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Are you mining your soil organic matter?

New research suggests that conventional corn-soybean systems managed with synthetic fertilizer alone do not produce enough plant material to sustain soil organic matter levels, reducing crop yield in the long term.

Are you mining your soil organic matter?

New research suggests that conventional corn-soybean systems managed with synthetic fertilizer alone do not produce enough plant material to sustain soil organic matter levels, reducing crop yield in the long term.

Iowa State University agronomist Michael Castellano, who led research on a recently completed project funded by the Leopold Center for Sustainable Agriculture, studied soil properties at four field sites in Iowa, each with continuous corn and corn-soybean rotations. Plots at each site have been maintained for 13 years under specific fertilizer regimes ranging from zero to 240 pounds of synthetic nitrogen per acre per year, including some maintained at the agronomic optimum of about 150 pounds per acre.

Results of the two-year study show when managed at optimum levels, continuous corn systems generally produce enough crop residue (about 3 tons per acre per year) to maintain or increase soil organic matter. Soybeans, however, create half that much, so corn-soybean systems overall do not produce enough bulk matter to replenish soil organic matter stocks in the long term, even at optimum fertilizer levels. Also, increasing synthetic fertilizer inputs beyond optimum levels does not appear to impact soil organic matter because it does not increase crop residue inputs and is rapidly lost to the environment.

Losing organic matter

“The corn-soybean system in Iowa is losing organic matter when managed with synthetic fertilizer only, even in the absence of soil erosion,” says Castellano. “Our data highlights the importance of getting as much organic matter into that system as possible.”

Funded in 2011 by the Leopold Center’s Ecology Initiative, the project was conducted on sites with little or no slopes to control against erosion, otherwise the biggest factor in soil organic matter loss. The carbon and nitrogen content of soil samples collected in 1999 were compared against samples from 2009.

More detailed analysis was also done on the continuous corn systems to understand what percentage of the soil organic matter was chemically available for plants to take up, and what percentage formed stable aggregates resistant to breakdown by soil microbes. The soil aggregates, which resist decomposition for many years, sometimes centuries, are critical for long-term carbon and nitrogen sequestration.

Best remedy is manure

On corn-soybean operations, Castellano says the best way to increase soil organic matter, after controlling for erosion, is to return livestock manures to the soil, since they contain carbon in addition to nitrogen.

Cover crops may also increase soil organic matter levels. At the ISU Marsden Farm near Ames, where research agronomist Matt Liebman has run a diversified cropping systems project since 2002, Castellano collected soil data from a two-year corn-soybean rotation versus a three-year corn-soybean-oat plus red clover cover crop system.

Early analysis confirms past research suggesting the three-year rotation has more N available for plant uptake, resulting in greater yields than those observed for two-year rotations.

Castellano hypothesizes that high-quality manure or crop residue benefits soil quality. He’d also like to test the hypothesis that soils with low organic matter have lower fertilizer and nitrogen use efficiency than well-managed soils with high organic matter.

Answering these new questions dovetails with the goals of the Iowa Nutrient Reduction Strategy. That is, to reduce nutrient loading of waterways. While overfertilization results in nutrient pollution to air and water, Castellano’s work shows underfertilization is also detrimental, since it reduces the amount of soil organic matter that acts as a reservoir for nutrients put onto the field. Meanwhile, the “optimum” level of synthetic nitrogen fertilizer inputs is always variable, depending on factors such as site conditions, management goals and weather.

Cover crops also help

“There are many sites that don’t respond to nitrogen fertilizer in one given year, and we don’t even know how to predict those,” says Castellano. “There’s always going to be uncertainty surrounding whether farmers are managing their organic matter optimally.”

Given this variability, he says, manure inputs or cover crops provide a level of insurance against nutrient losses. By improving the organic matter content of the soil, they serve as a buffer against year-to-year uncertainties.

“Cover crops are particularly promising,” says Castellano, “because the Iowa Nutrient Reduction Strategy has identified cover crops as a big tool to reduce nitrate loads into lakes, streams and rivers. So if cover crops also can increase or neutralize the loss of organic matter in a corn-soybean system, those are two very important factors that really can contribute to long-term sustainability of Iowa’s farmlands.”

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Iyer is a former ISU grad student who worked for the Leopold Center for Sustainable Agriculture.


CORN IS SHORTER: These people are standing in research plots that haven’t received synthetic nitrogen fertilizer. All other nutrients are added based on soil tests. The fertilized plots are in the background.


N NEEDED: In the foreground is corn grown without synthetic nitrogen fertilizer. The plots in the background received N fertilizer at the optimum rate.

This article published in the October, 2014 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

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