## 3 Results and Discussions  
### Fertilizer Characterization  
The characteristics of the P-BC-LDHs prepared in this study are listed in Table 1. The P-BC-LDHs had a high initial pH of 9.02, and as such, the use of these materials in soil would exert a positive liming effect, which can remediate soil acidity and lead to improved soil fertility [23]. An understanding of the amount of soluble salts in P-BC-LDHs is important because its application to soil or growing media at high concentrations may adversely affect salt-sensitive plants [24]. Owing to a lack of available data corresponding to the EC values of biochar-based functional materials, the EC values of P-BC-LDHs (1.36 dS/m) were comparable to those of biochar samples, which were mostly grouped into 0.04 to 54.2 dS/m [24]. The moisture content and water-holding capacity were 2.20% and 65%, respectively. It was expected that the high surface area and porosity of biochar in this composite were responsible for its high water retention, which was significantly higher than that of sandy soil after organic amendment through the addition of clay (4-13%) [25]. The high water-holding capacity of the composite will enhance the efficiency of water use in agricultural production in terms of the water supply for plants. It was observed that P-BC-LDHs also had a high nutrient retention capacity, with a CEC of 23.1 cmol/kg. This was because of the combined effect of specific surface area, porosity, and the chemical functional groups of biochar [26]. Moreover, the elemental analysis demonstrated that approximately 5 mg/g of phosphorus, 138 mg/g of magnesium, and 65 mg/g of aluminum were accommodated into the P-BC-LDHs. Phosphorus and magnesium have been widely acknowledged as essential macronutrients that are required in large amounts for plant growth and development. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline pH & 9.02 \(\pm\) 0.020 \\ \hline Electrical conductivity (EC) (dS/m) & 1.36 \(\pm\) 0.016 \\ \hline Moisture content (\%) & 2.20 \(\pm\) 0.286 \\ \hline Water holding capacity (\%) & 65 \(\pm\) 0.000 \\ \hline Cation exchange capacity (CEC) (cmol/kg) & 23.1 \(\pm\) 1.274 \\ \hline Phosphorus (mg/g) & 4.67 \(\pm\) 0.055 \\ \hline Magnesium (mg/g) & 138 \(\pm\) 2.807 \\ \hline Aluminum (mg/g) & 64.7 \(\pm\) 2.220 \\ \hline
**Pollutants** & **Values** & **Maximum Acceptable Levels** \\ \hline Cobalt (mg/g) & ND & 0.15 \\ \hline Nickle (mg/g) & ND & 0.18 \\ \hline Copper (mg/g) & 0.0325 \(\pm\) 0.005 & - \\ \hline Zinc (mg/g) & 0.0519 \(\pm\) 0.003 & 1.85 \\ \hline Cadmium (mg/g) & ND & 0.02 \\ \hline Molybdenum (mg/g) & ND & 0.02 \\ \hline Lead (mg/g) & ND & 0.50 \\ \hline \hline \end{tabular} Data presented in the table are represented as the mean \(\pm\) standard deviation of the triplicate measurements. ND: not determined.  
\end{table} 
Table 1: Basic physicochemical properties and heavy metal concentrations of P-BC-LDHs obtained from this study. Raw materials used in the manufacturing process of common commercial fertilizers are usually insufficiently purified, and this will lead to the inadvertent addition of heavy metals, such as cadmium, nickel, and lead to the environment and the transfer of these metals to the human food chain. The assessment of heavy metal contamination showed that no hazardous elements, including cobalt, nickel, cadmium, molybdenum, and lead, were present in these products. However, copper and zinc were detected, probably because biochar has inherent heavy metals within the structure derived from its source materials, which may be concentrated in ash fractions during pyrolysis [27]. The concentrations of these two elements were extremely low (less than 0.1 mg/g) and below the Canadian standard adopted for heavy metals in fertilizer products [28], indicating that the application of P-BC-LDHs is not considered to have detrimental effects in terms of heavy metal pollution. The elemental composition analysis also confirmed the accumulation of aluminum in dried seedlings with fertilization (Figure 2).