Characterization of castor plant-derived biochars

Climate change and population explosion have almost equitably contributed on the intensification of agricultural production. In return, soil nutrients have become depleted, therefore, requiring intense fertilizer application to offset the nutrients deficit. Biochar has for a long time being investigated as a potential source of nutrients and as a substitute to fertilizers, however, there is limited use of biochar due to inadequate information concerning its effects on plant productivity. Although both the former and the latter have negative impacts in the soil, the benefits gained from biochar outweigh their inefficiencies. Biochar can be derived from quite a variety of crop biomass such as wheat, castor seeds and maize. Unfortunately, little has been reported on the use of castor derived biochar for soil amendment.

Researchers from Institute of Applied Biosciences and Chemical Process and Energy Resources Institute in Greece explored the potential of residual de-oiled castor cake and castor stalk field remnants in the production of biochar using castor and tomato as indicator plants. They hypothesized that castor biochar could be used as soil amendment for increasing soil fertility and enhancing plant growth. Their work is currently published in the research journal, Biomass and Bioenergy.

The team initiated the experimental studies by cultivating castor plants which were then harvested and pressed mechanically for oil extraction. The resulting castor cake was obtained and utilized as feedstock along with the castor stalks that had remained from plant cropping. The dried biomass, comprising of both castor stalks and castor cake, was milled and subjected to slow pyrolysis. The resulting biochar was characterized. Eventually, data and soil analysis was conducted after having tomato and castor plant seeds germinate and grow for 8 weeks on the premixed biochar.

The authors observed that the addition of castor biochar affected root development in young seedlings of castor and tomato plants, depending on the type and level of biochar. From the data analyzed, the researchers realized that the biochar provided good effects on soil cation exchange capacity, EC and availability of essential macronutrients along with a slight increase of soil pH. Generally, it was seen that the beneficial effects of castor biochar types on seedlings at a modest level of 1-5% were good, suggesting that it had the potential be used to improve soil and promote a well-developed root system which is fundamental for plant establishment.

The study presented successfully the production of biochar by slow pyrolysis at 550^° C from castor stalks and de-oiled castor cake, both by-products of castor oil production, in view of sustainable valorization of a potential large scale cultivation of castor beans. It has been seen that castor biochars improved castor seed germination, achieving 90% success rate earlier when compared to control, but failed to show any significant effect on the germination of tomato seedlings. Therefore, biochar has the potential to be used as a soil amendment material and its utilization depends upon feedstock production, pyrolysis cost, and the value of carbon offsets.

‘Castor bean cultivation generates large volumes of residual biomass which remains in the field and can be valorized through slow pyrolysis to produce biochar which may support plant growth when used as soil amendment. This is a sustainable agricultural practice with eco-friendly materials’ Said Zoe Hilioti, first author of the paper.