How biochar benefits your garden

The pores on biochar protect micro-organisms in the soil from predation and drying out, while providing them with ...
Andrey Kuzmin/123RF stock photo

The pores on biochar protect micro-organisms in the soil from predation and drying out, while providing them with carbon, energy and nutrients they need to survive

Any gardener interested in improving their soil – that's all of us – should be tuned in to the arguments for, and against, biochar.

There is no one type, as it varies depending on the materials used, but in technical terms, biochar is a fine-grained charcoal high in organic carbon and resistant to decomposition. It is the product you get after the pyrolysis of waste biomass, such as wood, feedstock or other plant material. 

Pyrolysis happens when organic materials are heated at very high temperatures with little or no oxygen. Because oxygen is absent, the materials do not combust; instead, the chemical compounds decompose into combustible gases and charcoal.


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The first chemical reaction that happens when you burn wood is pyrolysis; the visible flames are the result of the combustible gases released. Another example of pyrolysis is what happens when you cook a roast in the oven.

So how does this knowledge help us in the garden? According to the International Biochar Initiative (IBI): "As a soil amendment, biochar creates a recalcitrant soil carbon pool that is carbon-negative, serving as a net withdrawal of atmospheric carbon dioxide stored in highly recalcitrant soil carbon stocks."

"Recalcitrant" in this case means the charcoal is resistant to microbial attack. Biochar has been shown to remain in the soil for a very long time – we're talking thousands of years – so it's good for home gardeners and farmers alike. Studies have shown that the properties of biochar that aid plant growth continue to improve after its incorporation into soil.

When fossil fuels, such as gas and oil, are burned the reaction with oxygen produces carbon dioxide (CO²), which is then released into the atmosphere. That makes it carbon-positive. Processes that result in no change in atmosphere CO² are called carbon-neutral. Biochar – the product created from plants grown by absorbing carbon dioxide, then pyrolysed – is carbon-negative as it captures the carbon from the CO² in a stable form, reducing its concentration in the atmosphere.

That's good news for all. A world expert on biochar, Professor Johannes Lehmann of Cornell University, has estimated that if it was spread over just 10 per cent of global cropland, the effect would be to sequester 29 billion tonnes of CO² equivalent – or the same amount of greenhouse gases we emit, worldwide, every year.

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But the benefits don't end there, according to a report from the IBI. Biochar reduces the need for fertilisers because of its ability to attract and retain moisture and nutrients, including those difficult-to-retain minerals like nitrogen and phosphorus. In regular soils, these tend to be washed away, upsetting the ecosystem in streams and riparian areas.

Biochar itself doesn't add those nutrients to the soil – or not enough to make a difference if the soil is already depleted. But it does retain them, which is useful in those soil types that can't hold onto nutrients or water very well. 

The report continues: "Char-amended soils have shown 50-80 per cent reductions in nitrous oxide emissions and reduced runoff of phosphorus into surface waters and leaching of nitrogen into groundwater."

N²O is a significant greenhouse gas that is 300 times more potent than CO², so anything that can reduce concentrations of this gas in the atmosphere can only be good.

Biochar can also absorb residues of pesticides and heavy metals in the soil and purify water. 

How does it do all this? Biochar retains nutrients in the soil directly through the negative charge that develops on its surfaces. Most plant nutrients – including nitrogen, potassium, calcium and magnesium – are positively charged ions, known as cations. Cations in the soil solution attach to the surface of biochar, both internally and externally, as biochar has the ability to absorb and adsorb. Its larger pores absorb water, air and soluble nutrients like a regular sponge.

Adsorption, though, works more like an electric sponge, with atoms, ions, molecules from gas, liquid and dissolved solids sticking to the surface. It's a bit like the static you get when you take synthetic clothing out of the clothes dryer and it clings to you. So all up, biochar is very good at retaining nutrients.


Biochar may be a relatively new term, but it may well have been around for as long as humans have used fire. Pre-Columbian civilisations in the Amazon jungle used a combination of charcoal, bone and manure to enrich the relatively infertile soil. The result was a rich, highly fertile, dark, stable soil known today as terra preta de índio (Indian black earth). The productivity of crops in terra preta is estimated to be twice that of crops grown in untreated soils.

