Monday, March 18, 2013

No, Cows Don't Make Fertilizer

Fertilizer that comes from cows or other animals does not really originate with them.  Manure from cows and other animals has been used as a crop fertilizer for millennia, and it is still used today for about 5% of US crop acres and for a high proportion of organic acres.  It is often spoken of as an alternative to "outside inputs" for crops and as a superior option relative to "synthetic fertilizers."  However, just as Obama said about businesses and infrastructure, "you didn't build that," when it comes to fertilizers from animal sources we must also say, "they didn't make that."

Don't get me wrong, I think that cows are wonderful.  It is only because their complex, ruminant digestive system houses certain bacteria that we humans have access to the most abundant form of plant-stored solar energy - cellulose.  Animals also do a rather good job of absorbing the mineral nutrients like nitrogen, potassium and phosphorus in their feed, but they are not 100% efficient at utilizing their dietary input.  So, their manure still contains nutrients which can fertilize a crop.  But the animals didn't "make" any of those nutrients.  For instance the ~2% nitrogen in cow manure came from whatever they ate (grass, corn, soybeans...) and those crops, except for the soybeans, were mostly fertilized with "synthetic nitrogen." The cow is just passing a bit of that along.

Why Organic Needs Conventional

Organic farmers are limited to non-synthetic sources of nitrogen fertilizer.  They can supply some of that through biologically fixed nitrogen produced by legumes as main crops, cover crops or green manure crops.  Still, the reality is that many organic crops (fruit, vegetables, corn, wheat...) are fertilized with animal manures or composts thereof.  Organic growers are allowed to use manures/composts that come from conventional animal sources.  This is necessary because otherwise, organic farmers would be severely nitrogen limited.  Organic agriculture is actually quite dependent on manure from animals fed with non-organic crops.

Manure Isn't Such A Great Fertilizer Anyway

Manure is also a non-ideal fertilizer in many ways.  Cows and other animals are better at getting the nitrogen out of their feed than the phosphorus, so manures generally have too much phosphorus if they are applied at rates sufficient to supply a crop the nitrogen it needs.  That phosphorus can then become a ground or surface water pollution issue. The best way to utilize manures is to supplement the nitrogen with synthetic sources to better balance with the phosphorus.

Manure is also a source of bacteria that are human pathogens.  That is why it needs to be composted to be safer for use on human food crops.  But there are enough methane emissions during composting to be highly problematic from a greenhouse gas point of view.  When manures are incorporated into farmed soils they can also lead to more greenhouse gas emissions as methane.

Also, much of the nitrogen in manure is not in a form that the plant can use until it is "mineralized" into the ammonium or nitrate ions that plants can absorb (also what is in "synthetic" nitrogen fertilizers).  Mineralization is dependent on the activity of soil microbes and they don't necessarily release the nitrogen at the time of the growing season that the plant needs it the most.  In a recent meta-study of research on organic vs conventional crop yields, the authors observe that to come closer to conventional yields, it was necessary for the organic program to use more total nitrogen so that during the period of peak demand it was not as limiting (sorry, it is behind a pay wall, but the authors were willing to share a full copy when I wrote them).   Of course, in such cases the extra nitrogen would also be mineralized at times when the crop was not demanding it, leading to more surface or ground water pollution or to greenhouse gas emissions (nitrous oxide).


So, cows (or sheep or pigs or chickens) are not actual net contributors to our demand for fertilizers.  They just temporarily retain a bit of the supply generated for their feed.  We need sources of nitrogen to enable crops to harvest solar energy and turn it into a form that we can use.  We need the nitrogen to make the proteins that we need for growth and brain development.  We need to make good use of the nutrients that are left in animal manures, but they are only ever going to be a small contributor to our overall fertilizer needs.

You are welcome to comment here and/or to email me at

Cows image from Wikimedia


  1. A lot of these discussions about manure from basically anti-organic positions such as yours talk as if conventional farmers don’t use manure and organic farmers use nothing but, and as if there are no problems with synthetic fertilisers. Not convincing.

