Soil organic matter’s yield benefits have a limit

Researchers discover that the higher yields that come from boosting organic matter in soil will eventually diminish

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

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

Soil organic matter (SOM) comprises plant or animal tissue in various stages of decomposition. While it is generally thought to be about 50 to 60 percent 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, boosts yields until concentrations reach two percent, the saturation point. Then the increasing yields can begin to diminish.

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

“We pulled data from published studies to look at the soil organic carbon-productivity relationship,” said Emily Oldfield, a PhD student at Yale and lead author of the paper. “While we found two percent as the saturation point for our overall model, we emphasize in the paper that this relationship may differ depending on climate, especially in more arid climates where getting to two percent soil organic carbon would be extremely difficult.”

Oldfield said that they were motivated to launch the study by a report put out by the Natural Research Council of the United States that stated while organic matter is considered the cornerstone of most sustainability and soil quality initiatives, they lack the data to understand how improvements in SOM related to soil function. A lot of policy around soil health was driven on the premise that the more organic material, the better. Oldfield said the goal of their paper was to better measure how much more organic matter was needed to make a difference and how much better it could become.

“The methods to increase soil organic matter are heavily context dependent, and we wanted to be careful not to prescribe a one-fits-all approach,” she said. “For instance, increasing organic matter depends on access to inputs such as compost, manure, and residues. We also highlight that in some instances a combination of both mineral fertilizers and organic inputs will be necessary to build organic matter to increase yields. The goal of our paper was not to prescribe certain practices, but rather to show that a general, positive relationship between soil organic matter and yield exists should a farmer or land manager be considering switching their practices or trying out ones that build up organic matter in soils.”

She wrote in the paper that although SOM is considered key to soil health, its relationship with yield is contested because of local-scale differences in soils, climate, and farming systems.

“We developed a quantitative model exploring how SOM relates to crop yield potential of maize and wheat in light of co-varying factors of management, soil type, and climate. We found that yields of these two crops are on average greater with higher concentrations of SOC (soil organic carbon). However, yield increases level off at around two percent cent SOC. Nevertheless, approximately two-thirds of the world’s cultivated maize and wheat lands currently have SOC contents of less than two percent.”

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

“This approach ignores the potential co-benefits that come with increasing SOM such as drought protection and potentially improved water quality,” she said. “But we do not want to prescribe specific practices, as those may be heavily context dependent. We mention in the paper that different regions and climate types will face different imperatives for building SOM. For instance, in the midwestern U.S., building SOM may be a good strategy for reducing fertilizer inputs and protecting against drought episodes while, in sub-Saharan Africa, building SOM may be critical for nutrient provision.”

As the global population increases, the pressure to increase crop production has resulted in expansion of land areas for agriculture and the uses of irrigation and fertilization. But a tradeoff is a degradation of land and water quality including the loss of soil organic matter.

Rebuilding that organic matter holds the promise of improving fertility, water, nutrient retention and soil structure, which in turn will promote better drainage and aeration with a minimal loss of topsoil from erosion. This could all add up to long-lasting crop productivity with less reliance on mineral fertilizers.

The report, which is called 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.

Co-authors were Mark Bradford, professor of soils and ecosystem ecology at Yale, and Stephen Wood, a scientist at the Nature Conservancy.

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