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By: Dr. D. Michael Shafer

This article is from ECHO Asia Note # 35.


A huge body of experimental literature describes biochar and how well it works. Many good reports have also resulted from field tests in the developing world.1

However, if you work in the developing world, with smallholder farmers, you should take all of the reported benefits of biochar with a grain of salt. I do not mean to imply that the data are not good. They are excellent (although skewed, like most scientific results, by the non-publication of bad and null findings). Rather, you must temper your excitement by never losing sight of where you work and with whom you work.

The World of Experimentation and You

Three characteristics of biochar laboratory experiments set them apart from trials you might conduct as a development practitioner. Those who conduct experiments for the purposes of publication in scientific journals must know: (1) the exact characteristics of the biochar; (2) the exact chemistry of the soil; and (3) the exact measurements of ingredients such as biochar, compost, manure and clays. In the context of a farm in the developing world, you and the farmers with whom you work do not and cannot know any of these things.

What to do?

Don’t throw out the science. Biochar is a great material; don’t be intimidated by experts who tell you that you can’t use biochar “unless you use this feedstock, pyrolyzed at this temperature, added to this soil, etc…”

Biochar can do amazing things for the farmers with whom you work.  However, the time you spend learning about and introducing biochar will be more effective if you learn to think clearly about what you are trying to accomplish with biochar; understand the requirements of good testing; and keep in mind the real limitations under which you work. In this article, I share suggestions for how to manage your work with biochar, based on our experience around Phrao in the Chiang Mai highlands of Thailand. I base these recommendations on four years of experience on the Warm Heart Experimental Farm, a 35-plot test field. Since 2015, we have tested biochar-based fertilizer variations against synthetic fertilizers, both on our farm and in field experiments conducted with farmers. Both kinds of tests have demonstrated that in local soils, biochar-based fertilizers made and applied by farmers outperform synthetic fertilizers as typically applied in this area.

Biochar quality, soil types, the utility of biochar, and measuring biochar in the field

When working with smallholder farmers to make biochar, you need to consider three factors: (1) the quality of the biochar you can teach farmers to make; (2) the characteristics of the soils they farm; and (3) how to measure biochar as you prepare it for use. (These are the very same factors that lab scientists fret over!) You have probably heard many explanations about why each of these factors is critically important, and why neither you nor the farmers with whom you work can be trusted to deal with them. Pay no attention.

At Warm Heart, we have designed and tested simple, low-tech biochar machines (now called “flame cap ovens” and “modified JRo’s”); commissioned broad-gauge soil studies; and experimented with field measurement systems. We are convinced that you can teach any smallholder farmer how to make quality biochar and use it successfully, provided you pay attention to what you are doing, where, for whom and why.

The Oft-Perplexing “Quality of Biochar” Problem

What is good or excellent biochar? If you don’t know, you are not alone. No one in the biochar community has figured this one out. The answer will depend on “for what” you intend to use the biochar and “for whom” you intend to make it. What is “good” in one application may not be so great in another.2 In this article, I consider “quality biochar” to be “biochar that delivers good yield increases and soil improvement for smallholder farmers.³ I also only discuss biochar-making methods that are used by the smallholder farmers who constitute the world’s poorest people.

The character and presumed quality of biochar will be determined differently in the lab than in the field. With the lab method, a researcher identifies one or more characteristics of biochar that are believed or known to be associated with, for example, improved yield. The experimenter establishes ways to test samples of different biochars, to identify which has more of the desired characteristic and is therefore “better.” In contrast, the field method involves putting different types of biochar in plots in farmers’ fields and measuring the resulting yields/soil improvements, to see which biochar had a greater positive impact. The former method produces a wealth of useful information, and offers the possibility of understanding the causal mechanisms by which biochar works its magic. However, the latter method tells you all you need to know about whether the biochar you are making is of a quality to do the job. (Though what “doing the job” is, is an ambiguous target: e.g. So you have achieved an extra 10% in yield gain, but can that be considered successful or not? If you made higher quality biochar, could you achieve 20%? This empirical question keeps some of us up at night.)