    It looks like the US regulations are laxer than the ones here in the UK about use of non-organic manure on organic holdings, though I don’t know how much sneaks through here under derogations. I guess it supports critics of organic ‘conventionalisation’ – the more the organic movement tries to play the game of large-scale industrial agriculture, the more it ends up looking like the farm system it set out to criticise.

    On traditional mixed farms, each type of livestock has an obvious role in accessing and cycling nutrients that would otherwise be difficult for the farmer to get at. An absurdity of modern agriculture is that we’ve done away with that and we’re using cheap energy to feed grains and soya to livestock in order to satisfy the meat cravings of the wealthy – to the likely detriment of human health, animal health, ecosystem health and global equity.

    But does organic agriculture depend on conventional? Or to put it another way, can we feed the world with organic agriculture alone? I don’t think anyone really knows, but I suspect the answer is probably no if by ‘agriculture’ we mean US or EU style capital-intensive arable farming, and probably yes if we mean labour-intensive horticulture. If we continue to roll out conventional capital-intensive arable farming (or indeed organic but ‘conventionalised’ capital-intensive arable farming), then someone is eventually going to have to come up with new sources of phosphorus and energy, a point that rarely seems to figure in these discussions. And meanwhile a lot of people will probably be hungrier.

    1. Well Chris, I had no idea that wolves, raptors, lions, tigers and fish were counted among the wealthy. How unprogressive of them.

    2. "...probably yes if we mean labour-intensive horticulture"

      So long as we are reducing the world to serfdom, I nominate you to be the one to work 14 hours a day on a 1000 calorie diet, and for you and your children to die young and emaciated.

    3. @Fred Z: if you have to work 14 hours a day to produce 1000 calories for you and your family you must be really bad at gardening. The key word in your response is 'serfdom', which is where people are coerced into producing food for others at the expense of their own wellbeing. Currently we produce about twice as many calories annually as are needed to feed the global population adequately, but a billion people still go hungry. What do you mean 'so long as we are reducing the world to serfdom'?

      @HMS Defiant: what a bizarre comment - but maybe wild predators that eat only grain-fed livestock could be considered 'wealthy'. How many of those do you suppose there are?

    4. @Chris S: Your comment about meat only for the wealthy struck me as some kind of rant opposing meat consumption due to the 'fact' that only the 'wealthy' could eat meat. Answer to your question is, how many need there be for them to have a place on the land too? Is one enough? Does the fact that it is the last meat eater consuming grain fed animals make it an endangered species worthy of extra protections and more slash and burn farming to sustain the herd? If some are still hungry surely we must grow more.

    5. Not sure I understand your point, but basically people have evolved to like meat because it's a nutrient dense food that's relatively hard for us to obtain in natural ecosystems. We’ve figured out how to produce it in vast quantities through agriculture, but in ways that are quite dysfunctional for human, animal and ecosystem health – and it's costly to produce, so it's largely a rich person’s food.

      “If some are still hungry surely we must grow more”. No, because the problem isn’t how much food we grow but who has the capacity to grow or buy it. That’s why so much of global agricultural land is devoted to grains to feed livestock to feed rich people, while other people go hungry. Growing more food will make no difference without political changes to the distribution of entitlement to food. Ed Carr explains the issues quite nicely on his blog:

  2. Chris,
    Fertilizers are always the biggest environmental challenge for any kind of farming because it is hard to time their availability to the crop's uptake pattern and so they are at risk for ending up where they are not wanted. Biological N fixation is obviously the best source, but not adequate for all our needs. I have recently learned about two groups that are working on small scale systems to make nitrogen fertilizer using something like wind power (make electricity to generate hydrogen to react with air). That would be a great solution to the energy/carbon footprint of its manufacture - but technically that wouldn't qualify for organic. Phosphorus is a big challenge too. We need efficient ways to recycle it from waste streams.

    I think it rather unlikely that we could return to the labour-intensive horticulture model you favor. I don't think most people would want to or be able to work that hard - particularly not as most societies get much older on average over the next few decades. I do suspect that those in the rich world could reduce their meat consumption to a health and environmental benefit. I was amazed to see what a large proportion of the wheat grown in Europe is used for animal feed.

