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Biochar & Terra Preta

In recent years there has been a lot of talk about biochar and terra preta from the Amazon being the answer to sustainable growing’s prayers. I’m trying to separate the truth from the hype that surrounds biochar.

BiocharBiochar – Terra Preta

What is Biochar / Terra Preta?

Terra Preta is Portuguese for ‘black earth‘ and refers to areas of dark, fertile soil discovered in the Amazon basin. The reason for the soil fertility, in an area where the soils are noted for their lack of fertility, is attributed by many to the use of biochar by the civilisation that thrived there some 2,500 years ago.

Biochar is the name given to charcoal made from plant materials and wood in the same way as ordinary charcoal is made with controlled low-temperature burning, also known as pyrolysis. This is usually ground into small particles. Sometimes the product is inoculated with microbial mixtures and matured prior to sale.

Benefits of Biochar

The enthusiastic advocates claim that adding biochar to soil will:

  • Increase soil fertility and nutrient availability producing better crops
  • Increase the microbial flora and fauna of the soil
  • Stabilise soils and after using for a number of years produce a permanent change in the soil to the fertile Terra Preta black earth.
  • Lock up carbon in the soil which would otherwise be CO2 in the atmosphere adding to the climate change problem

Many of these advocates sell biochar which may well influence their claims.

There is no doubt that the Terra Preta of the Amazon exists and is fertile but exactly how the pre-historic inhabitants achieved this or if the soil is due to man’s activity or a natural process is not known.

Certainly we can make charcoal from wood and various other plant materials such as dried corn stalks and weeds but there was probably more to the method than that if it was due to man. Or possibly the local soil type and ecology reacted positively with the plant charcoal to produce the black earth.

Some of the most enthusiastic advocates seem to be in tropical or semi-tropical climates which would support that theory.

Biochar Potentially Harmful

Biochar in temperate climates such as Europe and the UK may well react differently to the tropics and some studies have indicated that biochar, far from being beneficial may actually be harmful.

Acidity Levels of Biochar

The pH of charcoal is generally very high, 8.4 having been measured. This is an extremely alkaline product and whilst useful to sweeten acid soils would make naturally alkaline soils too alkaline for many plants. Most vegetable crops like a pH of around 6.5 to 7.0 at most. Adding large quantities of biochar could even make acid soils too alkali to support crops.

Using smaller quantities of biochar on an acid soil would be beneficial in that it would increase the pH in a similar way to adding lime to soil.

Microbial Action

I can find plenty of anecdotal evidence but no scientific studies showing increased microbial action. Adding ground charcoal to the soil will darken it in the same way that chimney soot was traditionally added to soil in the days when every house had an open fire. Darkening the soil will cause it to absorb more sunlight and warm up earlier in the spring and remain warm longer in the autumn which will increase the microbial population.

Stabilising the Soil

Laboratory tests and data from Institut für Agrarrelevante Klimaforschung in Germany suggest that the 1,000 year or even permanent stability benefits claim is vastly over-stated and the actual benefits range from 10 to 100 years which is of the same order as compost.

Effects on CO2 levels

Certainly biochar locks up carbon that would be released quickly by normal burning or slowly by woody materials rotting down. In natural eco-systems, however, the CO2 released by rotting wood would be taken in by the growth of new plants and trees. Commercial production of biochar can be an energy-intensive process and actually produce more CO2.

Another German study in 2009 concluded that converting 10% of all their waste plant materials to biochar would actually only offset 1.6% of their carbon footprint. At best the macro ecological benefits of biochar are marginal.

Effects on plant growth of Biochar

Most worrying for those using biochar is a study by Woods End Laboratories in the USA into the effects of biochar on plant growth have shown negative effects when compared with untreated soils.

Making Biochar at Home

You can make biochar at home on a small scale by digging a trench or hole and putting a mixture of dry wood and dried plant materials such as sweetcorn stalks or perennial weeds and roots into it.

Set fire to the material which will initially give off clouds of white smoke. This is mainly water vapour and not as bad as it looks. The smoke will then turn yellow as the resins in the wood and sugars in leafy material burn followed by grey smoke.

When the smoke turns grey, cover with turfs or soil leaving just a small hole for an air supply. This restricts the oxygen flow and the material will become charcoal rather than just ash. As with ‘proper’ charcoal production some skill and judgement is required.

Once it has become charcoal, wet down with a hose to put the fire out. Remove the covering and, if possible, put the cool charcoal through a shredder or use a hammer to break down large lumps. Leave to weather for a while and then use it.

There are ways to make charcoal using old oil drums and forced draft flues but for the ordinary gardener this trench method is sufficient.

Biochar My Conclusions

It would be foolish to say that using biochar has no benefit but I think it is often very overstated and in our northern climate probably has no more benefits than reducing acidity and darkening the soil which could be achieved with lime and soot anyway.

You can buy biochar and try it yourself, perhaps on one bed and then grow the same crop on a treated and untreated bed to compare. Alternatively you could make some and try that.

Do be careful not to overdo it, small is beautiful. Personally I’m very sceptical that it is worth it but I would be happy to be proven wrong.

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