Britain Facing Agricultural Crisis ... ?

The Independent reports:

Despite the traditional perception that there is a green and pleasant land outside the grey, barren landscape of our cities, researchers from the University of Sheffield found that on average urban plots of soil were richer in nutrients than many farms. 

Sampling local parks, allotments and gardens in urban areas, Dr Jill Edmondson showed that the ground was significantly healthier than that of arable fields. Allotment soil had 32% more organic carbon, 36% higher carbon to nitrogen ratios, 25% higher nitrogen and was significantly less compacted.

According to the researchers, Britain only has 100 more harvests left. Left until what is not clearly explained.

As I have followed news stories concerning possible crises, I feel that I have become jaundiced. Does the UK face a crisis, or merely the researchers' funding? I would expect that urban land would be potentially more productive than farm land for the simple reason that we mostly build our towns and cities on the better land, and farming gets pushed into more marginal land. Areas that were the richest farmland in my hometown when I was growing up are now housing developments and shopping malls.