Study shows soil-building benefits of manure and crop rotation, so why didn’t they say so?

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The secret to building soils is… wait for it… manure and diverse crop rotation. Underwhelmed? Researchers in Iowa (Delate et al, 2013) came to this result after conducting ten years of field research. Only this wasn’t their conclusion. Instead, the headline of the Leopold Center press release reads: “Iowa State study shows soil-building benefits of organic practices.” This is misleading for two reasons.

First, the researchers ignore the fact that manure application and diverse crop rotation, practices that improve soils, are not exclusive to organic production. There are plenty of conventional farmers here where I work, in the Columbia Basin of Washington, that use manure and have diverse crop rotations (just like the 3- and 4- year rotations in this study). Even in the Midwest, there are many conventional farmers who stray away from a corn-soybean rotation, and many of them use manure too. So, while manure and crop rotation might be more common in organic production, it is not organic production per se that improves soils.

Young maize plants growing in wheat residues. Image by CIMMYT via Flickr.
Young maize plants growing in wheat residues (in a “conventional” no till system that includes use of herbicides). Image by CIMMYT via Flickr.

Second, this study seems to have been designed to show that organic production comes 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, no one would have disagreed. Yet this experimental design compares a “conventional” system of a corn-soybean rotation, full tillage with no manure vs 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, too.)

Let’s stop here and ask: does this research, which took ten years, tell us which system will be better for the soil? Even if some herbicides and fertilizer were used, the improvement in soil would be evident. So why compare organic vs worst-case conventional when 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. Some soil properties that did not show differences after the first four years did show differences after ten years. Second, to move towards greater sustainability we need to discern which practices are valuable. 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.

*For many examples of farmers using cover crops to build soil and prevent erosion, see Plant Cover Crops.

Delate K., Duffy M., Chase C., Holste A., Friedrich H. & Wantate N. (2003). An economic comparison of organic and conventional grain crops in a long-term agroecological research (LTAR) site in lowa, American Journal of Alternative Agriculture, 18 (02) 59. DOI:

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Andrew McGuire is an Irrigated Cropping Systems Agronomist for Washington State University Extension. He works with farmers in the Columbia Basin of Central Washington in improving soils through cover crops and high residue farming systems.