a wonderful essay by charles waldheim which can be read in its entirety here.  An excerpt is below.

The categories of agrarian and urban are usually understood as distinct. Across many disciplines, and for centuries, the country and the city have been defined in opposition to one another. But today, in striking contrast, design culture and discourse abound with claims for the potential for urban agriculture. As environmental literacy among designers and scholars has grown, so too has enthusiasm for agricultural production in and around cities. Fueling this trend is rising public interest in food and its production and distribution in a globalized world. 
Contemporary interest in food is being shaped by various authors and interests, who argue for the more sustainable practices associated with local food production, reduced carbon footprint, better public health and the related benefits of pre-industrial farming techniques, including enhanced biodiversity and ecological sustainability. Among the strongest advocates for these goals are the slow food and locavore movements.
While much has been written about the implications of urban farming for agricultural production, public policy, and food as an element of culture, little has been written about the potentially profound implications for the shape and structure of the city itself. To date the enthusiasm for slow and local food has been based, on the one hand, on the assumption that abandoned or underused brownfield sites could be remediated for their productive potential; and on the other it has been based on the trend toward conserving greenfield sites on city peripheries — on dedicating valuable ecological zones to food production and to limiting suburban sprawl. But these laudable goals are not much concerned with how urban farming might affect urban form. This suggests that we need to probe further into the possibilities of agricultural urbanism: so these brief notes outline a history of urban form perceived through the spatial, ecological and infrastructural import of agricultural production. The choice of projects is based on the idea of agricultural production as a formative element of city structure, rather than as an adjunct, something to be inserted into already existing structures; thus this tentative counter-history seeks to construct a useful past from three projects organized explicitly around the role of agriculture in determining the economic, ecological and spatial order of the city.
read the full article HERE