Is More Soil Organic Matter Always Better?

Here's a press release from Yale's School of Forestry and Environmental Studies sharing some new information about soil organic matter's contribution to crop yields. While increasing soil organic matter provides numerous benefits (like carbon sequestration, improved water-holding capacity and reduced run-off and erosion), it turns out that some of the science about increases in production has not yet been done. I thought you'd like to know the latest on how much soil organic matter is required to increase production, and how that might inform policy and practice when it comes to improving soil health and our ability to feed the world. In recent years, policymakers across the world have launched initiatives to increase the amount of "soil organic matter," or SOM, as a way to improve soil health and boost agricultural production. Surprisingly, however, there is limited evidence that this strategy will actually improve crop output. A new paper by Yale researchers quantifies this relationship between soil organic matter and crop yields at a global level. Writing in the journal SOIL, they affirm that greater concentrations of organic mat

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15 thoughts on “Is More Soil Organic Matter Always Better?

  1. Here in the Northeast, it’s not at all uncommon for fields used for the production of corn silage (whole plant chopped and ensiled) to have soil organic matter in the 1.7 to 2.3 percent range. This would translate to soil organic carbon levels of 1 to 1.4 percent. Some are even lower.

    While a an increase of .6 to 1 percent to get to the 2 percent level where crop yields *start to taper off* might seem trivial, we should remember that we’re dealing with .6 to 1 percent of a very large number.

    A furrow slice/acre of soil weighs about 2 million pounds, so an increase in carbon from 1 to 2 percent means an additional 10 tons of carbon in the soil rather than in the air. Multiply that over the 300 acres of silage corn that a typical New England dairy grows, and you’re starting to talk about some real numbers.

    In reality, the question of where soil organic carbon stops having a positive effect on yields is the wrong one to be asking. I think we should be looking for the soil carbon level where yields start to drop off and encouraging (read paying) farmers to get to those levels.

    In the meantime, getting from 1.7 percent soil organic matter to 3.4 percent is a very worthy goal, and if it could be implemented across millions of acres of cropland we could be looking at removing gigatons of carbon from the atmosphere.

  2. Hi Kathy,
    PhD student in Natural Resources, with focus on Ag here. I read the paper and corresponded with the author and I don’t think anything in it is indicative of “more is not always better.”

    The paper simply does not test between “more is always better, but gets less better the more you have” or “more is not always better.”

    I’m by no means an expert statistician, but all the regression shows is that “SOC benefits saturate with respect to yield” is a better fit to the data than “Yield increases linearly as SOC increases.” (which is kinda obvious- no one says that going from 9%-10% has the same benefits as going from 1%-2%)

    But it doesn’t test against the hypothesis of diminishing, but positive returns forever (log function), which is arguably better supported by theory and practical experience than either a linear or quadratic model*.

    And 90% of the data is from below the threshold, and its impossible to tell the difference between a log model and a quadratic model with data below the threshold.

    If the data were publicly available, I would test the log, but alas…

    *For instance the model that they use suggests that 5% SOC would give lower yields than 0%, and that the drop between 5% and 6% would be even bigger than the drop from 4% to 5%. A model that suggests that there is an increase between 4%-5%, but that it is much smaller than the increase between 1% and 2% makes a lot more sense.

  3. More studies looking at stable 2% SOC. How disappointing. Many NZ farms were once well above this threshold, and are dropping. Most soils pre colonization would’ve been above 2%, yet this now becomes a threshold for yield as we’re dealing in poor biological disabled land systems.
    If they’re measuring a decline in yield it more likely points to an imbalance of C:N ratios or poor biological activities.
    Be great to do this research with regenerative practices who run above 3% SOM.

    1. Hi Nicole,

      Check out section 2.2 of the paper which discusses the interaction between SOC and N. They also talk about the importance of SOC for improving soil structure and to reduce fertilizer inputs and irrigation needs by using cover cropping, etc. I think you might like their conclusions as well.

      The Bradford lab folks, where this research originated, really are on your side. You can read more about the focus of their research at http://bradfordlab.com.

