Feb 19 2016

The Nitrogen Problem

Farming-2The farming systems we are putting in place now will need to feed the 9-10 billion people that will inhabit our planet in 2050. This is a huge challenge.

Many people speak about “sustainable farming,” which is a legitimate and important concept. Truly sustainable means that we need to track all of the inputs and outputs in the global food system and see that we can extrapolate that system indefinitely into the future.

One of the most, if not the most, important factor in sustainability is nitrogen. Plants need a lot of nitrogen to grow, and this is often the limiting factor in large-scale food production.

Thinking about where nitrogen ultimately comes from – the entire nitrogen cycle – is like thinking about where energy ultimately comes. It’s a very useful question to ask. For example, when people claim that they can run their car on water they are failing to ask this basic question. When you do you realize that the energy is not coming from burning the hydrogen and oxygen, but from whatever energy source you used the split the water into hydrogen and oxygen.

The same thought process applied to nitrogen is also illuminating.

Chemical Fertilizer vs Manure

About half of the nitrogen that ends up in our food comes from manure, with the other half coming from chemical fertilizers. Ultimately the nitrogen in manure or fertilizer comes from the atmosphere, which is 78% nitrogen. A small amount of nitrogen in the soil comes from other biological sources, such as nitrogen fixing crops and bacteria.

Plants will deplete soil of nitrogen, which has to be replaced in order for production to be sustainable. Let’s consider the two main sources of this nitrogen – manure and chemical fertilizer.

The green revolution was largely due to developing techniques for mass production of nitrogen fertilizer. The advantages of chemical fertilizer are that it can be mass produced, that production has a relatively small footprint, and the fertilizer is easy to store and apply and is cost effective.

There are, however, two main downsides to chemical fertilizer. The first is that it is often produced with petroleum, which serves as the source of hydrogen that binds with the nitrogen to make the fertilizer. Without getting into the “peak oil” controversy, I think it’s safe to say that using a fossil fuel as a source of fertilizer is not indefinitely sustainable.

The second big problem with chemical fertilizer is that it is very soluble (which is actually good for plant absorption), which means any nitrogen not absorbed by the plants will easily run off into ground water. This then gets into streams and rivers, and then into lakes and oceans. Once there it will cause a bloom of algae that feeds on nitrogen, dropping oxygen levels in the water and creating a dead zone.

Manure as a source of nitrogen has the advantage that it is essentially recycled nitrogen. Animals eat grass or crops which are the source of nitrogen, which then ends up in the manure, and is then put back in the fields to grow more crops. Of course, some of the nitrogen from crops also goes to humans (that’s kind of the point), it does not all go to feeding manure-producing animals. That nitrogen needs to be replaced.

Some of that nitrogen comes from grazing (essentially transferring nutrients from grazing fields to crop fields), but then we have to consider the sustainability of grass fields. In many parts of the world fields for grazing are losing nitrogen and phosphorous, and are not sustainable without further inputs.

There is also only so much manure to go around. If we want to maintain current production with manure only, we would need to essentially double the manure supply in the world, which is simply not possible.

Also we have to consider the overall footprint of manure farming. If we compare productivity per amount of land, we can’t just consider the land used directly for growing crops. We also have to consider the land used to feed the animals that were the source of the manure.

Output per unit of land is 20% less for manure-based farming than chemical fertilizer-based farming. That is not even considering the land necessary to produce the manure, however. Nitrogen-fixing cover crops or crop rotation may help, but then you also have to consider that land as well.

Using nitrogen-fixing plants also requires that those plants are tilled back into the soil, which releases CO2 into the atmosphere. Manure also releases nitrogen into the atmosphere through volatilization of the ammonia. It contributes to environmental nitrogen as well, although not nearly as much as chemical fertilizer.

The Solution

As is often the case with any complex problem, there is rarely a simple answer. If we want to optimize sustainable farming we need to consider the entire system, all inputs and outputs, all the land use, and all of the environmental impacts.

