That is the research question of this book. Or, put more simply, "So why don't more farmers change?" (p. 12). After dismissing the suggestion that farmers simply don't like to change (p. 12-13), he notes that some believe it's "the structure of agriculture" - "the technology, market forces, and government regulations" that prevent change. Furthermore, he describes a sort of survival of the fittest: those who are still farming are the ones for whom the current structure of agriculture works, and the way they farm works with it. Given that, why would the change and do something different? (p. 13 and a footnote referencing Blank 1998, Hart 1991, and Tweeten 2003)
Bell believes this is not true for two reasons. First, because the situation for those who still farm is precarious, so it's hard to say that the current structure is really working for them. Second, farming practices are so similar from one farm to the next that it's hard to make the case that only the most efficient remain, unless you mean they are the most efficient in getting subsidies. In a footnote, he notes that he's not calling any farmer inefficient. Rather, their practices are so homogenous that it's hard to pick out one as being more efficient than the others (p. 13).
Bell sums up the message of the first part of the book: "While it is true that the current structure of agriculture encourages farmers to farm the subsidies and not the land, many if not most Iowa farmers (and their communities, their environment, and their families) find themselves poorly served by this encouragement" (p. 14).
Then Bell comes to his point, which is worth quoting in full because it's a good one:
"This phenomenology of farming - this taking for granted of what works, even when you think it might not - is a matter of more than material and temporal investments, though. It is equally a matter of identity, of the investment of the self as a man, as a woman, as a farmer. What you know is who you are. The stocks of knowledge we each old within are stocks of self as well" (p. 14).
He provides more explanation on how knowledge translates to identity, and in a footnote he cites Bentley, Rodriguez, and Gonzalez (1994), Chambers (1994), Collinson (2000), Hassanein (1999), Hassanein and Kloppenburg (1995), and Kloppenburg (1991).
Bell writes: "We learn from, and with, others, and gain a sense of social connection thereby. Knowledge is cultivated within culture and culture's lines of difference and identity - what I will be terming the cultivation of knowledge" (p. 15). A little later he writes, "knowledge is a social relation" (p 15).
Then he relates it to why more farmers do not adopt sustainable methods: "To give up a cultivation of knowledge, to give up a field of knowing and relating, is to give up both a field of self and its social affiliations and a field of trust in the secure workings of the world" (p. 15).
Bell connects knowledge to power, invoking Foucault. One might also connect this idea to Skogen, Mauz, and Krange (2008) in which the authors contrast the narratives of the more powerful pro-wolf contingent and the less powerful anti-wolf one, pointing out that the narratives reflect differential power relations. Here they are not discussing the knowledge itself but the status of the narratives of each side, in which the more powerful side's narrative is seen as scientific fact and official truth, whereas the less powerful side's narrative is seen as rumors.
Back to Bell, he describes a number of farmers who experience "phenomenological rupture" in which both one's identity and one's cultivation of knowledge change at the same time, and do so quickly (p. 16).
In the last part of the book, Bell describes how changing your farming practices means changing your social relations. As he goes on to define what he means, he makes it clear that he does not simply mean that switching to sustainable agriculture switches the people you spend time with ("an alternative set of social relations, of friends and associates with whom one identifies"). He also specifies that it brings with it a different "social practice of agriculture" (emphasis in original, p. 17). That is, for at least the particular group of farmers he was studying in Iowa, "the relations of knowledge... have a different feel to them, a different way of experiencing others and of experiencing one's own self" (p. 17).
It sounds like he is saying that conventional agriculture is what the IAASTD report calls "locally black box." That is, knowledge travels from top down, with experts producing knowledge and disseminating it as a universal truth. Another way to produce knowledge is by one's self. Bell calls both of these methods "monologic." But the knowledge of the sustainable farmers who are the subject of his book is produced jointly, with one another, in a dialogue.