At Warm Heart, we have no scientists and we have no lab. We can do Hugh McLaughlin’s low-impact tests of biochar quality (McLaughlin 2010), but no more. The single most important test we do is the “look, mom, clean hands” test. That is, we handle the biochar and then wash our hands with water. If the biochar washes off with water, the production temperature was high enough to produce the critical layers of carbon rings that characterize good biochar. If our hands are still a greasy black, the temperature was too low and the char still contains oils, tars and other aromatics, indicating that the charring process did not proceed far enough. Our audience – local farmers – do not care about “real” lab results. They care about tangible results. They want to know three things in particular: Does our biochar increase yields? Is the crop visibly healthier? Is the soil healthier in terms of the basic measures of look, feel, and the presence of worms? To be able to draw any conclusions from a biochar test, whether in a lab or in the field, the raw material must be clearly specified. Without a baseline, the results of an experiment are basically worthless. In the field, an important question is, “Given the ways that farmers are likely to make biochar, what kind of stuff are they putting in the ground?” Farmers will use a variety of crop residues to make biochar; you can run your trainings in a place where people grow corn, for example, but if you are lucky, those folks will engage relatives who grow rice at home and so will make rice straw char. Farmers will also use a wide variety of methods to make biochar. You can teach farmers a very specific method for making biochar, but you can be certain that twenty-five teaching iterations down the line, very little of the methodology will be left as they share and change the method. A second important question is: “Does the use of homemade biochar make results worthwhile for farmers?”

The biggest question, in my mind, is: “Can you in good conscience tell farmers that they will improve their crops if they take the time and effort to make biochar and put it into their fields?4

Short answer: Yes.

How do I know?

I know because we at Warm Heart have taught farmers how to use TLUD  (Top Lit Up Draft) barrel ovens and “flame cap” troughs to make biochar, then sent them off to make their own biochar. Next we set up paired test plots – plots immediately next to each other with previous records of similar yields – and asked farmers to fertilize one with the synthetic fertilizer mix they would normally use and the other with the biochar they made, either plain or amended with manure, pig urine, EM and/or clay as we requested. We require all test plots to be adjacent to well-trafficked roads, so that community members can see them. We hang large banners over each plot to identify the treatment. We replicate these tests with biochars made with a variety of feedstocks, because farmers will use whatever feedstock they have available and it is important to know whether results vary appreciably by feedstock in a particular location.

AN35 Shaf Tab1

Table 1: Tests conducted in 2016 by Warm Heart with 10 smallholder farmers comparing yields from plots treated with biochar and plots using synthetic fertilizer and other mix.

The yield results are consistent – and positive. Table 1 shows the results of 2016 tests: 10 farmers growing Mali 105 (jasmine) and SanPaTong (sticky) rice.⁵ Note: the Mali 105 and SanPaTong species yield differently. The data reflects inter-species variation as well as inter-treatment variation. Measures are kilograms of yield per square meter. The treatments were as follows: Treatment 1: 400 kg biochar saturated with pig urine. Treatment 2: 15 kg 50:50 mix of 16-20-0 and 46-0-0 synthetic fertilizer with 6 kg biochar. Treatment 3: 400 kg compost and 400 kg of biochar saturated with pig urine. All biochar plots were given biochar at the rate of 250 g biochar/m².

AN35 Shaf Tab 2

Table 2: Soil tests taken immediately after harvesting from one pair of biochar/NPK plots
from Mai 105.7

In videotaped interviews with participating farmers conducted during this test program (Warm Heart 2017), farmers routinely commented that the plants in the biochar plots appeared healthier, and that the soil in the biochar plots improved noticeably over the course of the growing season. (Farmers were particularly happy with biochar’s impact on plant and soil health and on several occasions told us that this was more important to them than yield improvements. Because biochar could do these things that NPK obviously could not, they strongly preferred biochar.) Soil tests (table 2) taken immediately after harvesting from one pair of biochar/NPK plots from Mai 105 (Treatment 1) confirmed the impact of biochar on soil quality.⁶

Soil Types and Biochar

There are two ways of thinking about soil. In most developed world agricultural applications, discussions of soil focus on the specific characteristics of the soil in a particular field, or even in a portion of a field.⁸ In most of the developing world, such information is unavailable. However, biochar can offer huge potential gains (especially as a soil amendment) relative to what you can know about the soils that your farmers work.

On a broad scale, scientists classify soils by type, each with general characteristics that frame its overall agricultural potential. Maps of soil types are available for most of the developing world. You can help the farmers with whom you work by identifying the primary soil type or types where you are, and by making related recommendations. If you work with smallholders in the developing world, the basic soil type they farm is most likely deficient in ways that biochar will rectify.

How do we know?

Prior to committing to promote biochar, Warm Heart commissioned soil scientist Peter Elstner to prepare an overview of soil types in Southeast Asia. We then asked him to overlay the key characteristics of each soil type with the known benefits of biochar. From this, we could generalize about the expected high effectiveness of biochar as a soil amendment based on the global (largely tropical) distribution of particular soils.

