Growing a Revolution, page 23
Biochar and organic matter can also restore soil fertility fairly quickly—in years, not centuries. There is no better ready-to-go, cheap technology to address the multiple problems of eroding, lifeless, low-fertility soils and an atmosphere overloaded with carbon dioxide. Burying charcoal in the soil is an elegantly simple and economically feasible way to help reduce atmospheric CO2, one that is generally overlooked in favor of technologically complicated—and expensive—ideas for carbon sequestration, like pumping CO2 back down old oil wells.
There is also significant potential to link energy production to biochar. Here is how it might work. The raw material to produce biochar ranges from food waste and landscape clippings in cities, to crop and noncrop plant materials on farms, to slash from timber operations. Meza’s coffee drier is just one example of how to use this material to produce both energy and char to return to the soil. Another example is how a commercial power plant in Denmark uses straw to make biochar. When scaled up to full production, it will produce 10,000 tons of biochar a year, which can be returned to agricultural land to enhance soil fertility. About half of the carbon originally captured in the straw through photosynthesis is retained in the biochar. So the more energy it produces, the more carbon is scrubbed from the sky. Currently, this is the only energy-producing system with a negative carbon footprint.
However, there are trade-offs to consider. As we’ve seen, retaining organic matter from cover crops is essential for mulch and no-till practices. Continual removal of all crop residues to produce biofuels or biochar would lead back to soil degradation. Likewise, it makes no sense to cut down primary forest to produce biochar. But real opportunities do exist for repurposing rural and urban organic wastes to produce energy, sequester carbon, and build soil fertility. The greatest potential for biochar to increase crop yields is on highly weathered, nutrient-depleted soils in tropical regions.
Terra preta, like what is found around Amazonian villages, offers a model for how to build fertile soils wherever there is the potential to combine organic matter and biochar. It makes far more sense to turn organic waste from households, villages, and cities into biochar than to dump it in landfills. Adapting the principles that favored formation of terra preta to modern farming practices is another way to help make soil building a consequence instead of a casualty of agricultural production.
Still, even well-housed microbes need to be fed. And while biochar can help restore fertility to soils, it won’t offset the need to grow cover crops of green manure on farms. And this led me back to the heart of my own country. For Ohio, it turns out, is one of the best places to see the transformative potential of cover crops for American agriculture and how carbon farming could have a big impact on both agricultural production and climate change.
11
FARMING CARBON
I bequeath myself to the dirt to grow from the grass I love,
If you want me again look for me under your boot-soles.
—Walt Whitman
Seven years after I first met Rattan Lal in Washington D.C., I found myself sitting in his office at Ohio State University. I’d come to see his long-running experiment on how much carbon no-till farming could put back in the ground. At least that’s what I aimed to find out.
We jumped right in, talking about long-term food security and how legacies of past soil degradation have set the stage for regional strife and humanitarian disasters. Looking forward, we’ll need all billion and a half hectares of Earth’s cropland to be as fertile as possible if we are to reliably feed several billion more of us later this century.
Still spry at seventy, Lal became animated when talking about the power of mulch. On his desktop monitor he pulled up photos of a pair of Ohio cornfields during the 2012 drought—one field mulched, the other not. To provide scale, a six-foot-four student stood amid the corn in both photos. The plants in the unmulched field came up to his belt; in the mulched field, much greener corn reached up to his eyes. Lal’s smile said it all: Behold the power of mulch. Examples like this have convinced Lal that conservation agriculture can raise African crop yields and sustain intensive agriculture around the world.
As the conversation shifted to the results of Lal’s early plot experiments in Africa, it became clear why he is known for his depth and breadth of knowledge about conservation agriculture. Not only did he do some of the foundational work in the field, but beside me stood a large, six-level bookcase stuffed with books on the subject. I didn’t notice at first, but when he got up and pulled a tome off the shelf to show me a graph he was describing, I realized they all shared a common author—Rattan Lal.
He’d clearly been keeping busy since he’d returned to Ohio State in 1987. But his research focus shifted after he began finding it difficult to obtain support for basic research on soil erosion and practices that did not involve marketable products. Since then, he’s kept his experimental plots going in a decades-long study of the potential for farming practices to build up soil carbon and thereby reduce the amount of carbon dioxide in the atmosphere. Along the way, he’s found that the recipe for tempering the problem of erosion and enhancing drought resilience—don’t disturb the soil, mulch crops, and grow variety—also offers a potent combination for tackling climate change.
And whether researching solutions for agriculture or climate change, he’s disappointed when influential scientists, respected friends and colleagues, endorse gambling on yet-to-be developed technologies instead of encouraging wider adoption of practices already shown to work.
Part of the problem is that policymakers and scientists alike gravitate to silver-bullet fixes and high-tech solutions, like pumping CO2 deep belowground in the case of climate change or splicing new genes into plants in the case of agriculture. To illustrate this point, he launched into a story about how the Department of Energy shot down funding for soil sequestration work in a big project on which Lal was one of the principal investigators. The program started out with around $100 million for studying carbon sequestration in soil and studying deep borings drilled into bedrock. But the agency folks wanted a big project they could point to—50,000 tons of carbon in one hole makes a bigger impression than half a ton per hectare spread across 100,000 hectares of farmland. So Lal’s hand in the project got dealt out of the deck, and all the funding ended up going to investigate pumping CO2 down deep wells at coal-fired power plants. Meanwhile, Lal’s low-tech approach to distributing on-farm carbon storage languished.
