Unlike the others, I am going to evaluate this concept from the urban users' (who 'collaborates' with the farmer) side. I know a lot of colleagues and peers who are extremely interested in organic farming but due to the constraints of city-life and crazy work hours are unable to expend any effort in the direction. A niche lot I know, but a very critical user group you can tap into.
'Adopting' or as Johan said 'collaborating' with a farmer who lives in a village in a certain geographic proximity allows users to take weekend breaks and spend it with the farmer, allowing for a more physical interaction compared to the majorly 'virtual' nature that the concept currently insinuates, making the experience richer for both entities.
Users can inform farmers on food trends in their circles, advocate the cause of organic food among their peers and use enterprising solutions to help the farmer increase their profits :) If done right, this idea can evolve into one radical system.