Soil Types

If you work with developing world smallholder farmers, you are most likely in a hot area where the most common soil types are Acrisols, Lithosols or Nitosols (in mainland Southeast Asia, for example, these three soil types comprise more than 60% of total surface area. If you view all three world maps below, you will see how much of the developing world’s soils are Acrisols, Lithosols or Nitosols.)⁹ Acrisols, truly poor soils, are extremely common. Lithosols and Nitosols are preferable to Acrisols, but neither is promising for sustainable agriculture. However, the characteristics that make Acrisols terrible soils make them a perfect match for biochar. Biochar can help compensate well for many of the most important deficiencies of Lithosols and Nitosols.

AN35 Shaf Fig1

Figure 1: Map of the Global Distribution of Acrisols

Acrisols (figure 1) suffer “a general paucity of plant nutrients, aluminum toxicity, strong phosphorus sorption, slaking/crusting and high susceptibility to erosion [that] impose severe restrictions on arable land uses…. As biological activity is low in Acrisols, natural regeneration, e.g. of surface soil that was degraded by mechanical operations, is very slow” (FAO 2001).

AN35 Shaf Fig2

Figure 2: Map of the Global Distribution of Lithosols

Farming Lithosols (figure 2) “requires recurrent inputs of fertilizers and/or lime” and their “unstable surface soil structure makes Lithosols prone to slaking and erosion in sloping land.” They do “have higher base saturation and accordingly somewhat stronger structure than normally found in Acrisols [their] moisture holding properties…are slightly better than [those] of…Acrisols with the same contents of clay and organic matter….Lithosols are strongly weathered soils with low levels of available nutrients and low nutrient reserves. However, the chemical properties of Lithosols are generally better than [those] of…Acrisols because of their higher soil pH and the absence of serious Al-toxicity. The absolute amount of exchangeable bases is generally not more than 2 cmol(+) kg-1 fine earth on account of the low cation exchange capacity of Lithosols….The low absolute level of plant nutrients and the low cation retention by Lithosols makes recurrent inputs of fertilizers and/or lime a precondition for continuous cultivation. Chemically and/or physically deteriorated Lithosols regenerate very slowly if not actively reclaimed” (FAO 2001).

AN35 Shaf Fig3

Figure 3: Map of the Global Distribution of Nitosols

Nitosols (figure 3) are considered good soils in the humid tropics because they are stable and erosion resistant, and because they permit deep rooting, drain well and retain water. They contain more organic matter and chemical nutrients than Acrisols and Lithosols, but are still not very fertile so they are best used for undemanding plantation crops such as coffee, cocoa and rubber (FAO 2001).

Biochar and Soil Types

In the original version of “Soils of Mainland Southeast Asia,” Elstner (2017- available upon request) summarizes the primary benefits ascribed to biochar and then compares the pattern of benefits to the characteristics associated with each major soil type. His useful summary of biochar’s benefits notes that biochar:

  • Reduces soil acidity by raising soil pH
  • Increases cation exchange capacity (CEC)
  • Reduces leaching of nutrients
  • Improves soil tilth and reduces soil bulk density
  • Increases soil water holding capacity
  • Reduces aluminum toxicity
  • Supports soil microbial life
AN35 Shaf Tab3

Table 3: Characteristics associated with seven soil types in Mainland Southeast Asia. (Elstner 2017).

When he then summarizes the characteristics of each soil type as actually tested in Southeast Asia, it is clear that biochar is not a panacea in all places with all soils. However, when you examine the chemical and agronomic properties of the soil types (Table 3), it is clear that Acrisols, Nitosols and Lithosols (Lixisols) can all benefit from the use of biochar (Table 4).

To summarize, if you work with smallholder farmers in tropical or semi-tropical areas, you may not be able to fine-tune the type of biochar you use to the particular soil in each farmer’s field, but you can assess with some certainty whether biochar will have an impact. If your soils are Acrisols, you can expect biochar to have a big effect; if they are Lithosols, a good effect; and if they are Nitosols, a reasonable to good effect. You may achieve higher-than-expected results, but never overpromise and be sure to first test the production, application, and use of biochar yourself before you extend it to farmers!

AN35 Shaf Tab4

Table 4: The impact of biochar on the three most common and most problematic soils: Acrisols, Lithosols (Lixisols) and Nitosols. In the tropical developing world, these soils cover almost two-thirds of total land surface.

(International data suggests that this article’s assessment of biochar’s likely efficacy is too conservative. Trial data from experiments even in prime soils in the US and Europe show excellent results. However, since you and the farmers you work with cannot know soil specifics, the main takeaway is that, most likely, your soil is bad in ways that biochar is good at fixing.)


