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Is More Soil Organic Matter Always Better?

By   /  February 11, 2019  /  15 Comments

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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.

Co-authors of the paper are Mark Bradford, professor of soils and ecosystem ecology at F&ES, and Stephen Wood, a scientist at The Nature Conservancy. SOIL is an interactive open-access journal of the European Geosciences Union. Click to download the full paper.

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 matter indeed produce greater yields — but only to a certain point.

Specifically, they find that increasing soil organic carbon – a common proxy for soil organic matter – boosts yields until concentrations reach about 2 percent, at which level they tend to hit a saturation point. Thereafter, the researchers say, the increase in SOM begins to deliver diminished returns.

Even still, they find that roughly two-thirds of agricultural soils dedicated to two of the world’s most important staple crops – maize and wheat – fall below that 2-percent threshold, suggesting there is vast potential for agricultural policies that promote increased soil organic matter.

This map shows maize (a) and wheat (b) lands with less than 2% Soil Organic Carbon. The lighter the green, the lower the SOC level. Black areas are cultivated maize and wheat lands that have concentrations over 2% SOC. Approximately 2/3 of all maize (61%) and of all wheat (64%) producing areas are on soils with less than 2% SOC. (From the page 18 of the article found in Soil 5, 15-32, 2019.

 

“The premise for so many sustainable land management practices is that if you increase soil organic matter you’re going to increase production,” said Emily Oldfield, a Ph.D. student at the Yale School of Forestry & Environmental Studies (F&ES) and lead author of the paper. “But when you dig into the literature, there are very few empirical studies that actually directly quantify that relationship.”

“These results show that there is value in setting evidence-based SOM targets for many land stewardship initiatives,” she said. “They also suggest that we must move away from a qualitative ‘more is better’ approach to soil health policies and toward specific regional and local targets that can achieve measureable agricultural outcomes.”

It is well understood that building and maintaining soil organic matter is key to soil health. (SOM refers to organic matter found in the soil, including plant and animal materials that are in the process of decomposition.) It strengthens the capacity of soils to retain water and nutrients, supports structure that promotes drainage and aeration, and helps minimize the loss of topsoil through erosion.

For years, policymakers have emphasized the role of soil organic matter in a series of programs, including the “4 per 1,000” initiative of the Soils for Food Security – which emerged from the COP21 negotiations – and the U.S.’s “Framework for a Federal Strategic Plan for Soil Science.”

Yet when it comes to its role in promoting crop production, there’s been a surprising dearth of quantitative evidence, says Mark Bradford, the paper’s co-author and professor of soils and ecosystem ecology. For Bradford, this gap in knowledge has been a nagging concern for nearly a decade; a 2010 National Research Council report on sustainable agriculture described organic matter as the cornerstone of most sustainability and soil quality initiatives, he recalls, yet offered no information on how much was actually needed to increase crop yields and reduce fertilizer application.

“I was always telling people about how important soil organic matter was, and yet here was a national synthesis from our top scientific body saying that we did not have the data to say anything meaningful,” Bradford said. “Our paper is the first really synthetic attempt to put numbers out there to guide practice by helping to establish targets.”

To do so, they collected existing data on crop yields of maize and wheat that was paired with measures of soil organic matter at sites across the world. They found that the largest gains in yield occurred between concentrations of 0.1 percent and 2 percent. For example, yields were 1.2 times higher at 1 percent than 0.5 percent. But those gains tend to level off when concentrations reach 2 percent.

“The result is that we now have numbers, not just unverified ideas, that if you build organic matter you can improve outcomes — such as less fertilizer and increased yield,” Bradford said. “It’s a place to start to bolster soil stewardship efforts for a healthy planet and enhanced food security.”

The analysis offers valuable insights for policymakers and researchers as they evaluate the relationship between soil carbon and crop yield, said Wood, a Yale graduate who now is an applied scientist at the Nature Conservancy.

And while the research represents a global analysis, he said, the methodology will make it easier for targets to be identified at specific agricultural sites worldwide. “Because all locations will have different thresholds of how much a soil property can be changed and what level of a soil property is ‘good’ for that place,” Wood said.

Added Bradford: “We now want to work on refining these relationships for specific regions and even specific farms, and we hope to forge partnerships with agriculture companies to realize this possibility.”

One last note from Kathy. I reached out to Emily Oldfield to ask if she has done work yet on how this might relate to pastures and rangelands. She says this is something they hope to explore in the future.

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About the author

Publisher, Editor and Author

Kathy worked with the Bureau of Land Management for 12 years before founding Livestock for Landscapes in 2004. Her twelve years at the agency allowed her to pursue her goal of helping communities find ways to live profitably AND sustainably in their environment. She has been researching and working with livestock as a land management tool for over a decade. When she's not helping farmers, ranchers and land managers on-site, she writes articles, and books, and edits videos to help others turn their livestock into landscape managers.

15 Comments

  1. Bill Fosher says:

    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. Ben Dube says:

    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. Nicole says:

    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.

    • Kathy Voth says:

      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. Curt Gesch says:

    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.

    • Kathy Voth says:

      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.

      • Harold Schrock says:

        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. Bill Shockey says:

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

    • Kathy Voth says:

      I may not understand your question correctly. I did a quick search with google and found this page where one scientist asked a similar question and then got a variety of responses, including some equations he might use. Let me know if this link starts us down the path of what you’d like to know. If so, I can do more looking for you. 🙂
      https://www.researchgate.net/post/How_can_I_convert_percent_soil_organic_matter_into_soil_C

      • Bill Shockey says:

        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.

    • Harold Schrock says:

      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.

    • Jacob Quiring says:

      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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