Rice husks, a waste product generated in rice mills, can release its carbon as a greenhouse gas (GHG) to the atmosphere through burning or decomposition. Converting it into biochar and intercalating (filled) with urea can produce a slow-release nitrogen (N) fertiliser that improves N-use efficiency while minimizing GHG emissions. Rice husks are often considered as a waste, and its carbon is released to the atmosphere as carbon dioxide (CO2) which is a greenhouse gas (GHG) through either decomposition or burning as a biofuel.

However, rice husks can be converted into biochar – where its carbon is stable - with a large number of micro and sub-micron size pores in a honeycomb-like structure. Rice husk biochar was produced using an improved batch pyrolizer, “Kunthaniya”, at a temperature of between 450°C and 650°C. Pore spaces in rice husk biochar can be intercalated (filled) with urea and then, slow-release fertilizer (SRF) pellets can be produced through the use of a suitable biodegradable binder. This SRF has found to be more efficient in improving the N-use efficiency; hence the urea requirement of paddy fields in Sri Lanka can be reduced by about 25%, further contributing to environment sustainability. It has been well documented that biochar can improve soil physical, chemical and biological properties in a sustainable manner. This process is a contribution to creative recycling of agricultural waste.

The SRF technology was evaluated against current farmer practice in rice cultivated area in Mahakanumulla village, Anuradhapura district, Sri Lanka. The area belongs to the Dry Zone of Sri Lanka (mean annual rainfall <1750mm). Rice is cultivated during two seasons, yala (May-September) and maha (December – February): the yala season is generally drier. Farmers rely on irrigation water supplied from a small village tank. The SRF was transported to farmers’ fields and applied at 2 weeks (@ 100 kg/ha), at 4 weeks (@ 170 kg/ha) and 7 weeks (@ 145 kg/ha) after direct seeding.

Production of SRF: Rice husks were pyrolyzed using a locally modified “Kunthaniya” (a batch pyrolyser) to produce rice husk biochar. The temperature of the pyrolyser was around 450°C to 600°C with a heating rate of less than 20°C per hour. Pore structures were saturated using a urea solution through capillary action. The urea-intercalated rice husk biochar is then mixed with a biodegradable organic substance and pelletized using a medium scale pelletizer and dried to increase its mechanical properties such as resistance to disintegration and shear forces.

Field Experiment : Five paddy farmers were randomly selected from the command area of a small tank in the Mahakanumulla Village Tank Cascade System in the Dry Zone of Sri Lanka. The produced SRF was applied at a rate of 75% of recommended N in three split applications. Yields in SRF applied areas were compared against the current farmer practice. Experimental evidence showed that there is no yield reduction despite the reduction of nitrogen input into their fields.

Farmers indicated that the granule size was large and light, hence they had some concerns about even distribution of fertilizer. They perceive that plants receive N slowly compared to granular urea - suggesting the slow releasing nature of the new technology. They did not observe any yield difference. Obtaining rice husks in large quantities from rice mills to produce biochar can sometimes be difficult in some areas of the country due to competition for use in the poultry industry. Some farmers may be discouraged to implement this technology due to lack of knowledge: this can be overcome through extension officers operating at field level.

This new technology qualifies as a sustainable land management practice in number of ways. First it increases N-use efficiency in paddy fields, second it reduces the urea requirement by 25% while sustaining productivity, third it recycles agricultural wastes in paddy fields, fourth, repeated application of SRF improves soil fertility through rice husk biochar, and finally it reduces GHG emissions.

Compiler: Head Department of Soil Science, Faculty of Agriculture, University of Peradeniya

Co-compilers: Prof R. S Dharmakeerthi, Tharindu Kulasinghe (Department of Soil Science, Faculty of Agriculture, University of Peradeniya)

Key Resource Persons: Mrs. Lakmini Dissanayaka (Research Officer at Coconut Research Institute, Lunuvila)

Mr. Thilak Munasinghe (Research Officer, Rice Research Station, Amabalantota)

Land Users: Renuka Senarathna (Mahakanumulla Thirappane)

Lekam Ralalage Nandasena (Mahakanumulla Thirappane)

Vimalawathi Kirihamige (Mahakanumulla Thirappane)