    I'm not against the goals of organic, I just think that it is too limited by philosophical rather than science-based principles. I believe that there is an even better form of farming which is focused on building soil health but which also has the tools available to be as productive per unit of land as possible

  3. I don’t think the main problem with organic is philosophy vs science – it’s more that it’s inevitably a one size fits all regulatory approach. I agree that building soil health is key and that you can do that without following strict organic standards. It would be nice if we could do away with organic standards and trust farmers to make good decisions, but the problem is that farm economics don’t really favour building soil health when that militates against short-term profits, so the trust strategy would be unlikely to work in practice. It’s an interesting point about horticulture, labour and aging. I don’t know if you’re right. Where I live I see plenty of underemployed young people, plenty of happily gardening seventy-somethings, plenty of unhappy people toiling long hours in desk jobs they hate, and plenty of pointless expenditure. Whether all that could amount to a credible vision of a horticultural future I don’t know, but it would be good to see a few analysts taking it seriously and looking at it rather than dismissing it out of hand.

  4. Granted, my experience is minuscule compared to most farmers, but we have a couple of acres here on which we grow apples and vegetables for market, and we recycle the manures from our hobby cows, hobby horse, pigs, and chickens for compost to use in the gardens. I also have no qualms about supplementing the compost with bagged nitrogen, because I'm not of the holier-than-thou camp; I'm not making any claims about my methods being more "sustainable" than any other method. In fact, with 7 billion people on the planet, I don't see that it really matters what the f I do.

    I also don't care one iota for "organic" certification. They look more and more like crackpots the further I get away from them.

    For example, I was employed at an organic farm for several years and got to think a lot about the absurdities of the organic system. Skipping over the weirdness of having to be a trained pesticides applicator to work on the organic farm, I'll talk about the absurdity of composting imported manures and vegetables wastes at the farm.

    Cow manure was hauled in every farm from a local conventional so-called farm. (I just call them "farms" myself.) This meant that the "virtuous" "organic" nutrients in the compost were just as you say, Steve--recycled inputs from commercial fertilizers used on hay fields and on corn crops to feed cows. Sounds like an elaborate hoax, doesn't it?

    Also, in lieu of bagged fertilizers, the farm tried to make its own compost. This required hauling tons of materials to the farm from such far-flung places as cafeterias and restaurants. Usually the places themselves brought the stuff so they wouldn't have to dump it in the trash. This meant I had to spend a lot of time removing hamburgers and hot dogs; forks, knives and spoons; glass, aluminum and plastic from the containers of "vegetable waste."

    Also, neighbors would drop by with their bags of leaves.

    My job was to turn the stuff with the bucket loader to keep it hot, and to spread it on the fields once it had "made."

    What was this material? Commercial vegetable wastes. The chances of any of its being "organic" were about nil. Therefore, we were literally reusing the NPK that commercial farmers had used to grow the vegetables that ended up in the restaurants and cafeterias!

    What I learned from this experience is that "synthetic" fertilizers, synthetic nitrogen and such, are A-OK for "organic" farms--as long as it has been laundered through cows and cafeterias.

    1. Michael,
      Well, it sounds like you did some pretty absurd things on that organic farm!

    2. Rather, "Cow manure was hauled in every Fall from a local conventional so-called farm."

  5. Actually the problem with organic vs. science is that organic is not based on science. Rather, organic is based on marketing - organic rules exploit common misconceptions and pseudoscience to sell a premium priced product.

    There isn't enough animal manure to scale up it's use much beyond where we are at today because it is a byproduct. Nobody is going to get a herd of cattle for manure production. Fortunately, synthetic fertilizer is made with hydrogen, current produced with natural gas, but easily made with green electricity as well. One of the problems with renewable electricity sources is that they are intermittent. A 'green' fertilizer plant could produce hydrogen with excess power from these intermittent sources and effectively buffer electricity demand. Unfortunately the organic rules would not allow such a product, and among non-organic producers the cost would be prohibitive relative to conventional alternatives. In ways like this, the 'organic' industry has actually become an impediment towards sustainable agriculture because there isn't room on supermarket shelves for two classes of premium food that give people a warm fuzzy feeling. We should dump 'organic' for 'science-based sustainable'.