Unlike the slash-and-burn agriculture that happens elsewhere, the people of the Amazon practised "slash-and-char", which is a bit more labour-intensive but demonstrably better for the soil in the long run. 

At its most basic, slash-and-char involves setting fire to a pile of wood, covering it with earth, then adding vents in particular spots.

A form of biochar has been produced and applied in other parts of the world too  – in particular Japan and Korea, where dense populations called for intensive agriculture, which affected soil fertility. A Japanese encyclopedia from 1697 provides instructions for making it: "After charring all waste, concentrated excretions should be mixed with it and stocked for a while. When you apply this manure to the fields, it is efficient for yielding any crop."

Today, a lot of research on biochar's benefits is being carried out. Closer to home, the New Zealand Biochar Research Centre (NZBRC) at Massey University and the Australian and New Zealand Biochar Researchers Network have been set up to advance the understanding of biochar both for its agricultural benefits and for mitigating global climate change.

"Biochar has the potential to sequester carbon as it is much more stable than the carbon from the plant material it is made of, and it can remain in soil for hundreds or even  thousands of years," says Associate Professor Marta Camps, co-director of NZBRC.

"In New Zealand, we have high methane and nitrous oxide emissions as a result of the agriculture industry. Biochar technology may help New Zealand as a country in terms of meeting its international obligations to reduce greenhouse gas emissions."


From an environmental standpoint, biochar is not all good news. Scientists at Stanford's Global Climate and Energy Project warn that biochar production that relies on chopping down forests may result in a net increase in greenhouse gas emissions. But biochar made from waste biomass such as wood chips, sustainably harvested crop residues or crops grown on abandoned land that has not reverted to forest will not.

The problem with the latter method, though, is that any large-scale biochar manufacture that relies solely on biomass left over after harvest is unrealistic.

Part of the work at the NZBRC is to investigate how to produce environmentally safe biochars from New Zealand feedstocks.

There are other downsides to using biochar too – ones that affect home gardeners in particular. Biochar has been shown to inhibit germination, and because it has been used to reduce soil acidity (by increasing the pH), it will only be useful in certain circumstances – crops only tolerate a certain range of soil pH.

What's more, the fine ash of biochar could pose a risk for those with respiratory diseases, so caution is needed when handling it. It should not be spread in windy conditions, and a mask is recommended when spreading. Wetting the biochar before application can be helpful.

Some types of biochar have been found to have little or no influence on soil quality; for others, it can take up to a year to see results. It all depends on the material that's being burned, and the method of burning.

A report from the International Biochar Initiative notes: "The key chemical and physical properties of biochar are greatly affected by the type of feedstock being heated and the conditions of the pyrolysis process. Biochar made from manure will have a higher nutrient content than biochar made from wood cuttings. However, the biochar from the wood cuttings may have a greater degree of persistence over time."


While still in its infancy here in New Zealand, you can find bags of biochar at select garden retailers. Or you might consider making your own.

Since most home gardeners can't trudge to their own woodland and start a charcoal mound, a more practical approach is to burn wood and garden waste in a metal container, such as a metal barrel with a lid. (It's a good idea though to check first with your local council regarding bylaws around burning.)

* Cut or drill a dozen 2½-5cm holes in the bottom of the barrel. Place it on some bricks, because you'll want air to flow through the holes. 

* Start a fire in the bottom of the barrel – which is now basically a kiln – with a little paper, maybe some cardboard, but mostly dry kindling from old sticks and branches. Once your fire really gets going, add larger branches in a random pattern, making sure you leave gaps for the air to circulate. 

* Next, bank some earth around the base of your homemade kiln, leaving a 10cm space, then partially cover with the lid. If the smoking process begins to slow, bang on the kiln or shake it, to make sure everything it thoroughly burned. Once the smoke turns blue, seal completely the holes in the bottom with earth, and put the lid fully on your kiln. The fire will keep going for three or  four hours, after which it should be allowed to cool for 24 hours. 

* Once completely cool, you can tip the barrel over and start using your homemade biochar.

So how much should you use? Because both commercial or homemade biochar and soils are so variable, the IBI recommends testing several rates of biochar application on a small scale before setting out to apply it on large areas. Their experiments found rates of between 500g and 5kg per square metre to be the most successful. 

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 - NZ Gardener

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