Biochar, Soil

Biochar as a Soil Ammendment.

It is important that biochar is made and used correctly as it can be detrimental to plant growth under some circumstances.

Biochar should not be seen as a fertiliser, rather it helps to prevent nutrients from being washed out of soil or lost to the atmosphere. It is used as soil admix because it has a massive surface area and a slight electrical charge (cation exchange capacity.)

Nutrients which are otherwise soluble and easily washed out of soils by rain are instead attracted to and bound to this surface area. These nutrients, if washed out of the soil, cause pollution problems, rather than being made available to plants.

Biological activity is the key to making these nutrients available to plants. The large surface area, once saturated with nutrients, can then become a habitat for soil microbes, fungal mycelium and so on.

Traditional organic growing techniques usually require fertility inputs into the soil. These can be in the form of green manure crops (crops which are grown then dug back into the soil) or composted animal manures or certain restricted inputs (seaweed or comfrey liquid feeds for example). The soil is not ploughed as deeply as with conventional agriculture in order to reduce nutrient losses.

Ploughing the soil burries the weeds, killing the annual weeds. It also introduces air into the soil which results in increased bacterial activity resulting in rapid nitrogen release.

Adding biochar to the soil stabilises the nutritients in the soil to some extent, reducing the rapid nitrogen release and thereby making more nutrients available to crops.

Soil which is ploughed every year loses its structure. The exchange capacities (electrical forces which bind particles together in soil) are reduced resulting in the loss of the biological (carbon) components of soil.

Adding stabilised carbon (biochar) to the soil reduces this effect, resulting in a soil in which the particles are bound together with stronger bonds.

This may help to reduce soil erosion as well as nutrient depletion which result from ploughing or digging.

In my experience, however, it appears that biochar is relatively unstable in regularly ploughed soils. For this reason I try to combine using biochar with the conditions which support glomalin production.

For this reason, rather than viewing biochar as a nutrient ‘sponge’ which reduces nitrogen losses which result from ploughing, I use biochar as a means of encouraging conditions favourable for fungal rather than bacterial activity. (see the page ‘combining biochar and glomalin’).

Although I try to use biochar to stabilise and bind otherwise soluble nutrients in the soil, maximising stable carbon in the soil is my main intention.


Activating the Biochar with Nutrients.

This is important in order to encourage microbial activity. Biochar which is not rich in nutrients can remove soil nutrients, reducing crop yields.

Manure and other green waste should be composted before applying it to soils. Whether this is done in a small kitchen compost bin or in large piles, it is important to keep it covered in order to prevent rain from washing nutrients away.

Adding biochar to a manure or household compost pile as it composts is the simplest way to saturate it with nutrients, capturing the nutrients and encouraging microbial growth.

Horse manure and biochar, mixed and composted. 2011.

Liquid fertilisers such as comfrey liquid, seaweed extract of manure tea can easily be made. These fertilisers are very soluble and will easily be washed out of the soil. One way of applying these fertilisers is to soak biochar in them until they are absorbed by the biochar. The biochar can then be dug into the soil.

Comfrey-soaked biochar being dug in before a crop of tomatoes.

Biochar can be used on stable floors. Thiswayit gets crushed by the animals hoofs, it soaks up nutrients from the manure and urine and it provides a sterile bed for anilals hoofs.

Biochar on the stable floor. Biochar and manure around fruit trees.

The horse manure occasionally gets removed from their field in order to reduce disease risks. It usually ends up around the fruit trees where a few buckets of biochar hopefully prevent all the nutrients from washing out.

In order to encourage biological activity in biochar, I have grown green manures on biochar and manure compost piles as well as on ground in which biochar has been applied. Green manures are crops grown not for consumption but to be dug back in to the soil. The intention usually is to protect otherwise bare soil, to capture atmospheric carbon and nitrogen and to make soil nutrients available. I also believe that the symbiotic activity between microbes and plants becomes established prior to crops being grown. The biochar then becomes a habitat for these micro organisms.