## 2 Materials and Methods  
### Biochar Production  
The three sources of crop residues used as feedstock were corncobs, cassava rhizomes and cassava stems. Before heating the biomass, the materials were prepared to similar size distributions and moisture content. The cassava stems and corncobs were cut to less than 15 cm in length, while the cassava rhizomes were cut into lengths shorter than 10 cm. The materials were subjected to slow pyrolysis for 12-24 h in the Controlled Temperature Biochar Retort for Slow Pyrolysis Process (Patent Number 110100118, registered by [PERSON]) (Figure 1). The retort was designed to comply with the respective FAO guidelines [41]. In order to control temperature, temperature data were collected from inside the biochar retort and the end of the chimney at different times throughout the heating process [22]. During the pyrolysis process, the materials were heated to a maximum 500-600 \({}^{\circ}\)C for 30 min. The ratio of biomass volume to fuel volume was 1.0:0.6, i.e., the mass of fuel should represent 60% of the total biomass weight in order to achieve optimal conditions for pyrolysis [34; 41]. The Controlled Temperature Biochar Retort for Slow Pyrolysis Process is cost-efficient and can be built easily by farmers themselves for less than $US 50 using locally available materials and- most importantly- can use locally available biomass as feedstock [22; 34; 42; 43]. The patented design allows control over pyrolysis conditions under limited oxygen availability inside the retort, including air intakes and exhaust holes. The design is critical to produce high-quality biochar--the outer furnace with eight drilled holes and the inner steel biochar retort with four drilled holes, all with a diameter of 2.5 cm--showed a highest yield of biochar and used the least amount of fuel. Moreover, the diameter of the holes must be exactly proportioned as incorrect hole diameter could lead to excessively high temperatures, which may adversely affect the quality of the biochar produced. The retort was designed for farmers in rural areas. It is simple to build and easy to use. The retorts were introduced to local farming communities via projects initiated by organizations under the Royal Project Foundation, including the Huay Sai Royal Development Study Center and Padeng-Biochar Research Center (Pd-BRC). Pyrolysis temperature in the biochar retort can be controlled in the range of 500-600 \({}^{\circ}\)C. Slow for pyrolysis at a moderate temperature (350-550 \({}^{\circ}\)C) in the absence of O\({}_{2}\) and a longer residence time resulted in approximately 30% higher biochar yields than fast pyrolysis or gasification [24]. Higher yields of biochar are obtained, (up to 58-60 wt% of feedstock) as compared with similar oven designs. [PERSON] et al. (2018) [44] found the yield of corncob biochar produced in a \"Kon Tiki\" flame reached only 34.2 wt%. [PERSON] et al. (2016) [45] found similar low biochar yields of 22 \(\pm\) 5 wt% from Eurquadrium, wood and rice husk feedstocks using Kon Tiki kilns at Farmer Scale. These yield levels were consistent with those found by [PERSON] et al. (2017) [46] who reported an average biochar yield  
Figure 1: The Controlled Temperature Biochar Retort for Slow Pyrolysis Process. of only \(19.5\pm 05\%\) from Eupatorium feed stock in a Kon Tiki flame curtain and four subtypes of kilns (deep-cone metal kiln; steel shielded soil pit; conical soil pit and steel small cone). [PERSON] et al. (2013) [47] produced biochar from wheat-straw with a yield of around \(24\pm 4.7\%\). [PERSON] et al. (2000) [48] studied biochar yield from walnut shell using a Heinze retort with controlled temperature between 400-700 \({}^{\circ}\)C. The optimal temperature for their study was 500 \({}^{\circ}\)C and the highest biochar yield achieved was 28.9%. The controlled temperature biochar retort used in the current study can achieve yields of approximately double those achieved using other kiln designs.