Biochar

The term biochar does not refer to any single product; it is a catch-all term describing any organic material that has been carbonized 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 includes soft plant tissue, woody materials, sewage sludge, etc. The resulting biochar is carbon rich and doesn’t break down readily.

Our understanding of biochar and its properties is very much in its infancy with considerable scientific research underway. Many studies have focused on the application of biochar to soil as an additive. Positive effects reported include improving plant performance through water and nutrient retention 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.

Using biochar in the garden

Biochar is most commonly used as a soil amendment incorporated into garden soil or potting compost. It can be applied at any time of the year to the soil surface, planting holes or used in pots.

As with any soil improver, over application can be harmful (potentially permanently so). In the case of biochar, we would advise to use it sparingly and to follow the manufacturer's recommended application rates.

Effectiveness

The extent to which biochar can be a useful soil improver seems to differ depending on:

• Choice of organic material from which the biochar is made (if plant or tree material then this includes species and even cultivar effects)
• Organic material preparation (e.g. particle size, moisture content) before processing
• Production conditions (e.g. temperature, duration of heating, rate of heating and oxygen level)
• Cooling conditions
• Degree to which the resultant biochar is processed in order to be used (e.g. particle size)

The impact of any one biochar on the soil-plant system will also be affected by:

• Soil type (e.g. degraded low fertility soils often seem to show greater positive effects than fertile soils)
• Environmental conditions (e.g. tropical or temperate climates)
• Plant (e.g. species and cultivar) being grown

Is biochar application proven to be always beneficial?

It is important to note that there are many studies that show no effects of biochar application, or in some cases, actual detrimental effects of applying biochar. In all cases, positive, negative and no effect, these 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.

Unlike conventional forms of organic matter (such as garden compost) biochar takes a long time to decompose (centuries – millennia) thus it has been 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 waste stream organic materials and 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.

Q: Can biochar be produced at home?
A: Producing biochar at home is very difficult as being able to control the entire process, accounting for all of the variables described above, is a technical challenge. Most homemade biochar is probably more like charcoal.

Q: Can charcoal be used instead of or as a substitute to biochar?
A: No. Charcoal is NOT the same as biochar; it is not produced under the same controlled conditions, thus its properties are unknown and unpredictable.

Modern barbeque charcoal or briquettes also often contain additives or contaminants (tars, resins and other chemicals) that are not suitable for addition to the soil.

Charcoals also tend to be more prone to decomposition than biochars, thus are much less effective at sequestering carbon.

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.

Fertilisers
Fruit trees: feeding and mulching
Mulches and mulching
Plant nutrition: feeding plants
RHS climate change report
Seaweed products
Woody waste: using as mulch