A new groundbreaking study proves soils on organic farms store away appreciably larger amounts of carbons – and for longer periods — than typical agricultural soils. The important study, directed by The National Soil Project at Northeastern University in collaboration with the Organic Center, provides a new significant proof point that organic agricultural practices build healthy soils and can be part of the solution in the fight on global warming.  See below for detailed FAQ about the study, or read the press release here.  If you'd like to read the full study, you can view it in the peer reviewed journal, Advances in Agronomy.

Why should I care about soils?

Healthy soils are the backbone of our food system.  Without them, we wouldn’t be able to grow most of our food, because around 95% of the food we eat is produced by our soils.  It’s not just important for the food on our table, it’s also important for clean water, healthy ecosystems, and our climate.

Soil is the world’s largest water filter, and provides filtration via physical, chemical and biological processes that remove or degrade various pollutants in water as it passes through the ground. When water passes healthy soils water filtration is improved which means fewer pollutants in our rivers, streams, lakes, ponds and aquifers.

In addition to helping preserve clean water, soils are the base of the food web, and without them many organisms and the complex of life that depend of them wouldn’t survive. Soil dwelling insects and fungi are food sources for larger animals and the organic matter that decomposers break down is essential for all plant growth.

How do soils fit in with climate change?

Eighty percent of the Earth’s terrestrial Carbon is stored in soils and other than the ocean it is the largest pool of carbon on earth. Humans manage the majority of the Earth’s soils and research shows that our management practices can deplete the soil’s carbon stores – releasing it back into the atmosphere – contributing to global climate change. Agriculture in particular has been linked to large losses of soil organic carbon worldwide. In fact, a recent article published in the Proceedings of the National Academy of Sciences, shows that agriculture worldwide had resulted in a loss of 133 billion metric tons of carbon from the soil. They also found that the rate of carbon loss from agricultural soils has increased dramatically over the last 200 years as more land is converted to crop and rangeland.

Fortunately, a number of studies show that with proper management some agricultural practices can actually increase the carbon pool in our soils – making soils part of the climate change solution

Why is this study important? 

This study is important because it is one of the only to quantify the molecules important for long-term carbon storage in soils. We’ve known from previous studies that organic farming practices often result in soils that are higher in total soil organic carbon. However, not all soil organic matter is stays in the soil for long. As a result, total soil organic carbon is not necessarily an accurate proxy for understanding long term carbon storage ability in soil Measuring sequestration – the long term storage of carbon in the soil and the proportion of stable molecules that are responsible for that storage has been difficult and with previous methods it was not scalable to large studies such as this one. This study developed new methods to efficiently measure the molecules responsible for long term carbon storage in the soil.  These results are important because the stable molecules that make up total soil organic carbon are strongly associated with healthy soils and may have a role to play in climate change mitigation. It is vital to our climate and food security that we examine agricultural systems that can preserve carbon in our soils over the long term to help build our carbon reserves back up, and this study shows that organic can be a critical part of that solution.

What are Humic Substances, and why did you look at them in this study? 

The soil’s total organic carbon is made up of two distinct pools – the labile carbon pool and a pool called humic substances. The labile carbon pool is short lived and constantly reentering the carbon cycle – imagine a peach falling off of a tree, then a time lapse video of it decomposing.  Once bacteria and fungi have broken it down the carbon is released back into the atmosphere as CO2, then another plant will use that CO2 for photosynthesis, the plant grows, the plant dies or drops a fruit, decomposes and repeats – labile carbon is constantly being cycled in and out of plants and therefore doesn’t really contribute to atmospheric carbon reduction in the long run.  This carbon pool varies widely, even from season to season as carbon is added to and then leaves the soil.

Non-labile components of organic matter, on the other hand, called humic substances, are more stable and therefore a better measure of long-term impacts on the soil. Humic substances are made up of carbon (along with oxygen, hydrogen, nitrogen, and sulfur).  Together, they make up the soil humus (the humic substances are the components of humus), and are the major organic constituents of soil.  They are formed through organic matter (from plants and animals) biodegrading, and make up the largest component of soil organic matter.

