Biochar is an old acquaintance attracting new attention. The fertile dark earths of the Amazon region, the terra preta, owe much of their fertility to charred plant residues that people worked into the soil centuries ago. Today we produce biochar deliberately, from woody residues and with controlled quality.

But what exactly does biochar do in the soil? And why do some trials show large effects and others hardly any? In this article I summarise what research says today and what matters in practice.

How does biochar work in soil?

In pyrolysis, wood is heated in the absence of oxygen. The cell structure is preserved and becomes a fine network of pores. Depending on how it is made, one gram of biochar can have an internal surface of several hundred square metres. This structure explains most of the effects in soil.

Storing water

The pores absorb water and release it slowly. This is most evident in sandy, coarse-grained soils that otherwise lose water quickly. A meta-analysis by Razzaghi and colleagues (2020) shows that water holding capacity increases above all in such soils.

Holding nutrients

Nutrients such as nitrate, ammonium and phosphate attach to the surface of the char. They are leached out less quickly and remain available in the root zone. Kammann and colleagues (2015) showed that co-composted biochar retains nitrate in its pores, which helps plants grow better.

Promoting soil life

Bacteria and fungi find shelter in the pores. Active soil life converts organic matter, makes nutrients available and improves the crumb structure.

Balancing the pH value

Biochar is usually alkaline. On acidic soils it can raise the pH value, similar to liming. On soils rich in lime this effect is not desirable; here the right char is what counts.

What does research show?

There are now thousands of studies on biochar. Schmidt and colleagues (2021) evaluated 26 global meta-analyses. The picture is clear but nuanced.

Yield: large in the tropics, small in Central Europe

Jeffery and colleagues (2017) found yields around 25 percent higher on average in tropical soils. In temperate latitudes, including Germany, the average yield effect was small. The reason: tropical soils are often heavily weathered, acidic and low in nutrients, and that is exactly where the char helps most. Good Central European arable soils have less room for improvement.

Nitrate and nitrous oxide

The effect on nitrogen is very well documented. A meta-analysis by Borchard and colleagues (2019) shows on average around 38 percent lower nitrous oxide emissions and around 26 percent less nitrate leaching. This protects groundwater and keeps fertiliser where it is supposed to work.

Terra preta as a model

Glaser and colleagues (2001) studied terra preta. These soils contain many times more carbon than the surrounding soils and are still fertile today. They show how long the effect of biochar can last.

How do you use biochar correctly?

Charge first, then work it in

Fresh biochar is like an empty sponge. Worked in pure, it can even bind nutrients in the first weeks that the plants then lack. That is why it is charged beforehand: in compost, with slurry, with digestate or with fertiliser. Afterwards it brings the nutrients along with it.

The right quality

For soils and growing media, the char must be low in contaminants. The European Biochar Certificate (EBC) sets limit values for this, for example for PAHs and heavy metals. The classes EBC-Agro and EBC-AgroBio are intended for use on agricultural land. We produce our biochar in line with the requirements of the EBC.

Where it is particularly worthwhile

On sandy, dry and acidic sites, in growing media for urban trees, on green spaces and in livestock farming via bedding and slurry. Using biochar in the barn first and then bringing it to the field with the manure means using it twice.

In gardens and green spaces

In gardens and on lawns, biochar is best applied via compost or a ready-made substrate. When laying out new beds or lawns, it can be worked evenly into the top few centimetres. On existing lawns it can be spread together with sand or compost and raked in.

On arable land

In agriculture, the route via the barn is the most economical: biochar in bedding or slurry absorbs nutrients and moisture and then reaches the field with the farm manure. For use on agricultural land, the char must meet the requirements of the appropriate EBC class and of fertiliser law.

In the city: tree substrates

Urban trees often grow in compacted, dry soils. Tree substrates with biochar store more water and air in the root zone. Some cities already use such substrates for new plantings.

Common mistakes when using biochar

• Working it in pure: untreated char initially binds nutrients. Better to charge it first.
• Wrong quality: char without analysis may contain contaminants. Look for char produced in line with the requirements of the EBC.
• Expecting too much on good soils: on fertile soils, yield effects are often small. The benefit there lies more in water, nutrient retention and carbon storage.
• Dust when spreading: dry char creates dust. Slightly moistened, it can be handled cleanly.

How long does the carbon stay in the soil?

Plants absorb CO₂ from the air as they grow. Pyrolysis converts a large part of this carbon into a very stable form. In a meta-analysis, Wang, Xiong and Kuzyakov (2016) showed that the stable fraction of biochar in soil lasts several hundred years on average. Lehmann and colleagues (2021) therefore classify biochar as one of the methods that can remove CO₂ from the atmosphere permanently.

This storage can be measured and documented. If the whereabouts of the char are verified, CO₂ certificates can be generated from it.

Frequently asked questions

What does biochar do in soil?

It can store water and nutrients, offer soil organisms a habitat and improve soil structure. The carbon in it remains bound for a long time.

Can biochar be added to soil pure?

Better not. Uncharged biochar can initially bind nutrients. Mix it into compost or other organic fertilisers beforehand.

How do I recognise good biochar?

Look for production in line with the requirements of the European Biochar Certificate (EBC). Among other things, it sets limit values for PAHs and heavy metals and requires untreated plant feedstocks.

Is biochar approved as a soil additive in the EU?

Yes, in the EU biochar is approved as a component of fertilising products under the Fertilising Products Regulation (EU) 2019/1009. National requirements also apply to its use in Germany.

How long does biochar work in soil?

The char itself remains in the soil for centuries. Its effect on water and nutrients therefore lasts a long time and tends to increase over time as the surface becomes loaded with organic matter.

Is biochar the same as barbecue charcoal?

No. Barbecue charcoal is made for burning and is produced without control of contaminants and temperature. Biochar for soils is produced deliberately, analysed and assessed according to quality standards such as the EBC.

Can you use too much biochar?

Yes. Large amounts of untreated char can raise the pH value considerably and initially bind nutrients. That is why biochar is used in moderate amounts and charged.

Sources

• Borchard, N. et al. (2019): Biochar, soil and land-use interactions that reduce nitrate leaching and N₂O emissions: A meta-analysis. Science of the Total Environment.
• Glaser, B. et al. (2001): The Terra Preta phenomenon: a model for sustainable agriculture in the humid tropics. Naturwissenschaften 88.
• Jeffery, S. et al. (2017): Biochar boosts tropical but not temperate crop yields. Environmental Research Letters 12.
• Kammann, C. et al. (2015): Plant growth improvement mediated by nitrate capture in co-composted biochar. Scientific Reports 5.
• Lehmann, J. et al. (2021): Biochar in climate change mitigation. Nature Geoscience 14, 883 to 892.
• Razzaghi, F., Obour, P. B., Arthur, E. (2020): Does biochar improve soil water retention? A systematic review and meta-analysis. Geoderma 361.
• Schmidt, H.-P. et al. (2021): Biochar in agriculture: A systematic review of 26 global meta-analyses. GCB Bioenergy 13.
• Wang, J., Xiong, Z., Kuzyakov, Y. (2016): Biochar stability in soil: meta-analysis of decomposition and priming effects. GCB Bioenergy 8, 512 to 523.
• European Biochar Certificate (EBC): Guidelines for the certification of biochar.