David Tribe has an interesting post on organic crop yields. In short, he asks: from where does the nitrogen come? For organic crops, it's often manure. But, where does that come from? Often from animals fed conventional grains made possible by synthetic fertilizers. He also raises an interesting point about the need to count cover crops as an input into production of commercial organic crops when considering overall productivity.
It is an important point that has been addressed by others such as Adam Merberg who humorously noted now synthetic nitrogen is "laundered" to make it fit for organic agriculture:
“However much nitrogen exists in manure today, much of it has been fixed industrially before being taken up by corn plants and laundered through the guts of conventionally-farmed animals. Vasilikiotis does not explain how that manure might come to be in an organic world. To do so would require demonstrating the potential for sufficient biological nitrogen fixation”
A while back, I discussed some of the issues involved in comparing organic and non-organic crop yields, and included the following figure and explanation:
“Here is the basic point conveyed in the picture above: a non-organic farmer is free to use any of the practices available to an organic farmer (e.g., no-till or low-till farming, cover crops, etc) but an organic farmer can only use some of the practices that are available to a non-organic farmer. Thus, the range of possible production practices, costs, and outcomes for organic must be a sub-set of that of non-organic.
Being an organic farmer implies following a set of rules defined by the USDA. These rules restrict the practices available to an organic farmer relative to a non-organic farmer. Organic farmers cannot use “synthetic” fertilizer, Roundup, biotechnology, atrazine, certain tillage practices, etc., etc. It is a basic fact of mathematical programming that adding constraints never leads to a higher optimum.”