On-Farm Biochar Production: Quality control

In this is the final of 3 blogs exploring Climate Spheres’s recent on-farm biochar production and application project with Aqualate Farms, and the School of Sustainable Food and Farming and the Farmers Guardian. You can find the introductory blog here and one exploring the challenges in the production and application processes here. In this installment, we discuss the challenge of running the biochar kiln for the first time and share analysis of the biochar that was produced. 

1.Feedstock selection, preparation, and the burn

We began the project by discussing with the farmers the types of waste biomass that they most commonly have around the farm in significant quantities. We discussed the different waste streams and what opportunities for circular economy they had already identified. Together we determined that the most regularly available and underutilised material was waste oak materials collected from managing the trees around the farm. There was some concern that the residual dampness of the oak would have an impact on the quality of the biochar produced. This delayed the burn for some weeks as the oak needed to be stored undercover longer than predicted to dry adequately.

In starting the burn, some small pieces of waste wood were used as kindling, while these were not coated in creosote they may have introduced some impurities that would not have been represented in the sample taken for analysis. 

The kiln smoked heavily during the burn. Fumes released during this stage were not monitored. This is something we would want to monitor in future analysis to ensure safety for both human health and the environment. Data on this would be required for a full Life Cycle Assessment to be conducted that could give a proper understanding of the relative impacts and benefits of the agricultural production and application of biochar, and would be important in determining the best type of kiln for farmers to use.  

2. Performance and analysis results

After the burn, all of the wood had burnt through leaving biochar and ash with a fairly consistent quality across the kiln. This included some larger pieces than the farmer had anticipated, causing a reconsideration of how to manage the biochar and the processing that would be required. 

We sent a 1 kg biochar sample off to Eurofins Agro for testing. While the Eurofins contact was happy for the biochar to be sent in large pieces, it was partly broken down by the farmer to reduce packaging size. This was much more laborious and time consuming than anticipated, as with the relatively small amount involved he did this manually. On reflection, we would recommend using a mechanical approach to crush even a small sample of biochar.

The useful and informative analysis report received from Eurofins clearly set out the results from a broad range of tests. The biochar came back with a pH of 7.96, so a little alkaline but not enough to have a significant ‘liming effect' on the soil at the application rate used in this project. The fresh biochar (not dry matter) had 91% m/m organic matter, 53% m/m organic carbon, and a bulk density of 189 g/l.

The biochar was tested for the presence of a range of heavy metals (metal and metalloids with high density that are toxic even at very low concentrations), such as lead, cadmium, and mercury. Thankfully all of these passed with acceptable levels. Most came back with < 1% of the upper limit permitted. Cadmium was at nearly 7% of the permitted concentration, which suggests that this is something to be aware of and monitor, particularly if large quantities of biochar are intended to be applied on a regular basis. Cadmium is a carcinogen that can impair plant growth and health and is toxic to soil microbial biomass and its activity. Interestingly, a method used to alleviate cadmium toxicity in soils is the addition of biochar. 

The biochar was not formally tested for hydrophobia (how water repellent the biochar is), however this was taken into account. Our primary concern here was the potential challenge for nutrient uptake, as we wanted to be sure that when we mixed the biochar with farmyard manure the biochar would take up the nutrients well. Biochar hydrophobia is a particular focus when the biochar is used as a soil amendment with the intention of improving water management. Hydrophobia can break down in the soil over time, giving a delay to the intended positive outcomes.

As a simple and quick test for hydrophobia, water was poured onto the biochar. The water was observed to be quickly taken up by the biochar, leaving a dry feel. From this we had reasonable confidence that the biochar would successfully interact with the moisture in the farmyard manure and be able to take up nutrients from it. 

3. Conclusion

Having explored the results of this biochar analysis and discussed the data with both an agronomist and an academic interested in biochar, our farmers could go ahead with the application of the self-made biochar with full confidence that it posed no risk to their soil or crops.