Will cover crops feed the world?

A week before Thanksgiving, Tom Philpott wrote a blog post for Mother Jones about organic agricultural research, saying Yet Again, Organic Ag Proves Just as Productive as Chemical Ag. He was discussing a pamphlet (PDF) from Iowa State University’s Long-Term Agroecological Research (LTAR) Experiment, which compared yields and profitability of a “conventional” corn-soy cropping scheme with three different organic cropping schemes that rotated in oats, alfalfa, and/or wheat and red clover. What is otherwise promising research into crop rotations and management, however, was proof in Tom Philpott’s mind that Norman Borlaug, in particular, didn’t know what he was talking about when he opined on the limits of organic agriculture.

The ghost of Norman Borlaug, haunting foodies since 2009. Artwork in CIMMYT Seed Bank

I responded that contrary to such lofty conclusions, a combination of missing details, shortened quotes, and silver-bullet single-solution thinking was at play. The ensuing discussion was heard around the food blogosphere with Michael Pollan tweeting for people not to miss reading our exchange, and Mark Bittman advertising it as well. I would like to continue and expand the discussion here, and bring up some things that have been glossed over and forgotten in this discussion.

How much Nitrogen?

The main thrust of our disagreement was over the issue of the source of nitrogen for growing crops that are going to feed the world. Tom quoted Norman Borlaug as saying that organic would not be able to feed the world, and tried to address it with the ISU brochure. But as I pointed out, Tom cut off the quote, avoiding a key phrase that indicates he is talking about nitrogen production. Here is the full quote:

That’s ridiculous. This shouldn’t even be a debate. Even if you could use all the organic material that you have–the animal manures, the human waste, the plant residues–and get them back on the soil, you couldn’t feed more than 4 billion people. In addition, if all agriculture were organic, you would have to increase cropland area dramatically, spreading out into marginal areas and cutting down millions of acres of forests. At the present time, approximately 80 million tons of nitrogen nutrients are utilized each year. If you tried to produce this nitrogen organically, you would require an additional 5 or 6 billion head of cattle to supply the manure. How much wild land would you have to sacrifice just to produce the forage for these cows? There’s a lot of nonsense going on here.

This key phrase underscores the perennial problem of switching from fertilizers to an organic-only approach. The first question is where you are going to get the nitrogen that plants need to grow? It takes a lot of energy to pull nitrogen out of the air and break its triple-bonds to turn it into a form that plants can use. This is a major energy cost for conventional farming, but it also secures its higher yield. The only way that organic agriculture can get nitrogen is by harvesting it from other living things in one way or another. Nitrogen can be “fixed” from the atmosphere by legumes, which can be grown as a “cover crop” that is planted after the fall harvest, or in the spring to cover the land in an off-year and gather nitrogen that will be plowed into the soil. You can also plant a “catch” cover crop with a grain such as barley or oats, intended to capture excess nitrogen during the winter, which can be plowed into the soil in he spring. Or, you can gather nitrogen in the form of animal manure – which comes from previously-grown crops, and thus, previous sources of nitrogen. You could also go for fish slurry – and harvest your nitrogen from the ocean, or weirder still, argue over naturally-occurring deposits of Chilean nitrate (PDF) and their status in organic agriculture. In any case, the nitrogen has to come from somewhere. Ironically it would seem, nitrogen from human waste is not allowed. The ISU research that Tom was enthusiastic about was a little fuzzy on where the nitrogen was coming from:

The organic plots receive local compost made from a mixture of corn stover and manure.

Nitrogen Cycle, from landscapeforlife.org

Where did this manure come from? How many acres of land were required to produce this manure, and where did the nitrogen come from to produce it? These are questions that are not detailed, and it shows one layer to the complexity of long-term sustainability. Tom responded to defend organic agriculture with a paper that estimated that with cover crops alone (PDF), the world could produce enough nitrogen to replace all synthetic fertilizers. The Badgley et al. paper had many assumptions, but also some good information. Their basic approach was to estimate how much available nitrogen can be produced on all the non-forage croplands in the world. Essentially, how much can we gain by planting legume cover crops? But this is where the incompleteness of the paper began to unravel.

The paper assumed that none of the croplands currently in production were being planted with cover crops already. So the acreage of non-cover-cropped lands was overestimated. Next, it also assumed that legume cover crops would actually grow on all of these acres. Statistics about current practices are very hard to find, and the one that I could find (PDF), for New York vegetable growers (not grain), said that 50% of their acres had cover crops, and 20% of those were legumes fixing nitrogen. As I have learned, besides the timing of planting and the weather, certain cover crops can make pest problems worse, and if you follow a legume crop with a legume cover crop, you can have issues with rotting. Before you can estimate whether cover crops can provide enough nitrogen to replace fertilizers, you first have to estimate what can be practically achieved in actual cropping systems. Even the Rodale research did not plant legume cover crops every year.

