Study Shows Soil-Building Benefits of Manure and Crop Rotation, so why didn’t they say so?

Here is the secret to building soils – manure and diverse crop rotation. Underwhelmed? Researchers in Iowa (Delate et al, 2013) came to this conclusion after conducting ten years of field research. Only this wasn’t their conclusion.

“Iowa State study shows soil-building benefits of organic practices,” reads the headline of the Leopold Center press release on this research. This is misleading for two reasons.

First, it ignores the fact that manure application and diverse crop rotation, the practices related to improved soils, are not exclusive to organic production. There are plenty of conventional farmers here in the Columbia Basin of Washington that use manure and have diverse crop rotations (3- and 4- year rotations in this study). Even in the Midwest, there are conventional farmers who stray away from the corn-soybean rotation used for comparison in this study, and many of them probably use manure too. So, while manure and crop rotation might be practices required in organic production, it is not organic production per se that improves soils.

Second, this study seems to have been designed to show that organic production would come out on top in any soil building comparison. If before this study was designed (it started in 1998), I had stated that manure application and diverse crop rotation would build soils better than not using manure and having a short rotation, I doubt anyone would have disagreed. Yet the experimental design compares a “conventional” system of a corn-soybean rotation, full tillage with no manure, to 3- and 4-year rotations (including soil-building alfalfa) with one or two applications (per rotation cycle) of composted manure (I have other issues with use of manure in organic farming).

Let’s stop here and ask, do we have to do ten years of research to say which system will be better for the soil? Even if we took organic out of the picture by using some herbicides and fertilizer, the outcome, in terms of soil building, would be evident. Why choose this comparison if other better comparisons were available?

What if, instead of using the worst-case conventional system, they had compared the organic systems with the best conventional system? Consider a no-till system, with three or four crops in rotation (there are no-till farmers that grow more), with cover crops and manure. That, it seems to me, would be a better comparison, and would not waste resources confirming what we already know.

We can still gain a few lessons from the soil aspects of this study (pests and profits were also compared). First, it takes time to build soils. When soil properties were measured after the first four years of this experiment, they did not find the differences that they did after ten years. Second, to move towards greater sustainability we need to discern which practices are valuable. However, when practices are wrapped in labels like “conventional” or “organic”, the best way forward can be obscured. Researchers should throw out the labels and look at whatever mix of practices makes the most sense. After all, bulk organic amendments like manure, and diverse crop rotations, are good for the soil in any farming system.

Delate, K., Cambardella, C., Chase, C., Johanns, A., and Turnbull, R. 2013. The Long-Term Agroecological Research (LTAR) experiment supports organic yields, soil quality, and economic performance in Iowa. Online. Crop Management doi:10.1094/CM-2013-0429-02-RS.


Here is another example of claiming the benefits of organic farming, but it was really manure and green manures.

Williams, D. M., Blanco-Canqui, H., Francis, C. A., & Galusha, T. D. (2017). Organic Farming and Soil Physical Properties: An Assessment after 40 Years. Agronomy Journal, 0(0).


2 comments on "Study Shows Soil-Building Benefits of Manure and Crop Rotation, so why didn’t they say so?"
  1. So while amendments like manure are good for the soil in any farming system, not all farmers have access to manure. So the better question is how can any farmer best build his or her soil when this bulk amendment is not available.

  2. Al, you make a good point. As I see it, farmers who do not have access to bulk amendments have two strategies they can use. First, conserve what they have by minimizing tillage, erosion, and compaction (which leads to more tillage). Doing this will reduce the losses of organic matter already in the soil and make the best use of crop residues from cash crops. The other strategy is to grow their own bulk amendment in place with cover cropping.

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