  4. Right off, I can think of several examples of an excess of organic matter not helping production: 1) deep peat soils that are short of minerals; 2) forest mulch and “duff” that is never really makes it to a state of humus; 3) my top two fields that have about 10% organic matter but are short of moisture and nitrogen to keep micro-organisms alive (this may occur in other northern soils, too, I have been told).

    I doubt, however, that production increases are the only reason to work at increasing organic matter above 2%.

  5. I find it hard to believe that an increase in organic matter above the 2% level does not show an improvement in soil structure, water retention and porosity. With the increase in organic matter, there is also a correlation in better microbial activity and its association with mycorrhizae in the soil. If a soil is active in microbial activity, then the need for chemical fertility is reduced. Our soils are dead because of all the chemicals being applied in the hopes for additional production. Increased carbon sequestration also means more glomolin in the soil which helps to aggregate the soil allowing better water infiltration into the earth. Production increases should not be the only goal. Farmers should be more concerned with the biology in the soil rather than what chemical fertilizer to be added. Cover crops on the soil will also increase organic matter, hold the soil in place and keep mycorrhizae alive in the soil throughout the year.

    1. Hi Steve,

      The paper didn’t focus on SOM’s other benefits and when I corresponded with the Emily Oldfield she did point out that SOM is important for all the reasons you mention – soil structure, erosion prevention, water retention and more. With this work, she was focused on answering the question, “Does more SOM equal greater yields?” and her answer was that up to 2% the yields increase, and then after that, they continue to increase, but the improvements begin to level off. As she says in the paper, “Our overarching aim was to estimate the potential extent to which restoring SOC in global agricultural lands could help close global yield gaps and potential help reduce reliance on – and the negative effects of – Nitrogen fertilizer.”

      If you haven’t had a chance to download and read the paper, I think you’ll find agreement from them about what you’re saying. What I liked about the paper is that it was much easier to read and understand than many scientific papers, and they were very clear about their process, why they used it, how their modeling worked and correlated to the data, and how it might help us target efforts in those areas that need the most help so that we can make progress more quickly.

      1. I hear what you’re saying Kathy and agree that this paper is a step forward in understanding the relationships. I did thing it was rather disingenuous of them to start off on SOM and then switch to SOC to do the research. SOC is the proper form to evaluate but the link to SOM is not completely direct.

  6. Is there a reliable conversion factor between “soil organic carbon (SOC)” and “soil organic matter (SOM)”?

      1. Thanks, Kathy and Harold. The reason for asking the question on a conversion is that our soil testing laboratory measures Soil Organic Matter (SOM), and I was wondering how those results compare to the Soil Organic Carbon (SOC) results reported in the article. For my purposes I suspect that the 1.72 x SOC = SOM conversion should be sufficient for now.

    1. No, the relationship is all over the board. That is a shortfall of this research in that distinction is not clear. SOM will do positive things like increase water capacity and obviously if growing is pulling in atmospheric carbon, but SOM has no direct impact on crop fertility.

      SOC is a closer approximation of active fertilizer grade carbon. To get above 2% and keep a good C-N balance requires a very active soil biology, something that broad-based studies are not going to show.

    2. This is the conversion rate used by the study:

      “We used SOC (as opposed to SOM) for our analysis given that SOC is a common proxy for SOM. Carbon, as an element that is easily identified and measured within soil, is thought to comprise ∼ 50 %–60 % of SOM and is commonly reported in the literature (Pribyl, 2010). When SOM was reported, we converted it to SOC by dividing the value by 1.724 (Cambardella et al., 2001).”

      I find it interesting that when you look at the Pribyl article cited, it suggests a value of 2 would be more accurate. (To be fair I was only able to read the Pribyl article abstract).

      Unless I’m completely off base here, the conclusions of the Yale study would then translate to at least a 4% SOM (or 3.448 % SOM using the conventional value). When this On Pasture article says SOM is a common proxy for SOC, it does not mean a 2% SOC equals a 2% SOM.

      This may all be minutia but if On Pasture is trying to help us translate research into practices we can use, a little more clarity would be helpful in this case.

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