As much as Lal likes to talk about the potential to sequester carbon in soil, he’d rather show me. So, I fell in line when he rose and donned a wide-brimmed floppy hat emblazoned with the logo of the International Union of Soil Sciences. It was a fitting piece of headwear, given that he was their newly elected president. I followed him out to the parking lot, trailed by two of his staff, Jose Guzman, a postdoc from Kansas, and Basant Rimal, former undersecretary of forestry in Nepal.
We got into a silver Ford Fusion and left the university setting for what looked like a farm on the far edge of campus. When the car almost got stuck on the bumpy dirt road between rows of various crops, Lal joked that maybe we should have taken the truck. Soon we pulled up to his no-till research plots. They looked like any number of cornfields I’d seen over the past few months—except that in this one some rows were a lot taller than others.
The OSU no-till plots were part of an ongoing experiment, run with three distinct treatments since 1990. From the start, each row consistently received the same amount of nitrogen, though the source differed. Some rows received chemical fertilizers, others compost, and yet others cow manure. So differences in plant growth and health were not due to the amount of nitrogen added, but to how it was added.
Even a casual look revealed that crops were not equal after twenty-five years of cultivation under these treatments. The corn in the manured rows was a third taller and darker green than the conventionally fertilized rows. The composted plots were in between in both color and size.
The differences belowground were even more pronounced. Guzman and Rimal used a soil auger to drill up a core from the chemical fertilizer row and the manured row. The soil in the conventional control plot was light brown, with a dense, clayey feel. The loose-feeling soil in the manured row had a cap of plant litter and was blackish with organic matter. Looking at these cores side-by-side, I didn’t need fancy statistical tests to tell that adding compost or manure improves soil structure and increases soil carbon.
Yet never is carbon laid out as an integral part of fertilization plans. Usually farmers and researchers focus on nitrogen, phosphorus, potassium, and maybe calcium, sulphur, or zinc. That’s because plants don’t directly take up soil carbon. But it feeds their microbiome, the menagerie of microbial alchemists in the rhizosphere that, by the way, have partnered with plants in this manner since vegetation first colonized land. And a root microbiome well supplied with carbon from decaying organic matter and sugary exudates from plant roots is the key to ensuring that plants can acquire and take up an adequate supply of micronutrients and beneficial microbial metabolites (like those you read about in Chapter 3).
The next thing Lal wanted to show me was another experiment, this time comparing conventional fertilizer and compost. He’d started back in 1994 when he stripped all eight inches of topsoil off two side-by-side plots. He then began no-till cultivating the subsoil of both plots. He used chemical fertilizer on one plot and compost to deliver an identical amount of nitrogen to the other plot.
Guzman and Rimal waded into the cornstalks, pushed soil augers into the ground, and pulled up a sample from each plot. The chemically fertilized plot had an inch of light brown topsoil. The plot mulched with compost had six inches of dark brown topsoil above the khaki subsoil. New soil had formed six times as fast on the compost plot, at the rate of about a third of an inch a year. If you’re not a geologist, that probably sounds slow. But consider how, at that pace, it would take less than a century to make a couple feet of topsoil. Other long-term studies from around the world have also found that manure, cover crops, and diverse cropping systems increase soil organic matter. Here was a recipe for how to restore topsoil and reverse historical soil degradation—surprisingly fast.
Like others I spoke with, Lal cautioned that, when implementing this approach, it takes two to three years to make the transition from conventional practices to productive low-input, no-till farming. It takes time to start building up soil organic matter, and this can present an intractable problem for highly capitalized farmers paying off big loans. Several years of low production can mean losing the farm.
CARBON SINK
Lal’s plots were also demonstrating the largely untapped and underappreciated role of soil as a reservoir to take up and hold carbon from the atmosphere.
The world’s soils already hold at least twice the carbon as the atmosphere. Estimated to a ten-foot depth, soils contain more carbon than the combined amount in the atmosphere and all of the plant and animal life on Earth. Most soil carbon is held in the top several feet, due to surficial inputs of organic matter and the carbon-rich exudates that shallow roots push out into the soil. This means that changes in the organic-matter content of topsoil can significantly impact the amount of carbon in the atmosphere, and therefore global climate.
Every time we plow, it exposes soil to air, which speeds up decomposition of organic matter, releasing carbon skyward. Not just a little, but a lot. Since the start of mechanized agriculture, North America’s tilled fields have lost more than 40 percent of their original soil organic matter. Before 1950, plowing on U.S. farms contributed more to our national carbon emissions than all other sources combined. By the close of the twentieth century, a quarter to a third of all the carbon added to the atmosphere since the Industrial Revolution came from plowing. On the upside (I guess), a prominent climate scientist suggested that agriculturally driven loss of soil organic matter may have delayed the return of another Ice Age.
