Meeting these basic requirements does not guarantee a farm, but
not meeting one of these requirements guarantees there won't be
one. Other factors to consider: Farms need roads, but tend not
to develop when the majority of it is surrounded by roads. Having
only one or two sides of the farm with road access is ideal. Farms
will not develop if there too much pollution. Just keep industrial
buildings (destroy them if they start building where you have
intended for the farm) and other polluting structures away and
it'll be fine. Farms have a minimum size of 8x8 and a maximum of
18x18 tiles. Typically if you have a block of zoning larger than
18 tiles square, two farms will share the block. If there are gaps
of light industry left over from farms, destroy those buildings and
dezone the tile under it, as industrial buildings bring pollution.
If your city doesn't have a lot of high-tech industries, farms can be
beneficial since they will fill the demand for industrial zones without
bringing in the pollution. But farms do take up a lot of space because
they must be built far away from the pollution and higher land values
in the city, and are pretty big themselves.