## 3 Results and Discussion  
The aim of this study was to analyze the elemental composition and structural characteristics of high quality biochar derived from different feedstock types as a means of improving soil quality, enhancing crop productivity and sequestering carbon in the soil. The raw materials for biochar production were made of agricultural wastes such as corncobs, cassava rhizome, waste fruits peeling and weeds and also wood branches such as _Albizua myriophylla_ [PERSON], _Cornbretum punctatum_ [PERSON], _Leucaena leuccephala_ ([PERSON]), _Samantha saman_ ([PERSON]) or even hard wood as _Streblus licifolius_. The Controlled Temperature Biochar Retort for Slow Pyrolysis Process is cost-efficient and can be built easily using locally available materials and-most importantly-can use locally available biomass as feedstock [22; 34; 42; 43]. The biochar invented oven was low cost and used raw materials from the local area. It is about 50 US dollars [22]. The biochar was produced from the controlled temperature biochar retort for slow pyrolysis process could have a good product and successfully to soil improvement and higher products in rice [34], soybean [49] and vegetables farm [23]. The controlled temperature biochar retort for slow pyrolysis process can control the temperature between 500-600 \({}^{\circ}\)C. The invented oven can control air that passes through inside by pyrolysis condition depending on the size and position of pores around it. Biochar yield depends greatly on thermochemical technology. Slowpyrolysis was most effective; a longer residence time and moderate temperature (350-550 \({}^{\circ}\)C) in the absence of O\({}_{2}\) resulted in higher biochar yield (+30%) than the fast pyrolysis (12%) or gasification (10%) [24]. The process of the Controlled Temperature Biochar Retort for Slow Pyrolysis Process can produce high Biochar yields of around 58-60% from raw materials when compared to other similar ovens. [PERSON] et al. (2018) [44] studied the yield of biochar kiln which was produced from corncob in \"Kon Tiki\" flame curtain and found the yield to be only 34.2 wt%. So, in this study the invented oven can gave a 1.76-fold higher yield than their experiment. [PERSON] et al. (2016) [45] studied biochar from Eupatorium, wood and rice husk using Kon Tiki kilns on Farmer-Scale and found that the average biochar yield was only 22 \(\pm\) 5%. [PERSON] et al. (2017) [46] found an average biochar yield from Eupatorium feed stock of only 19.5 \(\pm\) 05% by using \"Kon Tiki\" flame curtain (four subtypes of flame curtain klins: 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; the yield is around 24 \(\pm\) 4.7%. [PERSON] et al. (2000) [48] studied biochar yield from walnut shell using Heinze retort with the controlled temperature between 400-700 \({}^{\circ}\)C. The best temperature for their study was 500 \({}^{\circ}\)C and the highest biochar yield from walnut shell was 28.9%. All experiments' biochar yields were still up to two times less than the yield from the invented oven with the controlled temperature biochar retort for slow pyrolysis process in this study. Therefore, the oven and the process for produce biochar from corncobs, cassava rhizomes and cassava steams can show the high quality of the biochar by measuring element of C, H, N, and O contents between 62.95-81.35, 2.24-2.73, 1.22-1.65 and 15.23-33.44% by dry weight, respectively. Moreover, surface area and total pore volume of the biochar were higher than raw materials. Total pore volume of corncobs, cassava rhizomes and cassava steams of biochar increased 1091.18, 560.47 and 2001.72%, respectively. Chemical analyses of the feedstock biomass before conversion into biochar (Table 1) revealed that corncobs contained the highest amount of C at 41.66%, while cassava rhizomes contained the lowest (37.60%). Corncobs contained the highest amount of H (6.84%) and cassava stems contained the lowest (6.04%).