Rice husks, wood remains, nutshells, manure and crop residues are regarded as agricultural waste, but recently such solid wastes have been transformed into biochar for the purpose of carbon sequestration. Biochar is commonly defined as charred organic matter, produced with the intent of being deliberately added to soil to improve its agronomic properties. On average, one ton of dry biomass can create 400 kg of biochar containing 80 to 90% pure carbon (Lehmann et al., 2009) at 300ºC to 700ºC, under low (preferably zero) oxygen concentrations.
Rice husk contains a high content of silicon and potassium, nutrients which have great potential for amending soil, while those with a relatively higher carbon content (e.g. wood or nut shells) are currently used for the production of activated carbon. The use of rice straw and rice husks in the field has been practiced for some time (Ponamperuma, 1982). Research has shown that incorporation of rice straw and rice husks can significantly improve soil properties by decreasing soil bulk density, enhancing soil pH, adding organic carbon, increasing available nutrients and removing heavy metals from the system, ultimately increasing crop yields (Williams et al., 1972).
Similar studies on cowpea, soybean, and maize (Yamato et al., 2006) have also supported the application of biochar as a way to increase crop yields. Asia, a principal rice-growing region, has abundant rice residues, estimated at about 560 million tons of rice straw and 112 million tons of rice husks, respectively. These residues could be a valuable resource for the production of biochar to increase soil fertility. Carbonized rice husks consist of a very light material with a micro-porous structure and a bulk density of about 0.150g cm−3 (Haefele et al., 2009). The carbonization process also improves the water-holding capacity of the rice husks (Oshio et al., 1981). Additionally, the widespread old practice of burning rice straw in the field indicates that black carbon from incompletely burned (i.e. carbonized) rice residues could be an important source of organic matter in rice soils, as has been previously shown for a range of other soil types (Schmidt and Noack, 2000).
The effects of the addition of biochar may vary from soil to soil. However, the following effects have been seen in experiments: a) the rice husk charcoal increases the soil pH, thereby increasing available phosphorus (P), b) improved aeration in the crop root zone, c) improved soil water - holding capacity and d) increased levels of exchangeable potassium (K) and magnesium (Mg) (FFTC, 2001). There is a need to highlight the agronomic properties and the effects of rice husks biochar on the growth of crops, to promote biochar use in the field by small landholders.
With several Asian countries applying the carbonized rice residues, their real outcome has not been clarified. More field work is required to indicate the relationship between the amount of biochar applied and the growth rate of crops. In our study, we examined the effects of rice husk biochar application and compared these results to wood biochar applied to increase the growth of water spinach (Ipomoea aquatica) in field conditions. Water spinach is a fast growing plant with strong system development. It originates from mainland China and is now widely grown worldwide.
Our study assumed that rice husk biochar could act as a soil conditioner, enhancing water spinach growth by supplying and retaining nutrients and thus improving the soil’s physical and biological properties. Our aim was to explore whether rice husk biochar (RHB) and wood biochar (WB), in combination with fertilizers, could increase the biomass yield of water spinach. Soil analysis, shared use of a scanning electron microscope, and heavy metal analysis were used to identify the properties of rice husk biochar. We hope that the results of our work may help to determine which of the biochars is more beneficial in boosting the production of water spinach.
Rice husk biochar added to soil.