I then had a thought. If you are going to plant a legume cover crop (as with any cover crop), you are going to need seeds. Those seeds have to come from somewhere, and will take up a certain amount of acreage to produce. Out of curiosity, I thought I would calculate how many acres of farmland would be required to grow the seeds necessary to cover the world’s croplands in hairy vetch, a common and highly regarded legume cover crop. The results were stark.

The Badgley paper estimated the total available croplands as 1362 M hectares (Table 4), and if all were planted with legume cover crops, it would produce 140 Million Megagrams of Nitrogen (or 140 Teragrams). The paper reports that the world uses 82 M Mg of Nitrogen (82 Tg), which means that according to these numbers, to exactly replace the amount of nitrogen being used by farms today, you would need 1362 * 82 / 140 = 798 M hectares of legume cover crops – so about 800 million hectares. How much seed would you need to plant that?

Hairy Vetch. Photo by neckonomania

The recommended seeding rates for hairy vetch are 30 pounds per acre. The only source I was able to find about seed production of hairy vetch reported that you can only get 200-540 pounds per acre of seed (PDF). This means that for every acre of cover crop, you would need 1/6 to 1/18 of an acre to produce the seed you would need. (You also need to produce the seed for the seed crop – making it slightly higher). Without knowing the true average for seed production, I just averaged the high and low-end of the range to arrive at 1/12 of an acre of seed fields to produce enough hairy vetch for one acre of cover crop. To plant 800 million hectares of hairy vetch cover crops, we need about 67 million hectares (or 164 M acres) of hairy vetch seed production to supply it. For seeds to plant the seed fields, add another 6 million hectares to give you 73 million hectares of land.

For perspective, I looked up the total cropland of my awesomely-productive home state of California, which according to the USDA, has 4 million hectares under cultivation. This means that we would need almost 20 California’s of cropland to grow enough hairy vetch seed to plant these 800 million acres, and if you converted all Californian farmland into seed production (goodbye meat, dairy, etc) you still only have 10 M hectares, and you would need the farmland of 7 Californias.

Where are we going to find this extra land? Or should we decrease the total cropland area in the world by five and a half percent? (73 / 1362 = 5.4%) This is the opposite of feeding the world, and it presents a real challenge for cover crops. But not the last challenge, either.

Another detail worth noting is that the yields of these organic plots can have higher total nitrogen applied when compared to conventional plots. In this paper (PDF) on nitrogen rates and leaching, also from Rodale, almost twice as much nitrogen was applied every year in the organic plots relative to conventional, in order to maintain their yields (Table 4). This translates, as admitted in the paper, into greater rates of nitrogen leaching into the surrounding environment. Nitrogen in the soil is a very mobile nutrient – it washes out easily. Nitrogen runoff from farmlands contributes to water pollution, leading to things such as the Dead Zone in the Gulf of Mexico. It turns out that according to more Rodale research (PDF), not only do organic farms leach just as much nitrogen as conventional farms, but farms with legume cover crops leach even more. 20% of the applied nitrogen leaches out of organic manure and conventional systems, while 32% of the nitrogen applied to legume cover-crop systems leaches out. There is a lot of research on nitrogen leaching and cover crops, including some that don’t sound so bad for leaching, but there is a shortage of good long-term leaching studies. There is also evidence that the cover crop can harm the yield of the following crop. Not only does the amount of nitrogen applied to maintain yields call into question the sustainability of these sources of nitrogen, but also the environmental sustainability of the downstream effects of legume cover crops as a silver-bullet solution to the world’s nitrogen needs.

So even post-mortem, Norm still beats Tom in an argument. Cover crops in an organic system have a long way to go to get to “feeding the world.” This is not to say there isn’t potential in cover crops – because there is. But one thing we must not slip into is silver-bullet thinking – nor excluding a tool from a toolbox because someone calls it a silver bullet.

