Charcoal, Ash and Biochar as Soil Amendments

Are charcoal, ash or biochar allowed as soil amendments in USDA organic farming?

By Brian Baker

The terms “charcoal,” “ash” and “biochar” are often used interchangeably, especially within the context of material review. The answer to whether these materials are allowed in USDA organic production depends on how they’re made. If these materials are produced by burning plants or animal materials (e.g., bones), they are permitted as a soil amendment on organic farms. 

Charcoal, ash and biochar produced by burning manure, sewage sludge, or treated wood products are prohibited. Fly ash (the residue from coal furnaces) is also prohibited as a soil input. The NOP rule also prohibits burning as a means of disposal of crop residues, unless the burning is used to suppress the spread of disease, or to stimulate seed germination (§205.203(e)(3)). Therefore, any burning of crop residues would need to be documented in the Organic System Plan consistent with those purposes and approved by the organic certifier. OMRI commonly sees wood and wood by-products used as raw materials and not crop residues. 

What is “biochar” and what can it do for my soils? 

A growing number of products historically considered as charcoal or ash products have in recent years began to be referred to as “biochar” or “terra preta.” Biochars are specific forms of charcoal produced by pyrolysis in reduced (low oxygen) conditions. The soil organic matter (SOM) of biochars are reported to be more stable as a result (i.e., carbon less susceptible to mineralization and subsequent release into the environment as carbon dioxide).

In theory, the process withdraws atmospheric carbon dioxide and stores it in the soil, potentially serving to build SOM and reduce carbon emissions from agriculture. Proponents claim that widespread use of the material may help prevent climate change. However, evidence is not clear about the effects of biochar on net carbon emissions—techniques used to sequester more carbon in the soil through the application of biochar may be offset by what is released during its production.

This article originally appeared in the spring 2009 issue of the OMRI Materials Review newsletter, and was revised in November 2022 by Technical Director Doug Currier.