“Biochar is a charcoal-like material that degrades over hundreds to thousands of years,” explained Dr. Deborah Aller during the 2023 Hudson Valley webinar and field day series. “It’s produced through pyrolysis – the thermochemical decomposition of biomass at high temperatures in a no-to-very low oxygen environment.”
While biochar looks like charcoal, it is not a fuel like charcoal, and it has a wide range of potential environmental and agricultural benefits.
Aller is an Extension associate in soil and crop sciences at Cornell University and program coordinator for the New York Soil Health Initiative. She also serves on the Board of Directors for the U.S. Biochar Initiative (USBI).
Biochar is made from organic waste materials, referred to as feedstocks. Examples include crop residues, dry manure, woody material from dead or dying trees, vineyard and orchard prunings, corn stover, etc. It can be produced on many different scales from large commercial units with energy co-generation to the at-home small kiln.
Biochar is highly stable, often comprising between 60% and 90% carbon depending on the feedstock and pyrolysis conditions. “If you look under a microscope at biochar’s structure, it looks like a sponge. There are larger pores and smaller pores, with a carbon backbone. Its high fixed carbon content and stability make it a long-lasting soil amendment,” said Aller.
The recalcitrant (or “very dead”) pool of soil organic matter to which biochar belongs is one of three general pools of soil OM. The other two pools are active and dead. The active pool is composed of living soil organisms and plant roots that are easily decomposed and readily available. It includes bacteria, fungi, nematodes, earthworms, mites, springtails and plant roots. The dead pool is a slower pool that includes manure, recently dead soil organisms and partially decomposed crop residues.
Adding to the recalcitrant pool through soil amendments, such as biochar, can increase soil water holding capacity, nutrient retention and carbon sequestration. The benefits provided by biochar play a role in building soil health, improving crop yields and helping producers mitigate and adapt to climate change.
Aller said, “Over 33% of global soils have been degraded, but degraded soils can be restored by land management practices that increase soil organic carbon stocks.”
Using biochar as a soil amendment is not a novel idea; it was in part inspired by the Terra Preta de Indio soils of the Amazon. According to the Smithsonian Environmental Research Center, slash-and-char farming by Indigenous peoples between 2,500 and 500 years ago left areas of dark, highly fertile soils in the otherwise infertile red and yellow tropical soils. There is also evidence, said Aller, that regular prairie fires in places like the U.S. Midwest helped build soil carbon levels, producing highly productive soils for agriculture.
Despite ancient evidence and a litany of research – nearly 30,000 peer reviewed publications related to biochar in the past decade – the use of biochar as a soil amendment is still overlooked by most producers. Aller mostly sees it being used on higher value specialty crops in vineyards, urban agriculture, small-scale vegetable production and the horticulture industry. She attributes this mostly to its high upfront cost, but anticipates that prices will drop as biochar production continues to increase and more people become aware of its many benefits.
If a producer is interested in applying biochar, Aller said it’s always a good idea to collect some up-to-date soil tests. Then, she supports taking a similar approach to the 4Rs of nutrient stewardship – right source, right rate, right time and right place. A new “Biochar Guidelines for Agriculture Applications” factsheet published by USBI and partners discusses this approach (at biochar-us.org/type-resource/fact-sheet).
Aller said, “We know biochars are not one-size-fits-all, and we don’t want someone to just spend money on biochar just to use it. We need to think about what is the reason we are using it – identifying the issues we are having and our management goals. Figuring out what’s the right biochar for the right soil is important.”
A critical factor when biochar is applied to soils intended for ag production is that biochars need to be inoculated or charged (sometimes called blended, mixed, activated, matured or cultured). Biochar is a relatively inert material that serves as the carrier, or backbone, of nutrients and shelter for microorganisms. Mixing it with a nutrient-rich material, such as manure or compost, is important because evidence shows that applying biochar exclusively can initially suck up nutrients in the soil, leading to immobilization and potentially detrimental effects on crop yields in the short term.
“Where we see the greatest benefits of biochar is when it’s mixed. There’s a synergistic effect with the more readily available nutrients coming from materials like manure or compost, and then the more recalcitrant carbon structure with the biochar,” Aller said.
A new NRCS conservation practice standard (code 336) is available to qualifying producers to help lower the costs of biochar adoption. The practice must accomplish one or more of the following purposes: improve or maintain soil OM; sequester carbon and enhance soil carbon stocks; improve soil aggregate stability; and improve habitat for soil organisms.
Aller highlighted a new “Dynamic Soil Properties Response to Biochar” tool available on the USDA-NRCS Web Soil Survey platform. The tool rates the potential for an improvement in one or more soil properties leading to better crop growth following a biochar application to different soil types. It provides a rating of unsuited to excellent. The tool assumes a single application of a corn stover and manure waste biochar with a mid-range particle size, spread with a manure spreader and minimal incorporation with tillage.
“Biochar has numerous potential benefits within agricultural and environmental systems, but we know biochar is not going to work everywhere. We know from research that sandier and more degraded soils, where for example we’re looking to build organic matter, are areas where we’re likely to see a positive response in crop productivity following biochar applications,” Aller said.
by Sonja Heyck-Merlin