Most of the sources I have read acknowledge that we will need chemical fertilizer in order to maintain food production. Some pro-organic sites claim that we can feed the world with manure-fertilized organic farming, but the numbers just don’t add up. Where will that manure come from? Right now much of the manure used for organic farming comes from animals fed with grain from chemically fertilized crops – so the ultimate source of that nitrogen is also chemical fertilizer.

Keep in mind manure is not really a source of nitrogen but is rather a way of recycling some of the nitrogen back to the farmland or transferring nitrogen from land used for grazing to the farm.

Making optimal use of manure is important, but I find most compelling the analyses which argue that we cannot feed the world with nitrogen from manure alone.

We can also use nitrogen-fixing crops to take some nitrogen from the air and put it back into the soil. The main problem with this approach is that it increases land use. Some studies, however, have shown that using a winter cover crop can put some nitrogen back in the soil and reduce the need for chemical fertilizer without reducing yields.

Many sites agree that we need to optimize our use of chemical fertilizer. Farmers sometimes over-apply fertilizer, because they want to maximize yield. The cost of under-fertilizing is lost yield and therefore lost money. The cost of over-fertilizing is the extra cost of the fertilizer and the increased runoff into the environment. Apparently many farmers worry more about the former loss than the latter, and so tend to over-fertilize.

Timing is also important. If fertilizer is applied when plants are soaking up nitrogen quickly there will be less left over to get into the environment. There are also other timing issues, like avoiding applying fertilizer before a big rain.

It seems that a combined strategy using as much recycled nitrogen as possible, nitrogen-fixing crops, and optimally applied chemical fertilizer, can maximize yield per unit of land while minimizing environmental impact. This can be extra work for farmers, however, so they need an incentive to do this.

As the human population grows, however, it will likely become necessary to add new options. One intriguing possibility is genetically engineered crops that are able to fix their own nitrogen. Imagine a wheat variety that can fix nitrogen from the air – no need for nitrogen fertilizer.

Legumes are plants that can do this now. Actually it is bacteria that live on their roots that fix the nitrogen from the air. There is ongoing research to engineer cereals that are able to develop the same symbiotic relationship with these bacteria. This is a complex task, however, and estimates are that we are at least 20 years away from this goal.

Still, this is the kind of technology we need to be working on so that we do have more options in 2050 when there will be more than 9 billion human mouths to feed.


A thousand years ago we were essentially mining our world for nutrients, and the reservoir was so vast that we did not have to worry about the entire system. That changes with the industrial revolution and the explosion of the human population. This was followed by the green revolution made possible by synthetic fertilizer, which caused further population increase.

As we start to push up against the limits of the natural resources of our planet, we have to think in terms of entire systems – what are the inputs and outputs. Farming has lots of inputs and outputs, but one of the biggest is nitrogen.

When we consider the entire nitrogen system we can figure out strategies for optimizing the flow of nitrogen to maximize food production with the smallest environmental impact.

One of those environmental impacts is total land use. Every acre of wild field or forest we convert to farmland has a huge negative impact on the environment – much more than the differences in how we farm that land. Without synthetic fertilizer as a source of nitrogen the need for land use would increase dramatically.

But synthetic fertilizer is causing significant runoff which is having a negative impact also. What we need is smart land and nitrogen use that combines the best of all available approaches.

This is partly why I think the false dichotomy of organic vs conventional is harmful. An integrated evidence-based approach to best practices, without an appeal to an arbitrary ideology, is what we need.

21 responses so far

21 Responses to “The Nitrogen Problem”

  1. Karl Withakayon 19 Feb 2016 at 3:08 pm

    “Without getting into the “peak oil” controversy, I think it’s safe to say that using a fossil fuel as a source of fertilizer is not indefinitely sustainable.”

    If the green energy revolution (wind, solar, tidal, geothermal, etc + viable storage systems like better batteries, fuel cells, synthetic liquid fuels, etc) reaches its full potential soon enough, we could reach a point where the principle value of any remaining crude oil reserves is as chemical feed stock for petrochemicals. While it still wouldn’t be indefinitely sustainable, if we have enough green energy soon enough, those oil reserves might last us quite. /end delusional optimism

  2. Karl Withakayon 19 Feb 2016 at 3:10 pm

    Typo correction to last sentence: “While it still wouldn’t be indefinitely sustainable, if we have enough green energy soon enough, those oil reserves might last us quite -a while-.”

