Pollution on a particular tile will divide its trade by half, rounded up. Trade is broken up into Tax, Luxury and Science. Some buildings affect these totals specifically - see the building chart for that information. Food Food tiles are as above with the following modifications. These effects are done in this order.
Irrigation adds one food unit to any type that can be irrigated.
The city center always acts as if it's been irrigated. A Harbor will add one food unit to every ocean square in the city. Railroads don't increase the trade production in the default configuration. But this can be set by the server to any percentage. Despotism will subtract one food unit from any tile that already has more than 2 food.