Albert Klein correctly points out that the recent trend of urban farming is inefficient from an economic point of view and that in a best case scenario urban farming is a second best solution for eliminating food deserts. Vacant lots in dense urban areas are close to public infrastructure like roads, plumbing, trash pick up, electrical lines, etc. as well as a supply of customers. To use such valuable land for a low value use like farming, an activity which can be done efficiently far from city infrastructure, underutilizes resources.
However, his solution that city officials tax developers who fail to build and subsidize grocery companies in order to encourage them to build on these vacant lands is unnecessary. Developers would certainly prefer to have their land developed and earning them a profit rather than sitting empty. And the recent Wal-Mart in DC case shows that companies are often willing to build in underserved areas. In fact the DC case exemplifies what in my opinion is the biggest hurdle to urban development in many areas; too much government zoning and regulation.
Too often city planning departments and officials are captured by special interests that encourage them to keep certain companies out that they don't like. City officials that get out of the way and allow companies to build on land where they see the potential for profit would help to reduce the number of urban food deserts in this country with no subsidies or penalties required.