Charcoal May Help Improve Soil Quality

Researchers say that adding charcoal to soil may provide more benefits for long-term soil quality than compost or manure. It could also be used to sequester carbon captured from carbon dioxide emissions. Mingxin Guo discusses new applications for the technique, used more than 1,500 years ago in the Amazon basin.

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This is Talk of the Nation Science Friday. I am Ira Flatow. A bit later in the hour, we'll be talking about open access to biomedical research and children and sleep. But up first, poor quality soil. It's a problem for farmers around the world. Dirt stripped of nutrients by years of over-farming and chemical fertilizers. Well, this week there's new evidence that an old farming practice traced back at least 1,500 years to tribes in the Amazon basin can give new life to nutrient-poor dirt. It's called "black gold agriculture." The idea is really simple. You add charcoal from burned organic matter to the soil and the dirt holds on to nutrients and produces lots more crops.

This week, scientists at the American Chemical Society meeting presented the results of a controlled study of black gold agriculture. And they found that fertilizing with charcoal produced more crops and captured carbon from the air, right out of the CO2. So the practice could also combat global warming. Sounds too simple, too good to be true? We're going to talk about it. Our number is 1-800-989-8255. 1-800-989-TALK. Mingxin Guo is an assistant professor in the Agriculture and Natural Resources Department at Delaware State University in Dover. He joins us today by phone from his office. Welcome to the program.

Dr. MINGXIN GUO (Assistant Professor of Agriculture and Natural Resources at Delaware State University): Hi.

FLATOW: How do I pronounce your name?

Dr. GUO: Mingxin Guo.


(Soundbite of laughter)

FLATOW: I'm sure I'm not the first one who's had some trouble with it, but thank you for pronouncing it for me. Let's talk about poor quality soil, a big problem around the world. Why is that?

Dr. GUO: Yes. So deterioration and chemical degradation is a severe and worldwide problem. It is expressed as soil compaction, poor tubes, surface crafting(ph), slow water seepage, low water draining, low nutrients and a low nutrient retaining, and also decreasing crop productivity. This problem is mainly caused by long-term chemical fertilizer application and mechanical tillage. The level of organic matter determines the quality of our soil. All the soils have high organic matter content, say, six to 15 percent. But soil plowing makes the organic matter decompose quickly, while chemical fertilization doesn't incur any external organic matter adhesion. So year after year, farmland soils become low in organic matter and the quality turns poor. So currently, most of farmland soils have organic matter content lower than three percent.

FLATOW: Ah. So what does adding charcoal to the soil, why does it make it a better fertilizer?

Dr. GUO: Charcoal is a fine-grained, porous black carbon, and it is generated from plant materials. And it is non-toxic to plants. So there are many tiny pores in charcoal. So once applied to soil, the pores will allow air to diffuse into the soil. Plant roots need the air to breathe. And in the meanwhile, the tiny pores will hold water and nutrients and later supply it to plants. More important, unlike other organic fertilizers, charcoal is very stable and it will not decompose to carbon dioxide. So once applied, it will stay in soil for hundreds to thousands of years. So to summarize, the high stability and porosity make charcoal a better fertilizer than other organic materials.

FLATOW: And you've actually conducted tests showing this?

Dr. GUO: Yes.

FLATOW: Wow. Well, that's good. Our number, 1-800-989-8255. Let's go to Bob in Cleveland. Hi, Bob.

BOB (Caller): Hi. Hey, how you doing? I have a question. If I wanted to do this in my backyard where I have a garden, could I just buy a bag of, you know, like charcoal that they use, you know, when they barbeque something and add it to the soil? And, you know, what kind of quantity would I add?

FLATOW: Or what about the ash that's left over, too? That's a good question. Dr. Guo, could he just buy a bag of charcoal and...

Dr. GUO: Theoretically, you can buy some charcoal directly from a supermarket and grind it into small grains and apply it to the soil. And make sure you incorporate the charcoal into the soil by plowing. And typically speaking, we recommend that five percent of the charcoal to be mixed with the top 20 centimeters of soil. For example, if the soil density is 1.4 tons per cubic meter and according to that, probably you should calculate it according to the surface area. I mean, according to the size of your garden and calculate how much charcoal is needed. But generally speaking, five percent is good enough.