The Role of Genetics

Wheat under CA Modern genetics includes a whole range of tools that we have in our toolbox, all of which are going to be essential in the decades to come. Not only do you have your basic breeding, gene banks for diversity, and genome sequences to help you find important genes, but modern technologies such as marker-assisted selection and genetic engineering are playing an increasing role in crop improvement. One of the ways you can help a plant gather more nutrients from the soil so they don’t run off is to strengthen its root system and its ability to uptake nutrients. In the last few decades, fertilizer use has stayed about the same, while crops have been yielding more, which means that they have been bred to be more nitrogen-efficient. With nitrogen efficiency as a goal, you can increase the yield of a crop without requiring more nitrogen to be applied, or perhaps maintain the same yield while applying less nitrogen. For you breeders out there, this can mean testing out your new hybrid contenders in nitrogen-limiting environments to see just how much yield you can squeeze out of a drop of N.

In the genetic engineering arena, there is a nitrogen use efficiency trait developed by Arcadia Biosciences, which I understand they have licensed to several seed companies and for a variety of crops, and even a nonprofit technology transfer organization for Africa. Transgenic rootworm resistance has been linked to nitrogen use efficiency (because it protects the roots so they can take in nutrients), however a field trial going on at UW-Madison has not been able to observe a consistent benefit from it – sometimes it requires less nitrogen, but not always (PDF summary). Still, one can write an entire book chapter on the potential for genetic engineering to contribute to nitrogen use efficiency.

There is another way that genetics can play a role in the nitrogen needs of the planet, one that might not come to mind right away: breeding a better cover crop. Currently, cover crops are evaluated on a species-basis. Red clover or hairy vetch? Why not take a survey of red clover and hairy vetch germplasm, looking for those that fix nitrogen at high rates, have good winter survival, and decay at a reasonable rate to provide fertilizer for crops the following year, and then combine those traits? (And while you’re at it, you could try to do something about hairy vetch’s horrendous seed yield. Non-shattering trait, anyone?) This kind of research potential is not just limited to legume cover crops – as grains are often used to capture nitrogen from the growing season to mix back into the field the following year as mulch. Why not breed or engineer a cover crop grain plant that is really good at scavenging nitrogen in the soil?

The future of sustainable agriculture is going to look a lot more like organic than most of what we have today, however, there are ideological barriers within that approach that are limiting its ability to not only expand but to use new technologies that can actually help it reach its goals. Imagine a nitrogen-efficient high-yielding corn crop that follows a legume cover crop that fixes nitrogen at an accelerated rate, followed by a winter wheat that grabs the excess before it can leak into the Mississippi. If we were to actually have this system, as organic and sustainable as it sounds, ironically it would not likely be eligible for certification.

Many Pieces to the Puzzle

Kenneth SayreThis summer I visited CIMMYT in Mexico, and one of the most dynamic presentations was given by Kenneth Sayre out in the field, amongst research and demonstration plots of Conservation Agriculture (CA). This approach combines rotations and cover crops with reduced tillage to reduce erosion, increase soil carbon and nitrogen, and reduce water stress and weeds. Besides discussing the benefits of these approaches, it was also pointed out that CA does not suffer from limitations against judicious use of fertilizer, or even genetically engineered crops. Are there perhaps some limitations to this approach, and ways to improve it that have not yet been thought of? Yes, as with everything else. While usually the CA plots do better than the non-CA plots, this year at the station the reverse was true.

We need better crops, improved soils, more efficient water and land use, more rotations, better nutrient recycling, precision farming, and improved social and political structures to make it all work. Too often, questions in agriculture are popularly addressed with narrow, single solutions, with lip service to diverse approaches. “Organic is the solution” vs “genetic engineering is the solution.” Honestly, I hear more of the former than I do the latter, but they are both misguided. It is interesting that while Tom and I were debating the merits of nitrogen issues in organic agriculture, he framed it as him versus a “GMO enthusiast,” and Mark Bittman framed it as “organic vs conventional.” These misleading frames of reference are part of the problem because they keep discussion adversarial and exclude the practical middle-ground. To paraphrase Jon Stewart: Stop. You’re hurting us.

There are many pieces to the puzzle and when it gets set up as one worldview versus another we all lose – because all current worldviews are wrong. Whether you are talking about the nitrogen needs of the world or water, energy efficiency, pests and disease, there is a lot more that we don’t know than there are things we know. Starting with the answer and trying to support it is going to inevitably lead to failure, and so the best approach, as it  always seems to be, is to have an end goal in mind and let the pragmatic application of scientific research figure out how to get us there, using multiple interlocking and interacting approaches. Do you want to feed the world sustainably, securely, and healthily for generations to come? Let’s figure out how to get there.

Follow Karl Haro von Mogel:
Karl earned his Ph.D. in Plant Breeding and Plant Genetics at UW-Madison, with a minor in Life Science Communication. His dissertation was on both the genetics of sweet corn and plant genetics outreach. He recently moved back to his home state of California. His favorite produce might just be squash.