The Yield Benefits of Soil Organic Matter Have a Limit

Researchers find that higher yields that come from increased organic matter in the soil will eventually decline

For years, most farmers have understood that increasing the volume of organic matter in the soil will provide nutrients to increase crop and pasture yields.

New research suggests this is true, but only up to a point.

Soil organic matter (SOM) includes plant or animal tissues in various stages of decomposition. Although it is generally thought to be about 50-60% carbon, it also contains other essential plant nutrients such as nitrogen, magnesium, calcium and sulfur.

Researchers at the Yale School of Forestry and Environmental Studies in New Haven, Connecticut, found that increasing soil organic matter, or carbon, increased yields until concentrations reached 2%, the end point. saturation. Then the increasing returns may begin to diminish.

The study was global in scope, and the researchers used EarthStat’s global gridded data to develop their statistics. EarthStat provides geographic datasets that help researchers find ways to help feed the world’s population while reducing the impact of agriculture on the environment. This is a collaboration between the Global Landscapes Initiative at the University of Minnesota’s Institute on the Environment and the Land Use and Global Environment Research Group at Columbia University. -British.

“We extracted data from published studies to examine the relationship between soil organic carbon and productivity,” said Emily Oldfield, a Yale doctoral student and lead author of the paper. “Although we found two percent as the saturation point for our global model, we point out in the paper that this relationship may differ by climate, especially in more arid climates where achieving two percent would be extremely difficult. cent of soil organic carbon.”

Oldfield said they were motivated to launch the study by a report published by the U.S. Natural Research Council which said that while organic matter is considered the cornerstone of most sustainability and quality initiatives of soils, they lack data to understand how improvements in SOM relate to soil function. Many soil health policies were based on the principle that the more organic matter the better. Oldfield said the goal of their paper was to better measure how much organic matter is needed to make a difference and how much it could improve.

“Methods for increasing soil organic matter are highly context-dependent, and we wanted to be careful not to prescribe a one-size-fits-all approach,” she said. “For example, increasing organic matter depends on access to inputs such as compost, manure and residues. We also point out that in some cases a combination of mineral fertilizers and organic inputs will be needed to create organic matter to increase yields. The objective of our article was not to prescribe certain practices, but rather to show that there is a general positive relationship between soil organic matter and yield if a farmer or land manager considered changing their practices or to try those that accumulate organic matter in the soil. floors. »

She writes in the article that although SOM is considered essential to soil health, its relationship to yield is disputed due to local-scale differences in soils, climate, and farming systems.

“We developed a quantitative model exploring the relationship between SOM and the yield potential of maize and wheat crops in light of covariant factors of management, soil type and climate. We found that the yields of these two crops are on average higher with higher concentrations of SOC (soil organic carbon). However, yield increases level off at around 2% COS. Nevertheless, around two-thirds of the world’s maize and wheat cropland currently has SOC levels below 2%.

Oldfield suggested that an alternative to increasing SOM relies primarily on mineral nitrogen fertilizers, adding that studies have shown that if SOM levels drop, nitrogen fertilizer inputs can maintain yields.

“This approach ignores the potential co-benefits that come with increased SOM, such as protection against drought and potential improvement in water quality,” she said. “But we don’t want to prescribe specific practices, as these can be highly context-dependent. We mention in the document that different regions and climate types will face different imperatives for building SOM. For example, in the US Midwest, building SOMs can be a good strategy to reduce fertilizer inputs and protect against drought events, while in sub-Saharan Africa, building SOMs can be essential for nutrient supply.

As the world’s population grows, the pressure to increase agricultural production has led to the expansion of land areas for agriculture and the use of irrigation and fertilization. But a trade-off is degradation of land and water quality, including loss of soil organic matter.

Rebuilding this organic matter promises to improve fertility, water, nutrient retention, and soil structure, which will promote better drainage and aeration with minimal loss of topsoil due to erosion. . All of this could contribute to sustainable agricultural productivity with less dependence on mineral fertilizers.

The report, titled A global meta-analysis of the relationship between soil organic matter and crop yields, was published in the journal Soil, an open access publication of the European Geosciences Union.

The co-authors were Mark Bradford, professor of soil and ecosystem ecology at Yale, and Stephen Wood, scientist at the Nature Conservancy.

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