Today, most croplands have been under conventional practices for long enough to have reduced soil organic matter by more than half. In a 1999 study, Rattan Lal estimated that the world’s agricultural soils had already lost 66 to 90 billion tons of carbon, mostly due to tillage and the resultant erosion. He estimates that, since the dawn of agriculture, most cultivated soils have lost between a third and two-thirds of their original soil carbon. It would take fundamental changes in agricultural practices to restore soil carbon to near-historical levels.
But it could be done. Anthropogenic changes in soil organic matter are a two-way street—after all, people made terra preta. A 2005 assessment of the global consequences of land use in the journal Science reported that chemical fertilization and conventional cereal production decrease soil carbon by 0.5 to 1 percent a year, whereas the use of cover crops and farmyard manure increase soil carbon by 0.2 to 0.4 percent a year. Estimates of how much carbon conservation agriculture can sequester in different environments range from 0.2 to 1.0 tons per hectare per year. This really adds up when you consider applying it to all the world’s agricultural land. In 1998, Lal and a team of colleagues conservatively estimated that adoption of conservation agriculture on U.S. cropland could sequester enough carbon to offset emissions from half of the cars in America.
In addition to building up soil carbon, no-till farming and conservation agriculture also have lower fossil-fuel emissions than conventional practices. A 2003 comparison of average fuel consumption found no-till farming used a third less energy than plowing, due to fewer passes of equipment over the fields. And adopting cover-cropping along with no-till further reduces energy-intensive fertilizer use.
There are additional benefits to increasing soil organic matter, through its effects on a host of physical, chemical, and biological properties that influence soil quality and health. Farming practices that build up soil carbon boost microbial biomass and nutrient cycling, and improve soil structure, texture, and aggregate formation—all of which enhance soil fertility. The net result is that increasing soil organic matter increases crop yields.
It also increases drought resilience by improving soil moisture retention and water-holding capacity, both of which will become increasingly important to modern agriculture in a changing climate. Reducing the organic-matter content of soil from 4 percent to 1 percent, as happened on many farms over the twentieth century, decreases the soil’s water-holding capacity by about half. So the historical loss of soil organic matter on farm fields undermined the ability of crops to weather droughts. Reversing this loss would double the available water-holding capacity of the soil, vastly improving drought tolerance. Many of the farmers I met while researching this book described how yields from no-till fields consistently beat conventional yields in dry years.
Refilling the largest terrestrial reservoir of carbon—soil—offers a way to both offset a portion of global CO2 emissions and reduce the effects of climate change on crop yields. But by how much? Estimates of the amount of carbon that can be added to agricultural soils vary widely, and a global meta-analysis found that no-till practices consistently increased soil carbon stocks only where used in conjunction with practices that increased biomass production and multiple crops per year. The effect of no-till on soil carbon sequestration is highly dependent on the cropping system it is used in conjunction with—residue retention, cover-cropping, and crop rotation. Adoption of no-till alone may do little to increase soil carbon.
Rattan Lal conservatively estimates that worldwide adoption of all three elements of conservation agriculture could put enough carbon back into soils to offset 5 to 15 percent of global fossil-fuel emissions. He says his Brazilian colleagues tell him his numbers are far too low. Other estimates of the overall potential for conservation agricultural practices to reduce global carbon emissions range from 1.2 to 3.3 billion metric tons per year—up to a third of total emissions. And while that won’t solve our global carbon problem, even the low end of those estimates offers a good start on the road to doing so.
A more liberal 2014 estimate from the Rodale Institute suggests that the carbon sequestration potential of soils is even greater. By extrapolating reported rates from studies of regenerative organic agriculture to all global cropland, Rodale researchers estimated that carbon sequestration could offset between a quarter and about half of global greenhouse gas emissions, based on those generated in 2012. Their report further claimed that applying the potential maximum annual sequestration through regenerative practices on all the world’s grazing lands could offset 71 percent of global emissions. When I asked Kristine Nichols about the difference between her and Lal’s estimates, she replied that he assumed that soil organic-matter levels cannot rise above levels found in native ecosystems. At Rodale, soil organic matter went from less than 2 percent to over 5 percent in thirty years. They are now about back to their native soil baseline. By Lal’s accounting, they’d be done. But Nichols thinks that they can push soil carbon higher with cover crops and manure—a lot higher.
Although estimates vary, agriculture is directly responsible for somewhere around 15 percent of global greenhouse gas emissions. So drastically cutting agricultural fertilizer and diesel use by adopting conservation agriculture practices would cut the agricultural sector’s contribution to fossil fuel emissions by 5 to perhaps 10 percent. Add that to Lal’s conservative estimate of 5 to 15 percent sequestration potential from the same practices and we can crudely estimate a potential to reduce or offset 10 to 20 percent of global carbon emissions. In addition, estimates of the additional carbon sequestration potential of biochar range from about 10 percent to well over half of global emissions. Again, even at these low-end estimates, there is substantial potential for soil-building practices to sequester enough carbon to make a real difference—if we can act over large acreages.