  3. daedalus2uon 19 Feb 2016 at 3:20 pm

    Synthetic ammonia can be made using hydrogen from electrolysis. That has been done at hydroelectric facilities to utilize their “excess” electricity. If we had suitable levels of solar and wind electricity to meet peak demand, there would be abundant off-peak electricity to fix nitrogen.

    It is somewhat easier to apply synthetic fertilizers, and their application can be better matched to plant needs (which change over the growing system). Organic fertilizers are more difficult to apply and require soil bacteria to break them down into the ammonia and nitrate that plants can absorb. This is why nitrogen runoff from conventional farms tends to be somewhat lower than from organic farms.

    Ammonia in soil is a cation (ammonium) and is bound by cationic ion exchange sites in the soil (on clays and humic matter). When ammonia oxidizing bacteria oxidize that ammonia to nitrate, it forms an anion which is not held and so the nitrogen then leaches. Some farmers do use what are called nitrification inhibitors which inhibit ammonia oxidizing bacteria and prevent this leaching. These are synthetic compounds not available to “organic” farms.

    Biochar (charcoal made from plant residues) does have anionic ion exchange sites and can be used to reduce nitrogen leaching somewhat. Native Americans in the Amazon region made synthetic soils called Terra Preta (black earth) by carbonizing their food and agricultural waste. This would be a better option than composting because it sequesters some of the carbon long term (centuries) as refractory black carbon. There is no loss of phosphate and potassium in making biochar. There are fertilizer benefits to biochar, likely due to providing niches for beneficial bacteria and fungi and also providing aeration (plant roots can respire aerobically and use much less energy then if they must use anaerobic pathways).

  4. Khym Chanuron 19 Feb 2016 at 3:23 pm

    What about genetically engineering crops to host nitrogen fixing bacteria?

  5. carbonUniton 19 Feb 2016 at 3:23 pm

    Making optimal use of manure is important, but I find most compelling the analyses which argue that we cannot feed the world with nitrogen from manure alone.

    Sure we can! Steaks for everyone!!

    Seriously, does this recycling of nitrogen make meat production a little less bad? Or does a significant amount of nitrogen go with the meat?

  6. MikeBon 19 Feb 2016 at 4:42 pm

    “Truly sustainable means that we need to track all of the inputs and outputs in the global food system and see that we can extrapolate that system indefinitely into the future.”

    A million thanks for finally saying what “sustainable” actually is.

    And under the current system(s) (“organic,” “conventional,” “whatever”) it is unattainable. Nothing we do now is able to be extrapolated “indefinitely into the future.” Maybe one day it will be.

    I grow tired of hearing glib references to “sustainable agriculture,” as if it actually exists, when it is really a goal, not a process currently under way.

    As for the “peak oil controversy”: There is no controversy. Every single “peak oil” prediction has failed miserably. It turns out the people who thought they know something didn’t know a thing. This includes my own geology professor, a man I respect greatly and who chose not to join in the fray when peak oil was all the rage about ten years ago.

    This doesn’t mean oil is infinite: It just means the peak oil delusion is off the map now, and that nature and economics is much, much more complicated than we think.

  7. Willyon 19 Feb 2016 at 6:17 pm

    Quick correction: N2 is 80%-ish of the atmosphere, not 70%.

  8. Willyon 19 Feb 2016 at 9:22 pm

    Excellent article, Dr. Novella. The whole field of agriculture and eating has become overrun with cliches and foolishness. We face a very large increase in the population of the world in the next few decades and simple minded beliefs ain’t gonna hack it. In addition to nitrogen, phosphorus and land for agriculture are also difficult issues. According to a Nat Geo magazine series on agriculture and food (published in 2014), if we cannot find a way to better utilize our land and/or reduce food waste, the additional 2 billion people expected by 2050-ish will require additional land for livestock and crops equal to the area of the US. NG stated that we already use an area equivalent to both Africa and South America to produce our food.

    Common sense must prevail–and sooner rather than later.

