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.