In the Guide “The Science of Growing more Nutritious Vegs”, I shared 3 steps to improving the soil biology, boosting the soil fertility with mineral fertilisers, and bring nutrients to the soil & Vegs.
When boosting the soil fertility, I recommended using Biochar. But what is it, and how to use it to better grow Vegs??

Last updated with recent scientific research in April 2022

Biochar is a very pure, high-carbon form of charcoal that improves the structure, aeration, water-holding capacity and nutrient retention of soils and substrates while providing permanent refuge for beneficial microbiology.

Biochar is a charcoal-like substance that’s made by burning organic material from agricultural and forestry wastes (also called biomass) in a controlled process called pyrolysis.During the pyrolysis process, the organic material is converted into biochar, a stable form of carbon that can’t easily escape into the atmosphere. Biochar is black, highly porous, lightweight, fine-grained and has a large surface area.

Once you add it, it's permanent! It is believed that biochar has a long average dwelling time in soil, ranging from 1000 to 10,000 years, with an average of 5000 years. So you will reap the benefit for a while…

The application of biochar improves soil fertility through two mechanisms: adding nutrients to the soil (such as K, to a limited extent P, and many micronutrients) or retaining nutrients from other sources, including nutrients from the soil itself. However, the main advantage is to retain moisture and nutrients from other sources.

How to make Biochar?

Alternatively, you can simply buy some!

Don't forget to go local & Organic

• Biochar in UK: Check Carbon Gold, with a variety of Biochar products in different forms (added to compost or on its own) – highly recommended! (and their buckets are great for re-use!)

• Biochar in US: Check out the Directory on US Biochar Initiative, a not-for-profit organisation promoting the sustainable production and use of biochar through research, policy, technology and doing it!

How to use Biochar?

Biochar Soil amendment: How much biochar to add to soil?

Biochar won’t decompose unlike standard organic soil amendments - it is an amazing, permanent, soil amendment to increase microbial, improve soil structure, reduce compaction and improve oxygen levels in the soil.

So don’t worry too much about how much you’ll use - you are likely to run out of Biochar well before it becomes a problem!

Biochar is so high in carbon that it doesn't break down over time, meaning one application is all you need to permanently improve your soil:

• You can add to your compost mix or directly to the soil at any time of year to improve nutrient availability. Each time I'm planting Vegs or sowing directly in the ground, I first put a handful of Biochar at the bottom.

• If you want to add to a new Vegs bed, use the application rate of 1kg per square metre.

• It's easy to apply and, because of its water holding qualities, it means less maintenance of plants, crops and lawns. It also means you'll save a bit on those water bills.

• Biochar will increase water-holding capacity and cation exchange capacity of sandy soils; clay and silt soils with added biochar can favour soil aeration. Water-holding capacity in sandy soils can be increased by up to 22% and soil compaction ameliorated by 15%.

• It also provides the perfect refugia for beneficial microbiology like mycorrhizae and Trichoderma.

There are some issues to consider however, as shown in latest scientific research:

The results suggest that high doses of biochar in clay soils are likely to decrease available water content, and surface application of biochar to sandy soils likely increases erosion and particulate matter emissions. Furthermore, biochar may increase the likelihood of excessive soil salinity and decreased soil fertility because of an increase in the pH of alkaline soils causing nutrient precipitation. Given the diversity of effects that biochar may induce in soil, guidelines for future biochar use should adopt a structured and holistic approach that considers all positive and negative effects of biochar.

— 2021, A critical review of the possible adverse effects of biochar in the soil environment

That’s why I recommend using Biochar as part of a comprehensive approach to improve soil biology, minerals and nutrients.

Does Biochar work for gardening?

Well, the only way to find out is to setup your own experiments - but check out these tests…

Biochar Fertiliser: Benefits of Biochar

• Water retention, resistant to drying out: Because of the way biochar is made, it’s extremely porous and its sponge-like structure lends itself well to water conservation.