FLATOW: And also, you were talking about the fact that the charcoal will - is it absorb or adsorb the CO2?

Dr. GUO: I should say, it's a mix of absorb and adsorb. It's a mixture and we cannot distinguish the two terms. It is either physical adsorption or chemical absorption.

FLATOW: And the fact that it could absorb CO2 from the atmosphere...

Dr. GUO: Oh, yes. In that sense, it is not absorption. Actually, probably, you know that the original absorption of carbon dioxide from the atmosphere is carried out by plants, through photosynthesis.

FLATOW: Right.

Dr. GUO: But when plants die, the biomass will be decomposed quickly by microorganisms and release carbon dioxide to the atmosphere.

FLATOW: Right.

Dr. GUO: And once the biomass is converted into charcoal, and at least 50 percent of the carbon will be permanently fixed into that material, and it is resistant to microbial and chemical degradation. So that part of carbon will stay in the organic form and it won't be released as carbon dioxide to the atmosphere.

FLATOW: So, you could take all the tree leaves and the corn stalks and the wood chips and turn them into charcoal and lock up a lot of that carbon in the charcoal, and create a great fertilizer at the same time.

Dr. GUO: Yes. Theoretically, all organic residues and wastes, including the grass leaves, crop residues, animal manure and yard trimmings, and even, you know, some leftover from the kitchen.

FLATOW: Wow. You know, people will say, but you have to burn this stuff to make charcoal. Does that not release CO2 back into the atmosphere?

Dr. GUO: The burning process, actually, it is not a real burning process because the materials are packed in a closed container and are heated at a relatively high temperature, say 750 degree F or 450 degree C.

FLATOW: Right. So there's no oxygen in the reaction. You're doing it without air.

Dr. GUO: Without oxygen.


Dr. GUO: Now, you know, only a tiny amount, a fraction, only about ten percent of the carbon will be converted to carbon dioxide. And 50 percent of the carbon will remain charcoal. And another 40 percent will be remaining as a byproduct called bio oil, which can be harvested and used as another renewable energy source.

FLATOW: And so, I know this is an ancient technique that was discovered in pre-Columbian tribes from the central Amazon. They were doing this 1,500 years ago.

Dr. GUO: Yes. We actually, we learned this lesson from the pre-Amazon people. An archeological event disclosed the fertile, charcoal carbon-rich and highly productive soil in the central Amazon basin. And later, scientific studies revealed that this fertile soil was fertilized by the Amazon people 1,500 years ago with char produced by smothering plant debris and annual bulbs(ph).

FLATOW: Ah. So the char, the fertilizer they made, the char they made 1,500 years ago, was still working?

Dr. GUO: Yes. The soil is still highly productive, even after 1,000 years of crop cultivation without any other fertilization.

FLATOW: Wow. Can farmers do this themselves? Can they make this charcoal themselves?

Dr. GUO: Yes. And on a farm scale, it is very simple. And the growth of the farmers, just have to pack the organic residues into a metal container and heat it at a like 300 or 400 degree C, until no visible smoke emitted. Then that charcoal is ready to apply.

FLATOW: Ah. So there must be a kit they can buy or some sort of - they must know how to do this, then. It's not very hard to do.

Dr. GUO: It's very easy. And actually, you know, 2,000 years ago, the Chinese people started to make charcoal by themselves by this kind of a process. We call it smothering or some, you know, paralysis.

FLATOW: Right. It almost sounds like the method to make coke, how people make coke.

Dr. GUO: Yes.

FLATOW: I don't mean the drinking kind, either.

(Soundbite of laughter)

FLATOW: So where do you go from here in this research?

Dr. GUO: Currently, we are trying to extern this from the lab to the field scale. And we are going to do some demonstrations and to look at the long-term effect of charcoal fertilization, also quality improvement and the productivity enhancement. So when succeeded, we want to encourage all the farmers and growers around the world to practice the charcoal fertilization to improve soil quality.

FLATOW: Sounds like a simple thing to do.

Dr. GUO: Yes.

FLATOW: Thank you very much, Dr. Guo, for taking time to be with us.

Dr. GUO: Thank you. Bye-bye.

FLATOW: Bye-bye. Mingxin Guo, assistant professor in the Agriculture and Natural Resources Department at Delaware State University in Dover. We're going to take a short break and come back.

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