We looked at these humic substances rather than the total soil organic carbon, because, while total organic matter is critical to look at when examining soil health, it isn’t the best measure of long-term carbon sequestration, because it includes the fluctuating labile pool.  By looking at humic substances we get a more accurate view of the stable, long-term storage of carbon in the soils.

Organic and conventional farmers use a wide variety of practices – how can you compare the them when there is so much variation within each system?

Both organic and conventional farmers use a wide variety of management techniques.  From different soil amendments to different crops to different tillage strategies, farms vary widely in how they implement strategies that impact their soil.  However, because organic farms are regulated by the United States Department of Agriculture, they do have several things in common that are important for soil building.  For example, they cannot use synthetic fertilizer to provide nutrients to their plants.  Instead, they must use things like compost, manure, and nitrogen-fixing plants (that are then incorporated into the soil) to manage their soil nutrients.  These techniques can increase soil organic matter, and are relatively consistent across organic farms.

This study looks at organic and conventional farms in a way that incorporates all of the differences in farming practices, soil types, crops, and other variables to get a big-picture view of how organic impacts carbon sequestration in the soils. We have soil samples from around the country, from farmers that use different strategies to farm different crops in different soil types.  The one thing they have in common is that the organic farmers all follow the organic standards when it comes to soil management.  We wanted to know if that was enough to have a significant impact on soil carbon levels, or if the differences among farms within each farming system would swamp out any of the overall impacts on soil carbon sequestration.

What is the backstory behind this project?

The National Soil Project had been collecting soil samples from conventional farms for several years, but was finding that many of these conventional agricultural land soil samples had little to no humic substances.  This worried the directors, Dr. Geoff Davies and Dr. Elham Ghabbour, greatly, because it signified a potential threat to the degradation of our national soils.  They hypothesized that the primary reason that they were seeing such low levels of humic substances on farms was the toll that many conventional farm management practices have on soil health.  To test this they partnered with the Organic Center so that they could get organic farmers involved in their analysis.  They tested over a thousand soil samples from all over the country, comparing humic substance levels in organic farms in comparison with those on conventional farms.

Why was the study published in Advances in Agronomy?

The study was published in Advances in Agronomy, because it is a peer-reviewed journal that is read by agronomists, agricultural researchers and agricultural educators – the scientific audience most likely to be interested in these results.  Peer review is a critical aspect of the scientific process, because it allows for the analysis of research hypotheses, project design, and results interpretation by other scientists.  As a manuscript passes through the peer review process, unbiased, researchers that are experts in the field read the article critically and suggest that it be accepted, rejected, or revised and improved before it is published. In this way, the quality of scientific projects can be monitored.

Advances in Agronomy is a highly-respected journal.  The Soil Science Society of America Proceedings praise the journal, "Advances in Agronomy is a first-rate resource describing the latest research in agronomy. This prestigious serial contains major review articles dealing with the current topics of interest to agronomists, crop scientists and soil scientists. As always, the subjects covered are varied and exemplary of the myriad subject matter dealt with by this long-running serial. It has the highest impact factor among serial publication in agriculture”.

What are the main findings of this study?

This study looks at over one thousand soil samples from 48 US states, comparing levels of humic substances, the long-term, stable pool of soil carbon, in organically and conventionally managed soil.  The specific humic substances that are included in the project are humic acid and fulvic acid.  Humic acid is more stable than fulvic acid, and is one of the best measures of the long-term storage of carbon in the soil.  The study also measured total soil organic matter and humification, the process by which organic matter is transformed from labile organic matter into more stable humic substances.

The study found that organic soils had higher levels of all soil organic matter and humic substances.  Specifically, organically managed soils had:

• 13% higher soil organic matter
• 1 ½ times higher fulvic acid levels
• 44% more humic acid
• 26% more humification (i.e. long term carbon storage)

These results highlight the potential of organic agriculture to increase the amount of carbon sequestration in the soil, contributing to climate change mitigation.

