Bryan Markhart Fifth Term Math & Science

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Left - a nutrient-poor oxisol; right - an oxisol transformed into fertile terra preta

Affect of Feedstock Choice on the Effectiveness of Biochar as a Soil Supplement

Bryan Markhart

December 2, 2011


Pre-Colombian Amazonian agrarians would incorporate charcoal into their fields, as a soil amendment to the point where they turned the usually light brown to red soils a deep black, known as the terra preta soils, literally translating to black earth. Even to this day these soils are highly sought after agricultural real estate because of their dramatically higher yields. In the context of the growing movement for more environmentally conscious agricultural practices, which focus a lot on building healthy soils that can produce high yields with low inputs, it is wonder that scientists have become very interested these soils that still have enhanced fertility 600 plus years later.

A lot of work has been done lately to determine why charcoal, or biochar, enhanced soils increase fertility. There have been several studies looking at biochar use in different soil types, in both subtropical and temperate (Novak et al, 2009, and Laird et al, 2010). They have looked at the affect of biochar has on known fertility characteristics of soil such as soil organic carbon (SOC), cation exchange capacity (CEC), and soil pH, as well as nutrient retention. The literature shows that the additions of biochar to soil dramatically increases SOC, acts as a neutralizer for acidic soils, and decreases nutrient leaching.

Besides the many agricultural benefits, biochar is also praised from an environmental standpoint for its ability to sequester carbon and prevent excess nutrients from leaching into the water table.

Biochar looks to be a very promising soil amendment and is already being marketed to farmers at a commercial scale. While biochar has clear agricultural and environmental benefits regardless of its source, the commercialization for profit of a product that is easily produced on a small scale may not be in the best source for small farmers interested in closed loop practices.

While biochar as a whole has been shown to have many beneficial properties, the differences between various feedstocks in the production of biochar has not been closely examined. In this study I will look at the role that feedstock choice plays: first on the efficiency of production based on the feedstock:char ratio measure in kg and second on SOC, CEC, pH, and nutrient retentions of supplemented loam to silt-loam soils. I will compare biochar produced from regionally available feedstocks, including hardwoods, softwoods, and cornstalks.

I hypothesize that all three feedstocks will produce biochar that will increase SOC, neutralize soil pH, and reduce nutrient leaching, with little increase in CEC of the loam to silt-loam soils found on Bennington College campus. There may be a differences in the extent to which each biochar does each of these. In regards to the production efficiency I predict the highest for hardwoods and lowest for corn stalks, with softwoods somewhere in the middle. The objectives of this study are look at 1) the effectiveness of biochar as a soil supplement for loam to silt-loam soils in southern Vermont, while also looking at 2) the effectiveness of three regionally available feedstocks at producing biochar and whether one need giver preference to one feedstock over the other.


Biochar Production

As opposed to other biochar studies that source their biochar from industrial scale production facilities that use a very controlled process, this study will use a small-scale production method that is easily replicated with minimal capital investment. The design of my kiln is from New England Biochar LLC (New England Biochar, 2011). I will pack a 50-gallon drum with my feedstock and weight it, subtracting the weight of the drum. I will then turn it upside down inside of a larger metal container with air vents in the bottom. There will be 2-3 inches of space between the retort and the outer container that I will pack with kindling and burn it from the top down. I will then cover this and attach a chimney to create good airflow through the kiln. After pyrolysis is complete and the kiln has cooled, I will weight the whole retort again and subtract the tare weight.

Sample Preparation and Incubation

I will collect soil samples from five distinctly separate fields, on Bennington College (BC) campus, all classified by the USGS soil survey as loam or silt-loam and mix them together to create a broad-spectrum soil sample for BC campus. Separately I will mix finely crushed biochar from each feedstock with composed cow manure, at a 1:1 ratio and let incubate for two weeks. I will then incorporate each biochar-manure cocktail in with separate samples of the soil collected from BC campus from each at 4.0% by weight, resulting in soil samples with a biochar concentration of 2.0% by weight.

