Writing as a guest contributor on the Seedstock blog, Dan argues that the biggest barrier to urban agriculture businesses is land values, not training:

In most urban centers high land values are the biggest obstacle to large-scale adoption of urban agriculture, not a lack of professionalization. High land values translate to rent or mortgage payments that eat into margin and leave precious little revenue for the farmer. For example, even an optimistic gross profit projection of $6,000 per month is barely enough to rent half an acre in Los Angeles, Chicago or New York, much less pay a farmer a living wage. 

As a result, the crop of current urban farming ventures is led by those who creatively circumvent the high cost of land by using rooftops (like Gotham Greens in Brooklyn), backyards and schoolyards (like we do at Farmscape) or temporarily vacant lots (like Riverpark Farm in Manhattan). What is saved in rent shows up in the paychecks of farmers and bankrolls the growth of these ventures.

You can read the rest of Dan's article here.