Scientists, growers and gardeners alike are still learning about the benefits and drawbacks of biochars and charcoals. The name “biochar” covers a wide range of materials with different properties.



Quick facts

Suitable for: sequestering carbon in soil; potentially beneficial soil improver
Timing: all year round
Difficulty: moderate

What is biochar?

The term biochar does not refer to any single product; it is a catch-all term describing any organic material that has been carbonised under high temperatures (300-1000°C), in the presence of little, or no oxygen. This process (pyrolysis) releases bio-oils plus gases and leaves a solid residue which is termed biochar.

Virtually any organic material can be put through the man-made pyrolysis process to produce biochar. This is why it is important to know the starting material used to produce any given ‘char’. Examples include soft plant tissue, woody materials, and manures. The resulting biochar is carbon rich and doesn’t readily decompose.

What claims are made about biochar?

Biochar incorporation into soil provides an important route for CO2 removal from the atmosphere and terrestrial carbon sequestration (Intergovernmental Panel on Climate Change; 2018). Our understanding of biochar and its properties has developed much over recent years, and considerable scientific research is still underway (including trials at RHS Garden Wisley).

The application of biochar to soil as an additive is known to have various effects;

  • Positive effects reported include improving plant performance through neutralising acidity, providing improved water and nutrient retention (especially in sandy soils) and enhancing beneficial microbial populations. This idea was born out of observing the man-made ‘Terra Preta’ soils of the Amazon. The fertility of the poor, acid soils in this region is believed to have been transformed through addition of charred organic material by the area’s indigenous inhabitants which sustained population expansion across the Amazon region.
  • Negative effects are primarily related to excessive pH increases (where soils do not need neutralising, or ericaceous plants are grown). There are also concerns related to quality of material purchased. It is a good idea to ensure the origins of your biochar are traceable (e.g. FSC certified). For this reason, at the RHS we use only biochar made from European hardwoods.
Experiments are underway at Wisley to determine 1) If nutrient-use efficiency of Wisley soils can be improved through use of hardwood biochar, and 2) What the associated effects are on the growth of a range of plants.

Aspects to consider if using biochar

Biochar can be used as a soil amendment incorporated into garden soil or potting compost. It can be applied at any time of the year, dug into the soil surface, or planting holes. As with any soil improver, it is best to follow the manufacturer's recommended application rates.

Nutrient enriched biochars are biochars that have added nutrients (organic or otherwise) and are useful to avoid any potential nitrogen lockup. Some nitrogen immobilisation has previously been reported for non-enriched or ‘straight’ biochars. This occurs because a very small fraction of the carbon in biochar is available to microbes (microbes then use up nitrogen in the soil).

Nutrient enriched biochars should ideally be incorporated into borders, pots or planting holes in spring. Spring timing is recommended because this maximises the opportunity for root-uptake of the added nutrients and therefore minimises the chances of pollution. As with any fertiliser, pollution can be caused by a) nutrients being leached away from plant roots by winter rainfall (causing pollution of waterways), or b) soil biological conversion of the added nutrients to greenhouse gasses (GHGs) which contribute to climate change. Appropriate timing of applications can minimise these risks. Another way to minimise pollution is to ensure that all nutrient containing materials (including nutrient enriched biochars) are only incorporated into soils that are ‘well drained’ (i.e. those that have a good oxygen supply). Clay soils can be both well drained and have good oxygen supply if the soil structure is well-maintained. This normally requires a healthy soil biology.

‘Straight’ biochars (non-enriched) are most suited for aeration of soils which are at risk of periodic flooding or ‘problem soils’ which are too dense or poorly drained. Generally, as with all soil management, gardeners should ensure there is an adequate air-supply throughout the rooting zone (except for intentionally boggy areas or aquatic plantings). As a rule of thumb, sandy soils tend to have better air-flow properties than clays. The high chemical stability of porous biochars (e.g. biochar produced from soft/hardwood trees) make them a very efficient and potentially superior physical conditioner to improve air flow or reduce the bulk-density of heavy soils. This is because while composts and other brown organic growing media can rapidly improve a soil (and contribute more nutrients!) after digging in, they can quickly decompose, compact, and contribute to the 3 main greenhouse gasses: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).

