Mark here.  The video by Dr. Eric Brennan concerning the sustainability problems of using what is essentially repackaged synthetic nitrogen in organic agriculture that I posted a few weeks ago has sparked a lot of discussion in my social circles, most recently for me at a dinner with a big ag company and a local PCA specializing in organic consultation.  These conversations turn around the same point that reader Thomas Flewell (Thom - note you got your byline!) brings up below.  Best to go to the expert on this very subject so I asked Dr. Brennan if couldn't answer the question for all of us, and he graciously accepted my invitation to do so.

Follow along below:

Thomas Flewell: Preface-  As the world population approaches 9 billion within the next 100 years, it is clear that organic farming will not serve up enough food for everyone.  
Dr Brennan's reasoning toward the use of synthetic nitrogen in organic farming is persuasive but I see an error of omission. Dr Brennan failed to note that manure derived fertilizer will be produced whether or not it is used for agriculture. Cows poop. Chickens poop. Pigs poop. Goats poop. All god's creatures poop. And none of them are raised only to produce fertilizer. So the energy to produce argument seems weak. Also, while manures are used as pre-plant fertilizer and in some instances as a top dress or side dress for crops, fish fertilizers are also used. I have had very good results with a hydrolyzed (not emulsified) fish fertilizer with a very low N analysis. Perhaps just an academic point.  

Eric Brennan:

Hi Thomas,

Thanks for your interest in my ‘repackaged' synthetic nitrogen video and for your comments on it. I've heard others raise similar questions after seeing my video. I'm working on an opinion paper that will provide more details on the arguments that I raise in the video, and hopefully will bolster my arguments with citations, and examples that I was able to fit into the video. But in the meantime here are some ideas to consider and some articles to help with that.

I agree on the value and importance of recycling nutrients from manure and animal slaughter by-products; to my knowledge slaughter by-products are more important than manure in high-value organic crops California. However, I imagine that rendering process (extraction, grinding, heating, pressing, etc.) to convert this material into pelleted fertilizers takes a fair bit of energy to ensure that the material is safe to use, and easy to handle and apply. And transporting this bulky material to where it's used is also energy-intensive. Consider an organic system in a place like my home state of Hawai'i where there is relatively limited animal production. Think of the energy costs to move organic fertilizer made of chicken manure, meat, bone, and feather meal all the way from California, for use on an organic farm in Hawai'i! To me, this seems like a relatively inefficient way to get N in these systems.

Ideally these animal production by-products could be used in organic and conventional systems near to where they are produced and in systems that need the full suite of nutrients they contain; even better would be for the nutrients in these materials to return to the systems that produced the feed for these animals. But having to rely heavily or almost exclusively on this type of fertilizer material (as in often the case in organic vegetable production in many regions) for nutrients like nitrogen (N) can be problematic and quite expensive. As I mentioned in the video, one problem is that N in many of these ‘repackaged' synthetic N fertilizers often come with excessive amounts of phosphorous (P) that far exceed what is being removed from the soil in crop yields. I became well aware of this issue of excessive P inputs in my research when I collaborated with researchers from Stanford and UC Davis to calculate P budgets (P inputs versus P outputs) in two long-term organic studies in California. This is described in detail in the a few publications like this one available here. To improve our P budgets in our long-term study in Salinas we switched from using pre-plant fertilizers with a 4-4-2 analysis to an pre-plant with a 8-1-1 analysis which was more expensive per unit of N; from what I've seen with pelleted organic fertilizers, unfortunately the materials with lower P or no P have a higher per unit cost for N. Switching to 8-1-1 helped with our P budgets, but we still were applying more P than needed when we added yard-waste compost to these systems. This improvement in our P budget is very obvious in figure 2B in of the paper note above that shows a big decline from 2007 to 2008 in the P balance. This also highlights the importance using cover crops to add carbon back to the soil, rather than over relying on compost which can add too much P. Carbon added by cover crops represents ‘on-farm carbon production' that doesn't add P, but just recycles what's already in the soil to produce the organic matter. Furthermore, organic matter inputs from cover crops appear to be a more important driver of soil health improvement than organic matter from compost. Here's our recent paper that describes that issue

Getting back to the P budget issue, during 8 years of our long-term trial on high-value vegetable production, we added more than 400 lbs of P per acre than was needed to replace what was removed in exported yields !   It's important to highlight that unlike N that we can capture out the air with biological N fixation (using legumes) and by synthetic fixation (using the Haber-Bosch process), P is a limited, mined nutrient, and there is considerable concern about the worlds dwindling P reserves; this highlights why we should only apply P when needed. Here's a link to another paper from our long-term study that provides some information the biological N fixation potential from legume-cereal cover crop mixture in these systems. While legumes do fix N in these systems, this is limited by their ability to complete with non-legumes likes cereals that provide other important services like nitrogen scavenging from previous vegetable crops, … and there can be other challenges with legume-cereal cover crops that I highlighted in this video Are legume-cereal cover crops a good fit for organic vegetable production?

Another issue to consider with organic soil fertility management that is relevant to the arguments in my ‘repackaged' synthetic N video is the total supply of manure and slaughter by-product based fertilizers available in a region like California. I don't know the extent of this supply, but it seems likely that as organic agriculture acreage grows here and uses more of these organic fertilizers, this supply of fertilizer will become more and more limited unless animal production (that relies primarily on feed grown synthetic N) increases. This is concerning because the science of climate change indicates the need for people in regions like the U.S., with excessive protein consumption, to reduce consumption of animal products. This important dietary shift to less meat consumption would likely reduce the amount of manure and slaughter by-products available for recycling as fertilizers. Furthermore, relying on fish as source of N for organic agriculture could overtax the world's oceans that are already over fished in many regions. Perhaps I'm wrong, but I don't believe there is a sound scientific basis for a total ban on the use of pure synthetic nitrogen in organic agriculture. That's why I argued for what I call ‘SPorganic' (i.e., scientifically progressive organic) agriculture that would allow the careful use of synthetic N in it's pure form, rather than only in the ‘repackaged' synthetic forms that are currently allowed in organic systems. However, I believe that limiting the use of N inputs (from ‘repackaged' and pure synthetic forms) and greater adoption of best-management practices like cover cropping, make scientific sense. ‘Enriching the Earth' by Vaclav Smil is a book that I highly recommend for additional reading on the complex issue of nitrogen in agriculture and how the Haber-Bosch process has transformed our lives and accounts for at least 40% of the world's dietary protein. Here's link to an entertaining RadioLab podcast on the Haber-Bosch story that you might also enjoy.

My N video was first presented along with 10 other 5 minute videos at a 2016 symposium that I helped organize at the American Society of Agronomy conference; here' a link to the 11 videos. One of my intentions with my N video was to start a conversation around a complex issue that I believe limits the sustainability of organic systems like those that I work with in California. Thanks for joining that conversation.

Take care, Eric Brennan

Mark here - thanks Thom and Eric for a most informative exchange - really appreciate it!!