    1. Anonymous,

      Unfortunately the organic brand is so well established it will persist. I've been a part of broad stakeholder efforts to design sustainability metrics, but it usually turns out that the things you really would like to measure are too difficult.

      You are perfectly right about the green synthetic nitrogen option. I know about three efforts in that area and two are for small-scale units that could function at a farm level (or at a village level in Africa). I think the economics might actually work and I think farmers would like the idea of the independence it would give them from global energy price fluctuations.

  6. Can you folks cite MJ/kg figures for fertilizer produced from natural gas and fertilizer produced from electricity? And perhaps a figure for the energy embodied in total annual global fertilizer production (if it were made using electricity) as a proportion of total annual renewable electrical energy production? Much appreciated.

  7. The energy number I know for synthetic nitrogen energy use from natural gas is 24,500 BTU/lb (sorry about the dumb units). The number that the USDA uses for greenhouse gas emissions is 4.2 lbs CO2-equivalents/pound. If the nitrogen were made using hydrogen generated by electrolysis from something like wind power I'm just guessing that the energy requirement would be a bit larger because it is a two step process with some inefficiency. That may depend on the exact nature of the electrolysis because with the right catalyst it can be more efficient. The big difference would be in the carbon footprint (once you have paid back the footprint of making the devices).

    I have heard various estimates for the totals. It is often said that 5% of natural gas goes to nitrogen fertilizer production but I don't know if that is still true with more abundant sources. One also hears that the production of nitrogen fertilizer accounts for 1% of total global energy use.

    That is a start

  8. s.d.
    One thing you left out of this discussion was water absorption of the soil, which seems quite important when comparing the use of manure and cover crops to industrial efficient farming methods.

    When you say preserving soil, are you simply referring to no-till?

    Given good grass fed beef genetics like the Piedmontese which marbles its fat even without corn, if some of the more productive land in this country would produce grass fed beef instead of corn and soybeans which we have too much of anyway, I think we'd have a more sustainable system which preserves the soil. I agree that labor and the desire to farm is an obstacle to idealism of organic farms, yet, that's also a problem in the new industrial systems. And the more efficient and larger the farm, the less we can entice good operators of this new technologic and expensive equipment because the less camaraderie and the less culture is out there amongst the dying towns caused by our efficiency at all costs mentality.

  9. Kay,
    I agree that water absorption of the soil is super important. I talk about that in this post

    and this one

    When I talk about what I wish was the agricultural norm I'm talking much more than no-till. I support the idea of serious soil building that involves crop rotation, cover crops, no-till and controlled wheel traffic. These are all things that make perfect sense to me as a scientist reading the literature. I fully understand that there are very real practical and economic barriers to the mass adoption of these options. I'm trying to find a solution to that.

    As for the issues of dying towns, I've been in these towns and seen some of that, but there is a certain degree of resurgence because of high grain prices. I'd like to see a sustainable form of that with a new generation of people becoming farmers. It is a long-shot because most of this generation has been convinced that they should do organic.

  10. I’d be interested to know the energy costs of the electrolysis, because my recollection is that they’re way higher than from gas, but maybe newer methods are more efficient? And what proportion of global energy comes from renewables? Once we’ve produced all our green synthetic nitrogen from it I doubt there’d be much left for doing much else...

    ‘Anonymous’s’ comment pretty much captures the way that notions of ‘scientific’ agriculture lead to pseudo-economics and pseudo-ecology. Ask a traditional farmer if they’re keeping their cow for meat, milk, manure or fibre and they’ll say all of the above, and probably conclude you don’t know much about farming. Only by throwing vast amounts of non-renewable energy into agriculture, a lot of which has ended up nitrifying our rivers and oceans, have we created a ‘scientific’ agriculture that maximises single outputs, and also creates a huge mess for other people to worry about.

    As Michael’s comment shows, there are plenty of absurdities in organic farming, but also in conventional farming, and why single out the organic farm for criticism in that story? At least they were trying to cycle nutrients. Why weren’t the conventional farmers doing the same? Maybe because it’s easier to buy another sack of fertiliser and not give a damn about where the nutrients end up? And we call that ‘scientific’ agriculture?

