## 2 Materials and Methods
### Soil Chemical Analysis before and after Pot Study  
The pot study was carried out using Topic Paleudults (Bekenu Series). The soil was taken from a secondary forest (latitude 3 deg12'15.45'' N and longitude 113 deg04'15.82'' E) at Bintulu, Sarawak, Malaysia. The soil samples were collected at a depth of 0 to 25 cm after which they were transported to a research complex at Universiti Putra Malaysia Bintulu Sarawak Campus, Malaysia where they were air dried at room temperature. Afterwards, the soil samples were crushed to pass a 5 mm sieve to remove twigs, plant roots, and ironstone concretions. Prior to collection of the soil samples, soil bulk density was determined using the core sampling method. The soil surface area was cleared, and three core samplers were gently hampered into the soil (within depth of 1 to 25 cm). Thereafter, the soil samples were dried in an oven at 105 \({}^{\circ}\)C until constant weight was attained, cooled in a desiccator, and weighed.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Macro Nutrients**} & \multicolumn{3}{c}{**Micro Nutrients**} \\ \hline
**pH** & **8.5** & **Av. Particle Size** & \multicolumn{2}{c}{**0.5-2 mm**} \\ \hline
**\%** & & & **mg kg\({}^{-1}\)** & \\ \hline Total organic carbon & 63.7 & Silicon & 2.3 & Magnesium oxide & 6.7 \\ Fixed Carbon & 61.2 & Aluminium & 1.5 & Arsenic & 2.1 \\ Nitrogen & 2.8 & Potassium oxide & 16.3 & Cadmium & 0.7 \\ Phosphate & 2.6 & Boron & 62 & Chromium & 9.6 \\ Potassium & 3.9 & Copper & 167 & Mercury & 0.06 \\ Calcium & 5.9 & Manganese & 1130 & Nickel & 14 \\ Sulphur & 0.59 & Zinc & 856 & Lead & 12 \\ Ash content & 23.7 & & & \\ \hline \hline \end{tabular} Source: [PERSON] et al. [15].  
\end{table}
Table 3: Selected chemical properties of Black Earth chicken litter biochar.  
The weight of the empty core sampler was subtracted from the final weight to obtain weight of the dry soil. Equation (1) was used to calculate bulk of the soil.  
\[\textbf{Bulk Density}=\left[\begin{array}{c}\textbf{Dry Wt. of soil only}\\ \textbf{Core volume}\end{array}\right] \tag{1}\]  
where  
\[Core\ volume=\pi r^{2}h=100.14\ \text{cm}^{3}\]  
The physico-chemical properties of the soil used in this present study (Table 4) were within the range reported of the Bekenu series (Typic Paleudult) [18].  
Soil samples were characterized for physical and chemical properties before and after the pot study. Soil pH in water and KCl were determined in a 1:2.5 (soil:distilled water/KCl) using a digital pH meter [19]. Soil total organic carbon was calculated as 58% of the organic matter using the loss of weight on ignition method [20]. Cation exchange capacity (CEC) of the soil was determined using the leaching method [21] followed by steam distillation [22]. Exchangeable cations were extracted with 1 M NH\({}_{4}\)OAc, pH 7 using the leaching method [21]. Afterwards, the cations were determined using atomic absorption spectrometry (Analyst 800, Perkin Elmer instruments, Norwalk, CT, USA). Total N was determined using Kjeldhal method [23]. Soil total P was extracted using aqua regia method whereas soil available P was extracted using Mehlich No.1 double acid method [24]. Water soluble P was extracted using deionized water following the Mehlich No.1 double acid extraction method [24]. The extracted total P, available P, and water-soluble P were then determined using spectrophotometer after the blue color was developed [25]. Soil exchangeable acidity, H\({}^{+}\), and Al\({}^{3+}\) were determined using acid-base titration method [26].