How much do organic farms depend on nutrients that originally came from synthetic fertilizers?


There's an interesting discussion of this issue at Biology Fortified blog. The post is by my first MS student, Andy McGuire, who also has interesting things to say on a blog from Washington State University, where he works.

Andy's post was apparently inspired partly by something I wrote last year, showing my estimates of how much nitrogen flows from synthetic fertilizer to conventional corn to chicken manure to organic farms.

I don't get too excited about whether organic farmers should be allowed to use manure from conventional farms, or whether we should "blame" them for the fertilizer used to grow the corn to feed the chickens. My point was simply that this dependence limits the potential for expanding organic agriculture. (So advocates of organic farming should be showering me with money for research on nitrogen-fixing legume crops and forages -- but that's another story.)

More interesting is this open-access paper, titled "To what extent does organic farming rely
on nutrient inflows from conventional farming?
" The paper has some actual numbers on nutrient fluxes to organic farms from conventional ones in France. While my earlier diagram assumed that an organic farm relied entirely on manure whose nitrogen originally came from synthetic fertilizer, the French study estimated that only 23% of nitrogen (though 73% of phosphorus) supply to organic farms came from conventional sources. But let's look at some details.

First, there are apparently some differences from US practices:

"European regulations recommend to use organically produced animal manure but allow the use of conventionally produced manure, provided that it is not the output of 'factory farming'."

I don't think we have that restriction in the US. Second, they found that:

"More than 80% of nutrient inflows through manures (82%, 85% and 81% for N, P and K, respectively)... came from conventional farming"

So 82% of N in manure came from conventional farms, but that was only 23% of estimated total N supply. The rest (almost 60% of total N inputs) was mostly estimated to come from biological nitrogen fixation:

"N fixation was estimated using the model proposed by Høgh-Jensen et al (2004)... based on the total N amount in leguminous crop biomass, multiplied by the ratio between the amount of symbiotically fixed N and the total N amount in the crop biomass."

That seems to be about all they said about biological nitrogen fixation -- I would have liked a little more detail. They also say that unsustainable "soil nutrient mining" (failure to replace nutrients exported in crops or otherwise lost from soil) was "not considered." A simple approach to that problem would have been to estimate nitrogen and phosphorus contents of products sold off-farm, and compared that to total inputs. If inputs exceed outputs, some of the difference may end up polluting groundwater, although some may be accumulating in soil, at least for a while. If outputs exceed inputs (including biological nitrogen fixation), over the long term, they will not be able to continue farming over the long term.

My conclusion is that organic farms that rely on manure from other farms (or on manure from their own animals, which are fed grain or hay from other farms) are dependent on conventional farms and therefore not a model that can be scaled up. That doesn't mean they are not making a positive contribution, if they are using manure that would otherwise end up polluting rivers.

If we can believe the nitrogen-fixation estimates in this paper, French farms are already getting a majority of their nitrogen from symbiotic rhizobia in the root nodules of their legume crops and forages. Maybe that could be scaled up. But if 75% of their phosphorus is coming from manure from conventional farms anyway, there may be little point in trying to replace the nitrogen in that manure with biological nitrogen fixation.


Ford, thanks for drawing attention to Andy’s post – for the reasons I outlined in my comment under it I can’t help seeing it as much more than another bit of organics-bashing ideology (surely the real story here is the wanton resource waste it implies in the conventional system), but obviously it does highlight a potential problem for scaling up existing organic approaches.

I’d love to shower you with money for your legume research, but like most organic (or in my case ‘organic in principle’) growers and organisations I don’t have the money that’s available in the conventional sector – I’d be interested in your views on research funding priorities for eg. legume research as compared to eg. synthetic fertiliser research.

I wrote a little on my fertility strategy for my farm here: – in a nutshell, very little in the way of imported synthetic inputs, but probably more labour inputs and lower yields...and possibly phosphate mining, I don’t know. The two key questions for me are (1) Can my approach be scaled up? Personally, I think yes – but perhaps only if we make the assumption that agriculture may not be able to furnish long-term whatever the global population demands of it (less meat, less catering to what people want and more catering to what they need), and the assumption that we need more, not less, people involved in primary production. (2) If it can’t be scaled up, then how sustainable is existing conventional agriculture long-term? Will it also end up requiring less furnishing of wants, and more of needs? I’d be interested in your views.

In the short-term, research to reduce over-fertilization (without reducing yields) may be more important than research to improve the efficiency of biological nitrogen fixation (at least for rich countries). But the latter will take longer, so we need to get started.

We are planning to do both as part of a new network of long-term experiments here in Minnesota.

Is anthropogenically produced CO2 a synthetic? It is a fertilizer, so should it be regarded by organic growers as a potential contaminant?
There's plenty of evidence to suggest C3 plants benefit from higher levels of CO2 in the atmosphere.
Of course this classification gets pretty ugly in a hurry... just exactly which human CO2 generating activities should come under scrutiny. We all breathe, and by this generate CO2. Composting generates CO2, so if one brings leaves and stems onto the farm from a remote site and those leaves and stems grew in modern atmosphere with postindustrial levels of CO2 (let's say 390ppm vs 310ppm pre industrial, so that roughly 20% of today's CO2 might be judged 'anthropogenic') - is the carbon in the compost now only 80% organic??

Not trying to be a meanie, but sometimes I wonder how far we have to go in trying to be pure.

Clem, well I guess I agree with you that it’s impossible ever to be ‘pure’. But that’s why I think the intentions of McGuire’s post are wrong – he wants to criticise organic farming for being less ‘pure’ than he thinks it claims to be, in much the same way that anybody who ever takes an ethical stance on anything gets criticised for hypocrisy through inevitably having some investment in the status quo they oppose. Fine, but why get so hung up on notions of ‘purity’ which – as you cleverly show – are always subject to a reductio ad absurdum? Ultimately what really matters is efficient resource use and making good decisions about agricultural futures. The main story here that McGuire ought to emphasise is crazy economics: conventional farmers can afford to give away fertility to organic farmers, and organic farmers have few choices other than to use it if they want to stay in business. Is this sustainable in the long term? If yes, then organic farmers are surely doing a good job as resource re-users. If no, then we’ve got a problem – and taking organic farmers to task for their lack of purity won’t solve it.

Very nicely capsulized!! If conventional farmers can afford to give away fertility... Indeed, many a conventional livestock producer here in the States is now faced with manure being a liability instead of a resource. So if another farmer - organic or otherwise - can return manure to the resource column, all the better.

More evidence we should be working harder looking for solutions and less about ideological differences.

Ford, thanks for commenting on my blog at . Sorry, I somehow missed your comment - have now replied, with further questions!

Leave a comment