New research suggests that composting biochar before applying it to soil may significantly improve its effectiveness as a soil amendment, offering stronger gains in plant growth, nutrient availability, and microbial activity than simply blending biochar with finished compost.

Biochar — a carbon-rich material produced by heating biomass in a low-oxygen environment — has drawn growing interest in agriculture and soil restoration because of its potential to improve soil health while also storing carbon. However, researchers note that fresh biochar does not always deliver consistent agronomic benefits when used on its own. In some cases, it can temporarily tie up nutrients or contain residual compounds from the pyrolysis process that may inhibit plant growth.

To better understand how biochar preparation methods influence performance, researchers published a study in the journal Biochar comparing three application strategies: biochar used alone, biochar mixed with green waste compost before soil application, and biochar composted together with fresh organic material prior to use.

The team evaluated three biochars produced from different feedstocks, including beech wood, mixed hardwood, and digestate combined with cereal straw. Lettuce was grown over a three-month period in two soils with differing fertility levels while researchers monitored plant biomass, soil chemistry, and microbial activity.

According to lead author Irina Mikajlo, the findings demonstrate that the preparation process plays a major role in determining biochar performance.

Researchers found that biochar applied alone produced little improvement in lettuce growth compared with untreated soils. When biochar was mixed with compost, plant growth increased roughly threefold. The largest gains came from composted biochar, which boosted plant growth nearly six times over the untreated control.

The composted biochar treatments also improved several soil quality indicators. Researchers observed more balanced soil pH levels, increased dissolved organic carbon and nitrogen availability, and greater microbial activity, particularly in the lower-fertility soil tested in the study.

The study suggests that composting alters the physical and biological properties of biochar in beneficial ways. During the composting process, biochar particles become coated with organic matter and enriched with nutrients, while also developing characteristics that better support soil microbial communities.

Researchers also noted that composting appeared to reduce performance differences among biochars made from various feedstocks, resulting in more consistent functionality across products.

The findings could have practical implications for farmers, composters, and land restoration professionals looking to maximize the value of biochar in agricultural systems. Rather than treating biochar as a stand-alone amendment, the study indicates that “charging” it biologically through composting may provide more reliable agronomic benefits.

The improvements were most pronounced in degraded or nutrient-poor soils, suggesting composted biochar could become a valuable tool for soil restoration and sustainable agriculture initiatives focused on rebuilding organic matter and improving nutrient cycling.

Researchers concluded that integrating biochar into the composting process before land application may offer a more effective strategy for enhancing soil health and supporting plant productivity while also improving the utilization of organic waste streams.