These soil samples will then be placed in 10 cm diameter, 20cm tall PVC columns, held at the bottom by sturdy mesh so as to allow water to escape but prevent any soil loss. Five cm of headspace was allowed in each column, leaving 15cm of soil. I will pack each column so that each has the same dry weight. There will be three test columns for each feedstock. Each sample will be analyzed for starting SOC, CEC, pH, and measurement of bio-available plant nutrients including: N, P, K, Cu, Ca, Mg, Zn, Fe, Mn, Na, and S.

These samples will then be incubated for 120 days, roughly the time between last and first frost dates for southern Vermont. Each column will be leached using deionized water every 30 days starting on day one for total of five leaching events. The leachate will be collected from each leaching event and analyzed for concentrations of total organic carbon (TOC) along with the relevant plant nutrients listed above, as well as leachate pH.

A final analysis will be taken on day 120 to measure the SOC, CEC, and soil pH, as well as bio-available plant nutrients concentrations of the soils samples after 120 days of incubation and five leaching events.


There are several possible results to this experiment: first, it is possible that the results support the null-hypothesis, showing no significant difference between the feedstocks, if there is shown to be a significant difference between any of the characteristics of the different biochars then the preferred feedstock may be clear based on fertility characteristics. The most likely difference is in nutrient retention, as this is more dependent on the chemical geometry, which may be affected by the nature of the carbon in the feedstock as well as the presence of other compounds. The CEC may also be affected by feedstock for the same reason, but I know less about what specifically affects this. It is unlikely that SOC will be different between feedstocks simply cause all biochar is primarily carbon so the amount of carbon being put into the soil will be essentially the same for each sample, there is a possibility that one feedstock will, for whatever reason, yield carbon in a form that is more easily dissolved and lost during a leaching event.

If the soil tests and leachate analysis support my null-hypothesis then the quality off each feedstock will be based solely on production efficiency. The feedstock that produces the most biochar per weight feedstock will be the most of efficient use of feedstock and resources for a farmer or gardener interested in economic production of biochar.

Works Cited

Laird, D., Fleming, P., Wang, B., Horton, R., & Karlen, D. (2010). Biochar impact on nutrient leaching from a Midwestern agricultural soil. Geoderma, 158(3-4), 436-442. doi:10.1016/j.geoderma.2010.05.012

New England Biochar (2011), <>

Novak, J. M., Busscher, W. J., Laird, D. L., Ahmedna, M., Watts, D. W., & Niandou, M. A. S. (2009). Impact of biochar amendment on fertility of a southeastern coastal plain soil. Soil Science, 174(2), 105.

Preliminary work and notes:

Primary question The feedstock used in the production of biochar have an affect on its effectiveness as a soil supplement?

Generic (old) Biochar Question What affect does the use of biochar as a soil amendment in, Zone 4, loam to silt-loam soils (soil types in the proposed student garden sites on Bennington College campus) have on soil structure, and how does that affect agriculturally significant properties of that soil, such as soil organic carbon (SOC) cation exchange capacity (CEC), nutrient retention, and pH. Do the affects observed in campus soils differ from those observed in tropical climates? Does biochar alter the affects / retention of N, P, and K fertilizers.

Supplementary design question: What is the best way to make your own charcoal?

  • Can you re-frame any of these more in shape of a particular mechanism? Or possible expectation (hypothesis)? Might be helpful to focus on why you think it MIGHT be interesting to look at water and nutrient retention effects?Kwoods 00:49, 29 September 2011 (UTC)
  • Hey Bryan, I was just wondering if you had thought any more about this question since we talked about it, I'm still interested in where you might end up taking this. Gfredericks 13:14, 21 October 2011 (UTC)
  • It would be interesting to try and apply this to the ecological effects of slash and burn farming exhibited by many cattle farmers in South America. Does charcoal simply add retention to the soil? Or does it add something like nitrogen or make the soil pH change?

Other Questions:

Agriculture / nature integration

This may be more of a design question, but I am interested in determining the least ecologically disruptive way to integrate agricultural crops into a landscape?