• Biochar will increase water-holding capacity of sandy soils; by up to 22% and soil compaction ameliorated by 15%.

• clay and silt soils with added biochar can favour soil aeration

• Significant yield increases have been found where medium and coarse-textured soils have had biochar added, likely to be due to improved water holding capacity

• Studying 5 soils from a desert area of China, additions of biochar from mixed tree prunings were found to increase soil water content and reduce evaporation compared to untreated soils. Evaporative losses were reduced by larger particle size biochar (1-2 mm) compared to fine biochar (<0.25 mm), this was due to the higher surface area and water absorbing capacity of the larger particles.

• Biochar derived from maize cobs (slow pyrolysis at 350oC ) added at relatively low rates (0, 2 and 6 ton/ha) to basins growing maize and soya beans was able to increase Available Water Capacity and soil aggregate stability by 3% per 1% added biochar (sandy loam soil in central Zambia), and reduce bulk density of the soil by 3-5% per 1% added biochar. Authors commented that this was very important for “droughty” soils in the region where rainfall can be erratic.

• As part of another study plant, available water and pore volume significantly increased with biochar additions and bulk density decreased. Yield increased slightly for maize but was not significant with additions of biochar.

• Using a highly weathered silty clay soil in South Taiwan, an incubation experiment lasting 105 days with biochar from white lead trees mixed in at 0,2.5 and 5%, bulk density was reduced from 1.4% to 1.1%, porosity increased from 41% to 52%, saturated hydraulic conductivity increased from 17% to 33%, the mean weight diameter of particles increased from 46% to 50%. Microbial biomass increased by 50% during the trial, the authors ascribe production of gums and mucilage helping to bind together soil particles. In a simulated rainfall runoff trial soil erosion rate was halved with 2.5% biochar and reduced by nearly 3 times for the 5% rate. The authors recommend a 5% inclusion rate of biochar to amend soil properties and reduce soil erosion

• Biochars are alkaline in nature and are effective liming agents. Some biochars i.e poultry-litter derived chars are high in carbonates and are particularly effective. Overall soil pH increase in acidic soils was seen as one of the main reasons for crop yield increase with biochar addition.

• Biochars provide habitats for soil microbes and can also provide them with nutrients as well.

• soil organic carbon is enhanced by biochar addition and biochar can stabilise organic matter and reduce the rate of breakdown, by absorption of organic compounds and disruption of microbial enzymes which break down organic matter. An example given was of biochar added at 20 and 40 te/ha to a calcareous soil low in organic carbon, the biochar increased the % soil organic carbon by 25% and 42% respectively. In a separate paper Laird et al. found significant (+46% average 6 sites in the USA) increases in soil organic carbon for biochar plus fertiliser compared to fertiliser only. The effect was shown to last for at least 4 years (the length of the trial).

• Biochars, particularly wood and crop-residue based, improved a range of physical soil properties as follows: bulk density reduced by 7.6%, aggregate stability increased by 8.2%, soil porosity increased by 8.4%, available water capacity increased by 15% and saturated hydraulic conductivity increased by 25%. The above properties will enhance crop growth by improving drainage and allowing greater root penetration of soils.

• Absorptive properties – reduced availability of toxic metals, Persistent Organic Pollutants and herbicides

• You can even use it for Lawns:

Learn more: The Most comprehensive, Definitive list of Biochar scientific research papers

Yes, I read all these research papers – so you don't have to! – to make sure the Guide “How to make & use Organic Fertiliser at home, to grow more Nutritious Vegs” gives you the evidence behind the information I share, and consider carefully risks and issues relating to using Biochar. As you can see from the research below, it’s still a relatively new topic, which much research progressing around the world for using Biochar both for carbon capture and fertiliser.
If you are a Geek like me, read on!

If you’ve written research which I haven’t spotted yet and would like me to highlight here, please let me know!