Hi Mark: 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.

Hi Thom, you make a good point and I get it. All those animals, God's creatures as it were, aren't just being raised for producing manure! The fertilizer production is going to happen no matter what, and organic ag is deploying what is already there.  
 
I'll forward your comment to Eric and see what he says.  
Mark