The stability of biochars/charcoals also means that a single application should remain effective for many (possibly hundreds) of years. Soils amended with ‘straight’ biochars can be supplemented as required with organic or inorganic fertilisers. Poorly drained and/or sandy soils should always be fertilised with additional care to minimise pollution of air (dense soils) and water (sandy soils).

Effectiveness as a soil improver

The extent to which biochar can contribute to aeration depends on porosity and particle size. Larger particles (1 mm or larger) generally being better for aeration than dusty materials. The impact of any biochar on the soil-plant system will also be affected by:

  • Soil type (e.g. dense, unhealthy, acid soils often seem to show greater positive effects than fertile well-structured soils. Sandy soils also benefit from the improved water retention and cation exchange capacities of biochar)
  • Environmental conditions (e.g. frequently waterlogged, poorly drained soils are more likely to benefit)
  • Plant (e.g. species and cultivar) being grown

Effectiveness in greenhouse gas mitigation 

The impact of biochar on climate will also be affected by similar variables to those above. While carbon sequestration is thought to easily be achieved by incorporation of biochar into any soil, there are likely to be additional benefits to improving the structure (aeration) of poorly drained, dense soils that can otherwise emit significant quantities of GHGs (carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)).

Is biochar application proven to be always beneficial?

It is important to note that there are many studies that show no plant effects of biochar application, or in some cases, actual detrimental effects of applying biochar. Many studies refer to relatively limited experimental assessments and report on quite specific plant, soil and environment interactions. It would be unwise to make generalisations on the effects of biochar on plant species, soils and environments based on such assessments.

Biochar & climate change

Unlike conventional forms of organic matter (such as garden compost) biochar takes a long time to decompose (centuries – millennia) thus it is generally seen as an effective way to sequester or store carbon in the soil.

In terms of carbon accounting, it is important to recognise that its value as a carbon rich material to be buried is very much determined by its production method. Its production is considered to be most sustainable when it is produced under carefully controlled conditions, utilising dry ‘waste stream’ organic materials and where the resultant heat, bio-oils and gases are utilised as fuels etc.

Biochar would appear to offer some potential as a way of sequestering carbon and contributing to the mitigation of climate change. In terms of actual benefits to plants and soils, scientists and horticulturalists alike are still learning.

Frequently Asked Questions

Q: Can biochar/charcoal be produced at home?
A: Producing biochar/charcoal at home is not recommended. Producing biochar at home will result in variable quality material. The process is dangerous and it is very difficult to control all the variables. For example, temperature and duration have a significant effect on the quality of the final biochar. Residual charcoal from log burning stoves should only be used if it is known that plastics and other non-woody biomass have always been excluded from the burner. Wet wood can have unpredictable effects. Carbon monoxide gas is produced during combustion and is a deadly hazard. Concentrations of carbon monoxide are substantially increased during pyrolysis (heating in the absence of oxygen). Application of any incompletely combusted/charred materials will decompose and use up nitrogen in the soil.

Q: Can barbeque charcoal be used instead of biochar? 
A: No. Barbeque charcoal quality is variable and should be avoided. It can be difficult to distinguish between woody charcoal and BBQ briquettes. Modern barbeque briquettes usually contain additives or contaminants (tars, resins and other chemicals) that are not suitable for addition to the soil. Ashes can often be yellow in colour. Ashes from barbeques where briquettes have been used should only be disposed of when cool as domestic waste. Burned cooking residues in ashes can also contain contaminants (polyaromatic hydrocarbons; PAH’s). It is therefore safer to dispose of all barbeque residues as domestic waste.

Q: Is biochar a source of nutrients and does it need to be reapplied?
A: Biochar has some nutritional value but this depends on all the variables listed above. Check manufacturer’s recommendations for application rates for particular situations and plants or crops.  

Q: Does biochar application have effect on the soil’s pH?
A: As outlined biochar properties are influenced by a number of factors including the source of organic matter and production process. Some could potentially lower soil pH, but most are likely to increase pH of the soil. Check manufacturer’s recommendations for application rates for particular situations and plants or crops.

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