 2-3-fold that of silt, depending on the initial concentration (Figure 4). The main mineral components in sand were quartz, dolomite, and calcite (Table 5), unlike clay and silt. In contrast to other minerals, Al\({}^{3+}\) exists on the edge group Al-OH group of kaolinite [63], facilitating the exchange of phosphate ions on a relatively short timescale. [PERSON] [64] found that, during the reaction of phosphate and calcite, adsorption only occurred at specific positions, thereby forming surface clusters of phosphate ions, leading to the growth of hydroxyapatite crystals. However, the sites available for adsorption only account for 5% of the particle surface [64]. Quartz has a relatively smooth surface with fewer voids and fewer adsorption sites, thus resulting in a lower phosphorus adsorption capacity than calcite [65]. As such, the adsorption capacity of coarse sand was far weaker than that of clay. The mineral compositions of fine sand and coarse sand were basically the same, except for the contents of muscovite and calcite. The content of phyllosilicates in fine sand was higher than that in coarse sand, while the adsorption capacity of clay was also larger than that of coarse sand. An increase in phyllosilicates content can improve the adsorption capacity. \(K_{\rm F}\) (\(p\) < 0.01) and \(K_{L}\) (\(p\) < 0.05) were significantly positively correlated with kaolinite (Table 7). In summary, the fine fractions adsorbed more phosphorus than the coarse fractions, which was a function of particle size and mineral composition. On the one hand, phyllosilicates were concentrated in the fine fractions due to the smaller particle size, leading to higher phyllosilicates and higher adsorption; on the other hand, the smaller particle size resulted in a greater specific surface area and, thus, increased adsorption. HPO\({}_{4}^{2-}\) is the main form of phosphate ion in calcareous soil solution [57], and it can generate dicalcium phosphate with a small amount of exchangeable Ca\({}^{2+}\) on the surface of dolomite [13].