You will find it tricky to use and communicate standard measurements when working with smallholder farmers to make biochar-based fertilizer. In a lab setting, it is possible to work from absolutes, the best of which is dry matter or “dm”; to determine exactly how much biochar or compost you are using, you dry it out entirely and then you weigh it. Also in the lab, you can analyze the chemical content of wet ingredients like pig urine (which we use to charge biochar), and know exactly what you are putting onto/into your biochar per liter. However, these techniques are not available in the field. You might briefly consider that biochar does not expand when wet, so that volume might serve as a consistent way to measure – but then you will remember that every feedstock (and every method of pyrolysis and temperature at which it is produced) produces a different consistency of biochar—each of which packs to a different density, and crumbles differently over time.

So what to do?

First, remind yourself why you are measuring the biochar in the first place. You are not measuring for the purposes of precise scientific experimentation. You are measuring to achieve a degree of replicability. You want to know: if I do this, what will I get? If I do that what will I get? You want to know what impact a specific mix of biochar will have on yield, plant health, and/or soil quality. You need a way to measure that will allow you to compare treatment against treatment within tolerances that make relative sense, given the tools that your farmers will actually use.

And those tools are – buckets.

The farmers with whom you work will almost surely have access to a scale for weighing their rice or other crops, so they will be able to weigh their biochar recipe ingredients. However, without an idea of the moisture content of the ingredients, the weight measurements will not communicate much. For example, biochar made in a JRo will almost certainly have a lower moisture content than biochar made in an FC trough.

We at Warm Heart cannot measure moisture content with accuracy. However, international standards for biochar application are set in tons per hectare (without specifying a moisture content), which requires that we estimate grams/kilograms applied by square meter. Estimating a rate of application is hard enough when a farmer broadcasts biochar by hand; without a standard definition of moisture content – or the capacity to measure it – it is a farce to try to do so. The “international standard” for biochar application (based on studies around the world) is 10 tons per hectare or 1 kilogram per square meter. Scientific literature suggests that the better the soil, the more biochar will be required to achieve a given increase in performance. At Warm Heart, we have found the inverse to also be true; the worse soils are (eroded Acrisols, for example), the less biochar that is required to increase yields. We have never used more than 250 grams per square meter in our experiments, yet we have seen large increases, as we have sought to improve yields but minimize extra work for poor farmers who are often old and/or malnourished.

With moisture content so difficult to measure, we need a way to be able to compare between batches of biochar, compost, clay, or other materials, to determine if those variations result in yield differences in the field.

Rather than measuring by weight, it is far simpler to use buckets to measure the volume of ingredients. Start with a large amount of biochar and any other additives you plan to use. Mix all of your treatments at the same time, using the same materials and the same buckets. To the best of your ability, add the ingredients in constant proportions. If your farmers take the same approach—and you do the same over time—you will get about as close as you can to relative comparability.

Bucket tests will not allow you to say anything definitive outside of your own localized context (which can include soil type, method of char pyrolysis, feedstock, charging amendments, length of time the biochar sat, etc.). However, provided that you structure your tests properly – i.e. you manage your treatments carefully in this fashion and apply them in a proper, randomized and replicated plot pattern with consistency – you will be able to make strong claims about the value of each treatment relative to the others in your particular, local context. Note that this approach is not meant to be unscientific. Rather, it takes into account the real limits within which you must work and still meets the basic requirements of the scientific method. Your experiments may not generate publishable results, but they can generate meaningful, relative comparisons among treatments within your particular context, which can provide guidance to others like you operating in similar contexts. If you proceed in this way, you will be able to make confident statements to farmers that if they make biochar like this and mix it like that, they can expect to achieve a certain result.

The bottom line

Do not be misled by people’s claims about what constitutes “quality biochar.” Similarly, do not get hung up on “the right biochar for the right use” or on “the right way to measure.” Look for methods and materials that are “good enough” given your particular context, what you are attempting to achieve, and the resources that are available to you. Already in ancient times, Aristotle said something to the same effect: seek a “degree of precision in each kind of study which the nature of the subject permits” (Aristotle, trans. 1962).


Aristotle. Translated 1962. Niomachean Ethics. The Library of Liberal Arts. Translated by Martin Ostwald.

