This is a transcript of an episode of Public Address Science which was originally broadcast on Radio Live, 5th May 2007, 2 pm - 3 pm.
You can listen to the original audio version of the programme by clicking on the 'Play the audio for this post' link at the top of this page or the 'Audio' button at the bottom of this page.
A month or so ago, I wrote an article on Public Address in which I implied that it would be very difficult to reduce greenhouse gas emissions from New Zealand agriculture. Well, it turns out that perhaps I was being unduly pessimistic -- as structural biologist Alfred Harris has subsequently explained to me.
Alfred's good-news story begins -- like all the best stories -- with a mysterious substance hidden deep in the Amazon region of South America. Terra Preta de Indio or 'Amazonian Dark Earths' are areas which contain soil of exceptionally high fertility.
Naturally enough, scientists were very keen to find out what made these soils so special -- but no-one expected the answer that they got. It turns out that these soils were actually man-made.
Pre-European Amazonians had manufactured these soils by working charcoal and manure into ordinary low-fertility earth. Of course, the fertilizing properties of manure are well-known, but it was the addition of charcoal which dramatically improved soil fertility. So much so, in fact, that the soil was still extremely fertile hundreds of years after the collapse of the civilization that produced it.
And here's an interesting co-incidence -- go a third of the way around the world to New Zealand, and archaeologists discover that pre-European Maori also worked charcoal into the soil [although there is no archaeological evidence to suggest that Maori did this with a deliberate intent to increase soil fertility].
It seems like an extraordinary co-incidence. Structural biologist Alfred Harris takes up the story...
In Waimea, for example -- where there's hundreds of acres of what they call 'Maori soils' -- the experimental work was done on that site, and it was very clear that charcoal was mixed far further into the soil than would be expected simply from just the burning of the site.
It's fascinating that two cultures so far apart should both discover the effects of charcoal in terms of agriculture. At a scientific level, how the does charcoal actually improve soil fertility?
Okay, going back to basics: plant growth is absolutely dependent on a number of key elements -- and the major elements are nitrogen, phosphorus, and potassium. Each of those elements are highly soluble. So given a reasonable sort of rainfall, which you need for good plant growth, they'll move through the soil very quickly.
So the fertility of the soil is really dependent on the ability of that soil to retain those key elements. The soils where charcoal seems to have a particular effect -- and that's in South America, and in Japan, and here in New Zealand -- are derived from weathered volcanic ashes.
Charcoal in the Terra Preta [de Indio] soils in South America increase the fertility and maintain it over very long periods of time. It appears to have more to do with the retention of nitrogen in that situation -- but in Japan, in different types of volcanic soils, it appears the effect is actually to increase the levels of mycorrhizal fungi, which, in turn, makes the phosphate [that is] heavily bound in those soils available to plants.
Okay, right, so what you're saying is that the charcoal has at least two different effects in volcanic soils. In the case of South America it slows the rate at
which nitrogen is leached out of the soil by rainwater. But in New Zealand and Japan, there's a different mechanism, where it effectively unlocks the phosphate already in the soil -- so as to make it available to plants.
Yeah, I don't think there's actually a universal effect. I think what we're seeing is a combination of different effects, which we're only just beginning to understand.
So the exact mechanics of how charcoal increases fertility isn't necessarily fully understood yet, but it's been conclusively proven that it does actually work?
Beyond a shadow of doubt -- that's absolutely right. And what's been demonstrated (by the Japanese, primarily) is that the addition of charcoal can reduce the amount of fertilizer that you need to put in for the same fertility effect. And they're quite substantial differences... you're talking 20, 30, 40 per cent.
Right, which begs the question -- why isn't this already standard practice for agriculturalists?
Again, that's a really good question. See what they say all the way through, is that: "Hey, really great that it works -- but the cost of producing the charcoal, and actually incorporating it in the soil means that it's not going to work economically". And they're absolutely right, in [terms] of conventional agriculture, where you've got cheap petrochemical products, and you can fix your nitrogen from the atmosphere using [cheap] energy. But, of course, what's happening now is that the economics of all of that is suddenly changing very fast.
So the rising world energy prices have now, perhaps, made financially worthwhile the fertilizer savings which are possible from charcoal addition.
But what you're also doing with the charcoal addition, of course, is effectively sequestering atmospheric carbon in the soil -- because carbon charcoal
is derived from wood products, and therefore ultimately from the atmosphere. Does the carbon actually stay in the soil long enough to be useful from a sequestration point of view?
Well, these Terra Preta [de Indio] soils in South America were developed about the time that the Spanish conquered... When the Spanish came back there and settled those civilizations had vanished. [So] the suggestion is that [the carbon will stay in the soil] for hundreds, if not thousands, of years.
But the other thing that's really crucial and really interesting is that (back in 1996) some very clever scientists in America discovered that in [the standard practice of soil analysis] for humus content they were throwing out... 27 per cent of the soil carbon. Now what they found was that [this] carbon was produced by mycorrhizal fungi, and it's the glue of soils. And what it does is that it glues the clay particles together in such a way that bacteria can't raid them. [So] that [these carbon] molecule[s] will hang round in the soil... for hundreds of years
Wow, so you actually get a double effect in terms of carbon sequestration?
That's absolutely right. When you put charcoal in the soil, you're fixing carbon. When you put charcoal in the soil, you're [also] increasing the level of mycorrhizal fungi, which themselves fix carbon. So it's one of those wonderful virtuous circles where you actually get a double whammy. You [end up with] fixed carbon from two different sources.
In a country like New Zealand, which grows biomass, it seems to me to be an absolute must if we're going to change from a fossil carbon economy. You know, it [would] really put New Zealand agriculture back on the world stage, and on the front
foot. Because biomass is produced so fast here [that] we have an advantage over virtually every other country in the world.
Is there actually anyone in New Zealand who's seriously investigating the possibility of using charcoal to make our agriculture more efficient and environmentally-friendly?
I'm involved in a start-up company called 'Aotearoa Biocarbon', and our intention is to produce charcoal in New Zealand. And what we're looking at, [firstly], is... to reduce the amount of fertilizer input.
The other [possibility], of course, is that if charcoal can be used to hold nitrogen in the soil, it can also be used to pull nitrogen out of places where it's not wanted [such as] Lake Taupo. Now, what our company is looking at... is getting paid to take waste from the forestry industry, turn that into charcoal, and then get paid by the powers-that-be to remove nitrogen (particularly from lakes) -- to produce product [that] is a slow release fertilizer, which you can then sell onto farmers.
So it's possible that the use of charcoal in New Zealand agriculture could reduce fertilizer use, save energy, and sequester carbon all at the same time. With rising petroleum costs it seems like maybe the time has come to re-learn a few lessons about charcoal from the ancient Amazonians.