Wastes as resources:
This invloves the obvious use of organic waste in urban agriculture
(including composted sewage sludge and waste water), and the diversion of
solid waste from the municipal garbage stream through the building of
garden infrastructure (planters, windscreens, shades, coldframes, &c.).
Informal employment and economic development:
Locally produced food reduces the drain on local economic resources
associated with the purchase of imports. It also supports local input
supply and processing industries.
A term found in Freeman's look at urban agriculture in Kenya.
Basically the beautification and maintenance of otherwise neglected urban
properties. I've seen a few North Anerican examples of this, esp. with
regard to community gardeners cleaning-up and using neighbourhood eye-sores
The presence of agriculture in the city re-establishes our link
with our food sources. Kids and adults can see where food comes from and
how it is produced. Also valuable as a training ground for those
interested in cultivation their own gardens.
Preservation of knowledge:
Many, cultures have interesting gardening traditions and cooking
practices which urban agriculture can help preserve. I consider genetic
diversity another form of knowledge that urban gardens can maintain through
the cultivation of heritage and other open-pollinated cultivars.
The contributions to local food security are obvious. Equally
important is the potential generation of fungible income (discussed in
Freeman and the UNDP report). This is income generated by its release from
other obligations (in this case, food purchases). Money saved by food
growing becomes available for other uses (e.g. paying bills).
Unused or inefficiently used urban spaces can be used productively
for urban agriculture. Utility rights of way, institutional and industrial
properties, and rooftops. There is also a social niche it can occupy by
providing goods and services (food vendors at bus stations and on the
streets in the developing world, for example).
There are a few other benefits which spring to mind, including mitigation
of storm run-off, shade (urban forestry or fruit trees), and insulation
(vines on walls, roof gardens), and ecological diversity.
The only problems I am aware of are food contamination via polluted
soils or improper use of waste water (both easily prevented), and noise
pollution and potential disease spread through inappropriate animal
husbandry (which can also be prevented by education and the provision of
extension services). There are a number of social, cultural, and
administrative barriers to the activity, but these aren't really
costs or benefits.