Why do organically managed soils have higher levels of humic substances?

A number of studies have shown that practices commonly used in organic farming are linked to higher levels of total soil organic carbon.  These practices include extended crop rotations, rotational grazing, fallowing and the use of manure, compost and legume cover crops to manage soil fertility and are likely the same factors driving the increased levels of humic substances we found in organically managed soils.

What about tillage?  Do organic farms use more tillage, and wouldn’t this have a negative impact on humic substances?

Nowadays the thought that organic farms use more tillage than conventional farms is a myth.  Organic growers used to use tillage more often than conventional growers, because they needed it as a tool for weed control in the absence of herbicides.  More recently, however, research has provided organic growers with solutions for ways they can successfully reduce tillage while controlling weeds without herbicides.  In fact, recent statistics show that the proportion of farmers using reduced tillage systems is organic is the same as for conventional farms in the United States.

Additionally, even without reduced tillage research from the USDA Agricultural Research Service's Farming Systems Project found that organic farms have greater soil organic matter than conventional no-till plots.  Another study comparing organic full tillage systems to conventional no-till found that organic plots had higher levels of soil carbon after 9 years compared with conventional no-till systems, even when those conventional systems included cover crops. Together these results suggest that organic practices such as the use of manure, compost, and legumes to augment soil fertility can provide greater long-term soil benefits than inorganic N fertility treatments in spite of tillage.

Are there other benefits of organic farming I should know about?

Yes!  There are multiple science-supported benefits of organic farming, from human health to environmental sustainability.

Pesticide avoidance is one of the most important benefits of eating an organic diet. American consumers are exposed to dozens of pesticides each day – pesticides that could be avoided by eating organic. One study that examined the effects of eating organic found that within 15 days of converting to an organic diet, pesticide exposure markers dropped to non-detectable levels. This means that eating organic food can protect against exposure to hazardous pesticides that are commonly used in agricultural production.

Organic farming is also important for the health of farmworkers and agricultural communities, because it prevents pesticide exposure for people working on or living near farms.  Pesticides have been linked with health hazards such as cancer, lymphoma, Parkinson’s disease, and neurological problems, to name a few.  Children are at particular risk for pesticide exposure, because they have higher exposures to environmental chemicals than adults, spend time near the floor where house dust may be contaminated with pesticides, and have undeveloped metabolic pathways and thus often have a reduced ability to metabolize chemicals such as pesticides into non-toxic water soluble forms that can be excreted as urine.  Even before birth exposure to pesticides can have long-term impacts on children’s lives.  Prenatal exposure to chemicals may alter the development of networks that build our individual neural architecture.  Recent studies examining prenatal and childhood exposure to pesticides have shown that exposure may be linked to the development of obesity and metabolic disorders, developmental problems, poorer neurodevelopment, and decreased IQ among other health challenges.

The environmental benefits organic production provides are also well documented. Organic farms protect bee populations, and ensure the biodiversity of other pollinators, amphibians, plants, birds, and fish. Organic management also improves soil quality by increasing microbial diversity, boosting chemical and physical soil properties, as well as nutrient content. Two of the more concerning issues that we are facing today — nitrogen pollution and climate change —could be mitigated by organic management.  This is because organic production is less energy intensive and results in less reactive nitrogen runoff.

I’m not a farmer, but want to help. How can I help protect soil health and climate stability?

The easiest thing you can do to make sure that you are contributing to soil health and carbon sequestration is to choose organic.  Organic farmers are required to implement an organic systems plan that improves soil health.  They can’t use synthetic fertilizer, so rely on carbon-building soil amendments such as fertilizer and manure for nutrient management.  They also can’t use toxic, synthetic pesticides which can have a negative impact on soil organisms.  Organic farms are certified by the USDA, and closely monitored and audited to ensure their integrity, so you can be assured that when you choose organic you’re supporting farming techniques that focus on soil health.