High-quality soil is crucial to growing nutrient-dense food. Tragically, much of the soil in the U.S. and around the world has been depleted and degraded by modern farming practices using nitrogen-based fertilizers, tilling and monoculture where only corn and soybeans are being rotated. Prior to 1904, soybeans were grown only as forage crops,1 providing some livestock feed, but more often used as a conservation crop.2
Soil carbon losses also greatly contribute to a loss of biodiversity in the soil and depletion of nutrients that feed plants. Past studies that have attempted to quantify those losses have generated results that vary wildly in their conclusions.3 Generally, scientists have believed carbon losses were near 78 billion tons, representing a huge loss to the soil and nutrient base for crops. However, a recent study published in PNAS convincingly demonstrates this number may not be as accurate as once believed.
Soil Organic Carbon Loss Moving More Rapidly Than Anticipated
Soil is more than just dirt under your feet. It’s a life-anchoring, thin skin that covers the Earth. In the soil live countless microbial species that provide a complex ecosystem supporting agriculture and wildlife. As the composition of the soil changes, so does the biodiversity and nutrient value. Carbon is one of those compounds lost in ever increasing amounts, thereby taking a significant toll on the intricate ecosystem that lives in the topsoil.
The transition across the globe from natural vegetation to agriculture has produced a condition that encourages soil erosion. In the past 150 years,4 the planet has lost nearly half of the topsoil as monocultures do not conserve soil in the way natural vegetation does.
Researchers now believe carbon losses across the earth amount to 133 billion tons, almost double what was previously measured.5 Past studies used a bookkeeping type method to determine carbon loss. Researchers would measure carbon loss in a single plot of land and then estimate a total loss across the world.
In this study, researchers used a large data set with information on soil from around the world and applied it to a model of human land use and agricultural activity across 12,000 years. This model relied on another larger database of information and accounted for physical factors such as climate and topography. The results suggest that the most intense losses in the top 6 feet of soil around the world have resulted from planting crops.6
Researchers found these losses varied in different locations and over different time periods, with the greatest increases noted since the industrial revolution, particularly during the 19th century. The challenge to the agricultural community, and the world at large, is more acute when you understand areas of land available for growing food are also contracting.7 Urban growth, loss of topsoil and soil depletion are factors that reduce available land suitable for farming.
Carbon Loss Damages Soil Quality and Product Yield
In a comparison of long-term effects of conventional and organic farming since 1948,8 organic farms had higher organic matter. The topsoil depth was greater, there was less soil erosion and more polysaccharide content on the organic farms, all factors that increase crop yield and topsoil generation.
This indicates that in the long term, organic farming systems are more effective than conventional systems in reducing soil erosion and thereby improving soil productivity. One of the main causes of soil degradation, making it less hospitable for plant growth, is a loss of soil structure and fertility.9 These are a major threat to crop production for future generations. Biological processes that contribute to degradation of soil quality include a reduction in total and biomass carbon, and resulting decline in land biodiversity.10
Supporting bacterial and fungal soil growth has the potential to reinstate fertility and structure, as microorganisms increase the bioavailability of nutrients through nitrogen fixation. One of the basic tenets of organic farming is crop rotation, which promotes greater plant diversity and carbon sequestration, which increases positive microbial activity.11 These factors are interrelated and necessary for robust soil health.
Organic carbon supports the structure of the soil that in turn improves the physical environment for plant roots, helps absorb and retain greater amounts of water and reduces the susceptibility to compaction and erosion.12 Loss of carbon may limit the ability of the soil to provide nutrients for plant growth. In other words, loss of carbon may lead to lower crop yields and reduced amount of nutritional benefits in the plants that are harvested.13