  • No problem with design questions for me -- but what do you mean by 'ecologically disruptive'?Kwoods 00:49, 29 September 2011 (UTC)
    • Might be good to think about this in terms of soil chemical content/pH balances, that sort of thing; in which case, crop rotation comes to mind as a design schematic. I think you might also have to account for land placement, irrigation practices, possibly erosion(?), just to name a few factors that might be worth considering. Ebraun 03:04, 03 October 2011 (UTC)
  • This would be an important area to study, but the design aspect seems like it might be tricky to me, just in terms of how you are going to determine one such practice as less ecologically disruptive than another. Agriculture is so wide-ranging in terms of techniques, practices, and possible effects on the surrounding environment as a result. Getting a full view of how different crop integration strategies impact different aspects of the environment would be quite difficult to do thoroughly, unless maybe you just focused in one certain possible agricultural ecological disruptions. I agree with Kerry, clarifying "ecologically disruptive" is the first place to start. - Guy
    • To go back to issue of design questions: assessing whether or why a design works is certainly a research question. But here you can't say 'whether' until you say what specific ecosystem property you qwant to look at. So maybe start with question like "does this management practice change this property more or less than that practice" (notice the 'chjange' rather than 'disrupt').

Integrated pest management

What integrated pest management practices can be employed to significantly reduce Crucifer Flea Beetle (Phyllotreata cruciferae) damage on Brassicas? How affective / cost-affective are they?

Found this. Seems interesting, not on brassica, but canola. Are they simlilar? Also, what might be interesting is to look at defense mechanisms. Can they somehow protect themselves? Do brassica have any ways of deferring beetle activity? Can they be distasteful? Or have harshly textured leaves? I really don't know much about 'em! ~Emily

  • Yep, they're all genus Brassica. BUT more to point 'what' questions are not very hypothesis-generative. Need to make it more along lines of 'here are possible practices (maybe those suggested by somebody), and here's a design for testing which are most effective...' SO, START WITH WHAT PEOPLE ARE ALREADY SAYING/DOING? (And probably focus on one part of the questikon -- effectiveness, cost, or... -- not all at onceKwoods 03:16, 11 November 2011 (UTC)

Companion cropping

What are positive and adverse affects of various companion cropping systems and what causes these crops to be companions or antagonists?

  • Second part is the more basic question? If you understand that, does the first part follow more easily?Kwoods 00:49, 29 September 2011 (UTC)
  • This is an interesting progression: your motivating question is, what is the best way to run a companion crop system? But that leads into a (as Kerry said) more basic question: what is the mechanism of a companion crop? Why should it work? If you can get a better understanding of why a companion crop works, then you are in the position to make predictions: based on the mechanism, you might be able to suggest crop combinations that no one had tried before. From there it can go down towards more specifics: maybe people say that crop X and crop Y grow better together. An immediate question is, is it true? But maybe a more productive question would be, why would that be true? Has anyone suggested a mechanism? Can you think of one? Once you have a hypothesized mechanism, you can start thinking of experiments you could do to test whether it really works that way. If your mechanism works out, then, at least in principle, you could make predictions: if Y grows well with X because it lends element A, and you know Z makes even more element A, then Y should grow even better with Z (probably something more interesting than that but you get the idea). -- Andrew
  • In general, if you are trying to solve a practical problem, there are two ways to go. One is to just try a whole bunch of different crops together in different combinations and see what works. This is common in industrial research. The other way is to try to get at an underlying mechanism, as Kerry and I were suggesting above. The underlying mechanism is more scientifically interesting, and potentially more productive if you can get it to work. But sometimes we are so far from understanding the underlying mechanism that the industrial scattershot approach is the only option. And sometimes it is a bit of a mix: for example, in drug discovery, there are some general guesses about what will work for a problem (the compound should be a certain size, it should have a group on the side of a certain shape, it shouldn't be too "greasy"...), but the mechanism is fuzzy enough that there is a lot of guessing and random trying. A constraint on this is that, doing a student senior work project, you probably won't have the resources to try hundreds of different combinations in the hope that something will work out... -- Andrew
    • And further to Andrew's point; getting to a pointed approach may be best done by looking at what people are already suggesting/using, and asking how you'd assess whether it's effective (however you'd define that). Don't start with 'various'; start with a particular example. (There's probably no reason to think that different systems would have similar effects?) Kwoods 03:16, 11 November 2011 (UTC)