Seed Bed: Figuring out the benefits of biochar

Sunday, January 4, 2009

The value of char is obvious in tropical soils, where it allows continuous cultivation. Could it also to help improve soil health and enable carbon sequestration in Ontario?

by KEITH REID

There has been lots of chatter about biochar from the incurably enthusiastic as the "next big thing" in agriculture. So far, very little experimentation has been done with biochar under Ontario conditions, so we are in a situation where enthusiasm exceeds understanding.

Fortunately, though, there doesn't appear to be any risk of harming our soil by adding biochar, but it will take some work to determine if the benefits claimed for this product are real.

The excitement about biochar began with the discovery of the "Terra preta de Indio" the name for certain dark soils in the Amazon basin of South America.

ainforest soils are normally very low in organic matter and fertility, as the constant rain and heat rapidly leach nutrients out of the soil, but these pockets of soil remain dark and fertile in spite of the climate.

Further investigation found that these soils had received large quantities of char, created by burning the jungle vegetation at low temperatures and with limited oxygen. These soils were much higher in carbon than the surrounding areas, but very little of this carbon was in the organic form.

One of the basic tenets of improving soil health is increasing the carbon content by adding crop residues or manure which will end up as humus.

In this process, most of the organic compounds are used up as food for soil organisms, and what is left behind is a complex mixture of high molecular weight hydrocarbons, intimately mixed with the mineral particles in the soil.

Biochar is a totally different material. During the charring process, most of the oxygen and hydrogen is driven off, leaving almost pure carbon in the char. Unlike fresh organic materials, this is not good food for soil organisms, so it is almost inert in the soil.

What the char does have, however, is a huge surface area. The lattice of cell walls remains, but the contents are gone, so that the char acts like a microscopic sponge where soil organisms can use the structure as habitat, and nutrient elements can stick to the surfaces. This explains the persistence of the carbon in the Terra preta soils, where regular organic matter would be rapidly burned out of the soil.

The benefits of char are obvious in tropical soils, where it allows continuous cultivation. The alternative in these areas is slash and burn agriculture with long rest periods to allow soils to recover productivity. Unknown is whether there will be similar benefits to adding biochar to temperate soils, where organic matter levels are normally much higher and the soil minerals have a much greater capacity to hold nutrients.

The renewable energy sector has an interest in adding char to soils, because biochar is a byproduct of pyrolysis or chemical decomposition by heat. When organic materials are heated in the absence of oxygen, hydrocarbons are released which can be used to replace crude oil in some applications. The residue left behind is almost pure carbon, which could either be burned like coal or added to agricultural soils. Obviously, if the material can be sold to farmers at a price higher than fuel, it improves the bottom line for the pyrolysis process. 

The process of pyrolyzing crop residues and returning the biochar to the soil may also have the benefit of net carbon sequestration. Some estimates are that about half the carbon in organic materials would remain as char, and that 90 per cent of this would be stable when added to the soil.

If these figures are correct, each tonne of organic carbon used for bio-oil production would sequester about 450 kilograms of carbon to the soil. In contrast, that same tonne oforganic carbon worked into the soil would be degraded by the soil organisms until only about 100 kilograms of carbon was left as humus.

Both the soil health and the carbon sequestration benefits of biochar need to be tested in the field. Until then, treat the promises as speculation! BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, based in Stratford. keith.reid@ontario.ca