Biochar is carbonised organic material that is produced through pyrolysis, that is thermal conversion under low or no oxygen. The biochar is used as an amendment to farmland for ameliorating soil properties. Owing to the great number of micropores and charges on biochar, its application to soil boosts water and nutrient storage, and their delivery to crops. Biochar also helps neutralise acidity in soils, replacing the need for lime. All these positive effects make it ideally suited for acid, nutrient-poor soils in tropical/ sub-tropical regions. Even at low rates of 1 or 2 tonnes per hectare, biochar significantly increases crop production. Biochar hardly decomposes in soils, causing positive effects to last for several years without repeat application. This also provides very stable carbon storage on farms which helps mitigate climate change. Its stability also allows farmers to gradually add biochar to the entire farm, thereby increasing their food self-sufficiency and incomes.
Biochar itself is not a fertilizer, but a soil rejuvenating agent. Therefore, farmers must apply essential nutrients through synthetic or organic inputs to reap the full yield potential and maintain soil fertility. Yet, when biochar is combined with fertilizers, about 40% - 70% nitrogen and phosphorus can be saved over multiple seasons while maintaining the same productivity. Biochar is a promising solution to address low response of crops to fertilizer addition in tropical soils and to alleviate the impact of input price hikes and supply disruptions.
In the study presented here, farmers produced biochar as a residue from small gasifier stoves during daily cooking. These systems are highly energy efficient and reduce the consumption of firewood while improving indoor air quality, which, alongside crop yield gains from biochar, is perceived positively by farmers. The specific design of gasifier stoves requires them to be well-packed with appropriately sized tree prunings or maize stems. For larger meals it requires a refill, which users find less convenient than their traditional open combustion fires. Life-cycle assessment shows that households significantly reduce greenhouse gas emissions with the gasifier stoves when biochar is applied to the farm. For the field trials with maize and kale in this study, farmers applied their usual rates of inorganic fertilizers, animal manure, or a combination of both. Results from this multi-locational participatory assessment showed that maize grain yield increased by 0.2 - 0.7 tonnes per hectare for every tonne of home-made biochar that was added to the field.
One household can produce 300 kg biochar per year by cooking on the gasifier stove. With this amount a biochar dosage of 1.5 t/ha can be realized on an area of 0.2 ha per year. If biochar is not spread evenly, but along seed rows, its effective dosage can be increased. As the gasifier stove can be operated for 2-3 years, it is assumed that the costs of 50 USD, which are needed to purchase one gasifier, are good to equip 1 ha of land with an amount of biochar that results in increased crop yields.