Elstner, P. 2017. Soils of Mainland Southeast Asia. ECHO Asia Notes #30. Available: https://www.echocommunity.org/en/

FAO 2001. Lecture Notes on the Major Soils of the World. P. Driessen, J. Deckers, O. Spaargaren, and F. Nachtergaele (Eds.). Rome:
FAO. Available: http://www.fao.org/docrep/003/y1899e/y1899e00.HTM.

Hilber, I., and T. D. Bucheli. 2010. Activated carbon amendment to remediate contaminated sediments and soils: A review. Global NEST
Journal 12(3): 305-317.

International Biochar Initiative (IBI). 2018. Available: http://www.biochar-international.org/.

Jeffrey, S., D. Abalos, K. A. Spokas, and F. G. A. Verheijen. 2015. Biochar effects on crop yield. In: Biochar for Environmental Management:

Science, Technology and Implementation, 2nd ed., Eds. Johannes Lehmann and Stephen Joseph. New York: Routledge. Pp.

McLaughlin, H. 2010. Characterizing biochars: Attributes, indicators and at-home tests. In: The Biochar Revolution: Transforming Agriculture and Environment. Eds. Paul Taylor and Hugh McLaughlin. Lilydale, Australia: Global Publishing Group. Available: http://warmheartworldwide.org/characterizing-biochar/.

Rongjun, B., S. Joseph, L. Cui, G. Pan, L. Li, X. Liua, A. Zhanga, H. Rutlidge, S. Wonge, C. Chia, C. Marjo, B. Gong, P. Munroec, and
S. Donned. 2014. A three-year experiment confirms continuous immobilization of cadmium and lead in contaminated paddy field with
biochar amendment. Journal of Hazardous Materials 272: 121-128.

Warm Heart Foundation. 2017. Biochar Interviews with Farmers in Phrao (English Version). Chiang Mai, Thailand: Warm Heart Foundation. Available: https://youtu.be/eUSEE1-ueE0.

1The single best source of information about biochar, from bibliographic references to scientific papers, press releases from biochar companies or field reports, is the International Biochar Initiative (IBI), http://www.biochar-international.org/. IBI also publishes a series of excellent short papers about research methodology. In this article, I will argue that these are largely overkill for you, who are working in the field with farmers. What is essential about them, however, is that they make absolutely clear the core rules of the scientific method, the reasons why we do science as we do. In what follows, I will not sound as if I am a lab coat kind of guy – and I am not. But make no mistake about it, when I do studies I do science.
² Take a moment to scan the internet for companies that sell biochar. How much product differentiation do you find? If you are like me, I don’t see any differentiation.
³ At Warm Heart, we are currently testing the ability of the biochar produced by our TLUDs and troughs to remove heavy metals, pesticides, and other contaminants from the soil and runoff water from fields. The research is ongoing and the results will be the subject of a future paper. There is ample scientific evidence that biochar does this well. See, for example, Hilber and Bucheli (2010) and Rongjun et al. (2014).
⁴ It is important to emphasize this point. There are two, separate worlds of research. There are those for whom the research itself is the point of the exercise; then there are those for whom the potential of biochar to improve lives is the point of the exercise. This article is for the latter, although it is an effort to make the methods of the former applicable in the “real” world to ensure that “first, we do no harm” and “second, we do all the good we can.” I want to do this in an effort to escape the current situation aptly described to me by Hugh McLaughlin in a personal communication. “The researchers of the biochar world advocate making the perfect the enemy of the good - principally, and on principle, to promote the cause of “more research is necessary before we let go of this intellectual welfare pipeline. The salvation of mankind can wait and the suffering of the little people should persist until I get promoted to full Professor, or even emeritus if you are dumb enough to let me get away with it....”
⁵ According to the metadata reported in Jeffrey, et al. (2015), rice responds poorly to biochar. The global average response seems to be around 15%; the first year response in these fields was approximately 11%. Missing data in the table and the outsized response of SanPaTong to Treatment 2 reflect that several Treatment 2 farmers dropped out too late for us to rearrange the distribution of treatments and that a farmer growing Mali 105 with Treatment 2 suffered an early infestation of aphids that destroyed much of the rice in the biochar test plot.
6The These results lack even the most basic statistical references (e.g., standard deviations), because these are not supplied by government labs. The OM figures will seem high to anyone familiar with northern Thai soils. This is a rice paddy in which the farmer plows in stubble and roots and produces two-three crops per year.
⁷Copy of lab report available from Warm Heart upon request. info@warmheartonline.org.
⁸It is increasingly common for GPS guided machinery to change fertilizer applications while it moves across a field to adjust for minor soil variations.
⁹All specific references to the soils of Southeast Asia and the soil/biochar comparison tables below come from Elstner 2017.