Increased soil carbon increases crop yield — to a point

They found that the largest gains in yield occurred between concentrations of 0.1 percent and 2 percent of soil organic matter.
“…we now have numbers, not just unverified ideas, that if you build organic matter you can improve outcomes — such as less fertilizer and increased yield.”

Yale School of Forestry & Environmental Studies Read Science Daily article here 

  • Emily E. Oldfield, Mark A. Bradford, Stephen A. Wood. Global meta-analysis of the relationship between soil organic matter and crop yieldsSOIL, 2019; 5 (1): 15 DOI: 10.5194/soil-5-15-2019

While policymakers often tout the benefits of increasing soil organic matter as a way to boost agricultural yield, there is limited evidence that this strategy actually works. A new study quantifies this relationship, finding that greater concentrations of organic matter indeed produce greater yields — but only to a certain point.

Specifically, they find that increasing soil organic carbon — a common proxy for soil organic matter — boosts yields until concentrations reach about 2 percent, at which level they tend to hit a saturation point. Thereafter, the researchers say, the increase in SOM begins to deliver diminished returns.

Even still, they find that roughly two-thirds of agricultural soils dedicated to two of the world’s most important staple crops — maize and wheat — fall below that 2-percent threshold, suggesting the vast potential for agricultural policies that promote increased soil organic matter.

…It is well understood that building and maintaining soil organic matter is key to soil health. (SOM refers to organic matter found in the soil, including plant and animal materials that are in the process of decomposition.) It strengthens the capacity of soils to retain water and nutrients, supports structure that promotes drainage and aeration, and helps minimize the loss of topsoil through erosion.

For years, policymakers have emphasized the role of soil organic matter in a series of programs, including the “4 per 1,000” initiative of the Soils for Food Security — which emerged from the COP21 negotiations — and the U.S.’s “Framework for a Federal Strategic Plan for Soil Science.”

And yet when it comes to its role in promoting crop production, there’s been a surprising dearth of quantitative evidence, Bradford says. For Bradford, this gap in knowledge has been a nagging concern for nearly a decade; a 2010 National Research Council report on sustainable agriculture described organic matter as the cornerstone of most sustainability and soil quality initiatives, he recalls, yet offered no information on how much was actually needed to increase crop yields and reduce fertilizer application.