There is 10-12 ton per hector biochar to be dosed as suggested by most of the research paper for rice cultivation. However, it is quite impossible to add that large volume practically to be economically efficient practice.
Hi Bidus, the dosage you quoted is quite commonly reported in literature for freshly produced biochar but depends on soil fertility status and type as well as physico-chemical properties and history of biochar. If the soil is not fertile you may be able to add less biochar to obtain a positive result. If the biochar has been charged with nutrients (e.g. via composting, use as bedding material in poultry farms or in anaerobic digesters; http://ejtafs.mardi.gov.my/jtafs/44-2/biocharcoal.pdf ) you may again need less biochar. Density of biochar is critical in conventional paddy fields to prevent wash-out. Biochar-clay complexes which not only have a high cation exchange capacity but also density may be more suitable than say plain rice husk biochar. You also don't have to add all biochar in one go to all parts of the field. Target those spots where the soil is not fertile. In Kedah (Malaysia), the utilization of low grade rice husk biochar as polybag and rice transplanting medium (http://www.msss.com.my/mjss/Full Text/Vol 16/Theeba.pdf) is an established practice. The advantage here is that the roots of the rice plant have grabbed the biochar particles thus reducing the possibility of loss. I suggest you carry out pot trials with charged and non-charged biochar for soil of different fertility from your field to determine the most promising dosage. Hope this helps? Kind regards, Rob.