In the on-going quest to develop sustainable agricultural practices, growers are looking for new and inventive technologies. In this blog post, we'll focus on biochar, one such technology that has been a focus of intense research in recent years. Biochar is produced by burning organic material at extreme temperatures as high as 1600° F with little to no oxygen available. Oftentimes biochar is a by-product of energy production, but it can also be produced solely to be used as a soil amendment.

There's a few reasons growers might incorporate biochar into their cropping systems. Biochars' high surface area allows it to act as a reservoir of water while increasing the retention of nutrients such as calcium, magnesium, and ammonium. This is especially useful in more sandy soils with low cation exchange capacity. Biochar can also serve as a liming agent to increase soil pH, which increases nutrient availability in acidic soils. Additionally, biochars with high ash content can contain calcium and potassium that plants can use. Biochar inputs are also high in carbon. Stay tuned to this blog for another post highlighting the potential for biochar to increase soil carbon storage.

Feedstock – the organic material used to produce biochar – varies widely. Common feedstocks include wood chips, nut shells, and grasses. In California nut shells stand as a potentially useful source of feedstock due to the large nut industry. Biochar can also be produced from manures. Both feedstock and production temperature influence how biochar will behave in the soil. Dr. Sanjai Parikh's lab at the University of California, Davis has developed a biochar database that includes both of these characteristics.

Initial interest in biochar stemmed from the study of the Terra Preta soils in South America. These generally low fertility, acidic oxisols were able to sustain higher productivity than nearby non-Terra Preta soils while also accumulating organic matter. One of the reasons for this productivity was the addition of charcoal by indigenous farmers thousands of years ago. The hope was to mimic this in a modern agricultural setting.

Like most agricultural practices, biochars present some challenges for effective integration into a cropping system. Like compost or manure, it can be difficult to predict when nutrients from biochar will become plant available or how a char will interact with a particular soil. Different soil types require different rates of biochar application. For example, a clay loam would require more biochar to increase pH when compared to a sandy soil as a result of the clay loam's higher buffering capacity (see figure below).

UC Davis Soils and Biogeochemistry graduate student Deirdre Griffin is researching how soil microbes respond to biochar additions. She explains that “while biochars can sometimes serve as a source of labile carbon to spur microbial activity, some chars can give off inhibitory compounds that may reduce microbial activity.” In particular, she is looking at whether biochars with high sorption capacity (i.e. the ability to hold on to compounds in the soil) can interfere with signaling between legumes and soil bacteria that fix nitrogen and make it available to plants. She is careful to note that “others have found biochars to increase nodulation in legumes.”

All in all, “the leaders in the field recognize that while there are many benefits of biochar, there can also be negative impacts…There was a burst of [research] excitement followed by some backlash, and now things are starting to even out.” Biochar can serve as a tool for sustainable production systems, but it isn't appropriate for every situation. Continued research will illuminate what types of biochar are suitable for different soils.