This study compared the agronomic properties of rice husk biochar (RHB) and wood biochar (WB) and investigated their effect on the growth rate of water spinach in a field experiment. Each biochar type was tested at 5 different rates of 4.0, 3.0, 2.0, 1.0 and 0.5 kg m3, respectively. The results detailing the growth of water spinach showed that the application of rice husk biochar improves biomass production. The WB added to soil increased the plant weight of water spinach by increasing the root size and leaf width, whereas RHB added to soil increased plant weight by increasing the stem size and leaf length of the water spinach. In addition, the stem size of water spinach was proportional to the WHC/silt ratio; whereas the root size of water spinach was proportional to the OM/OC ratio of soil. We also proposed that the working mechanism of WB and RHB in soil would be such, that the decomposition of OC in biochar-added soil to OM resulted in increased WHC and decreased silt in biochar-added soil. The decomposition of WB biochar was faster than RHB under a lower dosage amount (< 1.5 kg m3), but this trend was inversed with a higher dosage ( > 3.0 kg m3).