    1. I would respond by saying that there are no "conventional" farmers. That's a fake category that only arose when the self--designated "organic" movement began.

      You say:

      "Maybe because it’s easier to buy another sack of fertiliser and not give a damn about where the nutrients end up?"

      This is another of those straw men set ups that reveals a mindset that wishes to heap calumnies on those outside the organic Holy Circle.

    2. No it's not. You were heaping calumnies on the organic farm because they were recycling non-organically certified nutrients. But at least they were recycling nutrients. Maybe the 'conventional farmers' or 'non-organic farmers' or whatever you want to call them were doing so too. But the pseudo-economics of agriculture has generally incentivised farmers to buy in new fertility and not worry much about where the old fertility goes. I don't particularly blame the farmers for that, but I do blame the farm systems that we've created. Where's the straw man?

    3. "You were heaping calumnies on the organic farm because they were recycling non-organically certified nutrients."

      Ah, no, I was simply describing what I did there.

      "Where's the straw man?"

      Right here: "Why weren’t the conventional farmers doing the same? Maybe because it’s easier to buy another sack of fertiliser and not give a damn about where the nutrients end up?" That's almost the definition of a straw man: Project the "other" side in a negative light and then excoriate it.

    4. You were "simply describing"? 'Crackpots', 'Absurdity', 'Elaborate hoax'. I don't think so.

      A straw man is when you misrepresent a position with which you disagree, not simply when you criticise it. If the non-organic farmers were also recycling vegetable waste in your area then I'll happily exempt them from my criticism. In fact, as I already said, I don't really blame the farmers. But where do you think the dead zone in the Gulf of Mexico comes from? Or is that just a straw man?

  11. Using manure as fertilizer is recycling in its purist form. I grew up on a farm at a time when most farms had at least some livestock and everyone understood this. Work has been done in the past to produce corn with a lower level of phosphorus to lower the level of phosphorus in the manure but I have lost track of how successful that was commercially. We all knew, as any livestock farmer would today that manure is the nutrients in the feed the animal didn't use.

  12. Question - does manure contribute to the soil other than through it's mineral content (i.e., N/P/K)? What about the organic components of it? Is a compost/manure mix along with synthetic fertilizers superior to purely synthetic fertilizers alone to the point that it's worth the time/effort/energy expenditure to do so?

    1. Manure or compost can help build soil quality in a tilled setting, but it does have greenhouse gas issues before and after application. The better way to build soil quality is to let it happen the way it happens in nature. That means no tillage and leaving the crop residues on the surface to break down slowly. It also means growing a winter cover crop so that the soil is constantly being fed. For crops that do require tillage, added organic matter from the outside is necessary to improve soil quality over time.

  13. Chris,
    The concept is that farmers could install new wind power to generate their own energy for making their nitrogen. In most farming areas such energy couldn't economically be part of the grid because of the cost of the transmission lines.

    Whether the nitrogen ends up being an environmental issue has mostly to do with how the land is farmed. If it is no-till with a cover crop and controlled wheel traffic to prevent compaction it won't be a problem.

    I don't think it is productive to think about ag as two monolithic categories - conventional and organic where the latter is perfect and the former terrible (as it is mostly presented and believed). All farming has environmental challenges, but there is a cutting edge that is dealing with this by paying attention to the science

  14. "If it is no-till with a cover crop and controlled wheel traffic to prevent compaction it won't be a problem." True enough, but how much of it actually is? Very little where I live, and our local watershed has just been declared a nitrate vulnerable zone. Hence the pseudo-economics and pseudo-ecology.

    I agree with your third paragraph, and I appreciate that you're not entirely against organic farming (and nor am I entirely in favour of it in terms of how it's practised), but some of the comments on here are equally and oppositely monolithic, eg. 'Anonymous' on organic farming as pseudoscience. If we want to overcome the polarisation then some open-mindedness is needed on both sides.

  15. I didn't have any idea about all these till date. I appreciate your research. I know about nano fertilizers that is also good one.


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