​Biochars and Their Uses in Agriculture

Biochars and Their Uses in Agriculture

Author: Sandy Vanno, Master Gardener Warren County CCE

Although it has been around since the times of ancient tribes, biochar is only recently becoming a popular topic of discussion among modern home gardeners, commercial farmers, land managers, and wastewater treatment operators. Originating from the term “terra preta” or dark earth, the use of biochar has been around for thousands of years. Many practices have been proven to increase soil health and fertility, such as manure application, cover crops, and no-till. Biochar, an emerging soil amendment, also shows promise in improving soil health and fertility.

What is biochar? It is a fine-grained, black charcoal material. It is different from traditional charcoal, mostly in that it is generally designed for use as a soil amendment instead of as a fuel for heating or cooking. When organic material, like wood, is burnt, it gives off smoke that contains methane, carbon dioxide, and carbon monoxide gases. These gases are then typically released into the atmosphere. During the process of creating biochar and charcoal, known as pyrolysis (high temperature 500°C or 932ºF), the smoke given off is captured in a closed system. That smoke is redirected back to the heat source to add more energy to the charcoal, or an alternative heat source then used as fuel for another process like generating energy. This reduces the amount of air emissions. The material used as a source for producing biochar is a key point in what makes biochar a popular topic. Biochar can be produced from a variety of materials- trimmings from woodland and yard maintenance, agricultural waste like corn stalks, and purpose-grown biomass such as bamboo. The pH and composition of the biochar that is produced are directly linked to the material used to produce the biochar, so if making your own, be sure to avoid contaminants such as treated or painted lumber.
*From Cornell CALS Soil and Crop Sciences Section

Benefits of Biochar:

  • Increased moisture retention in soils; good for plant growth when there is limited rainfall; good for water quality because it reduces runoff
  • Increased nutrient retention in soils; good for plant growth in soils with poor nutrient content; good for water quality because it reduces nitrogen leaching into groundwater and runoff into surface water. While biochar is not a fertilizer, research indicates it can help retain nutrients in the soil due to its charged surface and high surface area (from its porous structure), which allows it to absorb nutrients like nitrogen, phosphorus, and carbon. Some scientists have even found that when biochar is used in combination with compost, it can retain the nutrients provided by the compost and can help decrease the need for commercial fertilizers.
  • Stable storage of carbon; provides a way to sequester carbon from these various organic wastes instead of releasing the carbon into the atmosphere through traditional burning
  • Very absorbent surface with many functions; it can capture and filter pollutants out of the water, including metals; for this reason, biochar has been successfully used in the wastewater treatment process as well. It is also exceptionally well suited for restoring degraded soils, such as those found near mining sites, because it tightly binds toxic heavy metals and neutralizes unnaturally acidic soils.

Not all soils are well suited for biochar. Research at Michigan State University (MSU) is currently examining how well-drained sandy soils experience biochar benefits such as increased soil moisture retention, but moderately well-drained and/or clay soils may not always respond as positively. MSU advises before applying biochar to your fields or gardens; it is important to consider how the biochar was produced as well as the conditions of your soil. According to the University of Arizona Cooperative Extension, all biochar is not created equal. Each material used to make it gives it differing properties. Biochar made from straw is different than those made from coconut husks, yard waste, or wooden pallets. Also, the temperature at which they are charred will create varying chemical and physical properties. In addition, biochar effectiveness is highly dependent on soil characteristics, such as texture, organic matter content, and mineral nutrient levels present.

Biochar may be a soil amendment you could try for enhancing your crops or backyard gardens or reducing your impacts on the environment. At the very least, you will be helping put carbon back into the soils where it can reside for hundreds of years. It may also be benefiting our local waterways, which are significantly impacted by nutrient pollution. If you decide to use biochar, monitor your plants in the first few months for signs of nitrogen deficiency. Small experiments in your garden may help determine if biochar could benefit your soils.

Pre-packaged biochar can be purchased from garden centers and agricultural supply stores, or biochar can be made at home by buying or building a biochar kiln for home use. Many low-cost designs can be found online and enable the conversion of biomass (wood) into charcoal to amend the soil.

If you have access to fireplace ash, this can be another ready source of charcoal and other beneficial soil amendments. Wood ash is a good source of potassium, lime, and micro-nutrients (which are taken up from the deep roots of hardwood trees used for firewood, such as oak and maple). It is best to add ash to the compost pile rather than directly to the garden, as the high concentration of lime can alter soil pH and may burn sensitive plants. The alkaline nature of wood ash can also have a neutralizing effect on compost, as the decomposing materials in a compost pile, can become somewhat acidic. Do not use ash from a charcoal grill, as this ash may contain chemical residue from the lighter fluid. Because of the alkaline nature of wood ash, avoid using it around acid-loving plants such as azaleas, rhododendrons, and blueberries. Wood ash can also be used as a natural repellent for pests such as snails and slugs, who will resist crossing a line of wood ash laid around their favorite plants (the salts in wood ash are an irritant to these soft-bodied pests). The wood ash needs to be reapplied after rain or watering to remain effective.

Carbon in the soil is the key to soil life. Growing plants exude carbon into the soil through their roots in the form of sugars produced through photosynthesis. These secretions attract and feed beneficial bacterial, fungi, and other microorganisms that live and die in the area surrounding plant roots, providing plants with the nutrients they need to be healthy. The addition of carbon to the soil, in the form of charcoal, can provide additional material for this symbiotic relationship to play out in the garden soil. And in doing so, we can play an important role in removing carbon from the air and returning it to the soil through our everyday love of gardening!


Penn State Extension “Using Biochar for Water Quality”

Michigan State University Extension “Biochar” An Emerging Soil Amendment”

University of Arizona Cooperative Extension, Backyard Gardener; “Using Biochar in the Garden”; 1/8/2020

Last updated November 9, 2021