  9. BillyJoe7on 19 Feb 2016 at 11:37 pm

    Actually it’s 78%

  10. BBBlueon 20 Feb 2016 at 1:09 am

    Many thanks for discussing this topic. Those who understand food production issues are far too few, and you are doing yeoman’s work in bringing issues like this to the attention of the skeptical community.

    In California’s Central Valley, we are faced with the challenge of maximizing nitrogen use efficiency in order to minimize the amount of nitrate found in groundwater, which is our primary source of drinking water. Nitrogen that percolates to groundwater does not find its way into streams and oceans, at least not to any significant degree in inland valleys. Nitrate pollution is a serious issue in our area, and so in addition to economic and environmental considerations, nitrogen use is constrained by water quality concerns.

    Increasing the concentration of soil organic matter, and thereby increasing cation exchange capacity is an important strategy in maximizing the use efficiency of applied nutrients and that process is agnostic in terms of nutrient source and has nothing to do with organic versus conventional; it’s all about best practices and maximizing the benefit of inputs while minimizing negative consequences.

    The Green Revolution did spawn an era when synthetic fertilizers and cheap fossil fuels encouraged a number of myopic production systems, but now the pendulum seems to have swung as many embrace an appeal to nature.

  11. Steven Novellaon 20 Feb 2016 at 8:57 am

    I corrected the nitrogen figure to 78%. Thanks.

  12. Willyon 20 Feb 2016 at 7:49 pm

    BJ7–Thanks for the more precise value for N2–I couldn’t remember if it was 78% or 79%, hence I said 80-ish.

  13. BillyJoe7on 20 Feb 2016 at 10:31 pm

    Willy (and RC in the other thread) – Google is your friend. 🙂

  14. BBBlueon 21 Feb 2016 at 1:46 pm

    Well if y’all want to be precise and stuff, you need to define terms and conditions. Percent by weight or volume? Moisture content of air? Approximately 78% by volume is correct for N and dry air, but air is seldom dry, so more often than not, actual N in atmosphere is less than the 78.09% by volume of dry air that is often cited.

  15. BillyJoe7on 21 Feb 2016 at 2:49 pm

    To be even more precise:
    Nitrogen constitutes 78.084% by volume of the atmosphere excluding water vapour, the percentage of which varies between roughly 0.001% and 5%.

  16. BBBlueon 21 Feb 2016 at 3:49 pm

    All very well and good, and now that we nailed that down, there is still something more appealing about the number 80 compared to 78, and especially compared to 78.084, right Steven?

  17. hardnoseon 21 Feb 2016 at 5:22 pm

    “That changes with the industrial revolution and the explosion of the human population. This was followed by the green revolution made possible by synthetic fertilizer, which caused further population increase.”

    “As we start to push up against the limits of the natural resources of our planet, we have to think in terms of ”

    Dropping contraceptives from helicopters.

  18. Willyon 21 Feb 2016 at 7:27 pm

    hn: I’m all for it. Why don’t you go make it happen?

  19. Bill Openthalton 22 Feb 2016 at 6:19 am

    hardnose —

    As we start to push up against the limits of the natural resources of our planet, we have to think in terms of …

    … Stop feeding the trolls :).

  20. a_haworthrobertson 22 Feb 2016 at 8:38 pm

    You may have seen/heard about this:

  21. Willyon 03 Mar 2016 at 5:03 pm

    The March Nat Geo issue has an interesting article on food waste.

    I am a Master Gardener (not as impressive as it sounds–it’s a way of blowing smoke up the skirts of unpaid volunteers) and, as such, have occasion to talk with Extension and University folks around the states of AZ and NM. An extension agent from Yuma, where a huge amount of the nation’s lettuce comes from in winter, says that a lettuce field after harvest often looks almost like it did before harvest due to the large amounts of lettuce left behind. Reasons include inadequate size, appearance, and bird doo. The growers actually employ folks to walk the field looking for droppings. They then mark the spot with a flag. Many distributors stipulate that their lettuce cannot be picked within 5 feet! of a flag. Some are more stringent yet. She also says the entire area, very dependent on agriculture, lives in fear of a contamination outbreak traced to Yuma produce, so the threat is taken very seriously.

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