I agree with Caroline about how hard it is to say HOW FAST ammonia will dissipate - there are too many variables, like how much you apply, what the pH of your soil is, how much rain you get, and how fast your soil drains or runs off.
Maybe a better question to start with is HOW MUCH and how concentrated it has to be, to be an effective herbicide. It will make soil basic or caustic , if you have enough. Were you planning to start with household ammonia (5-10% ammonium hydroxide), or an industrial chemical like anhydrous ammonia?
Another thing too consider is that classic chemical herbicides are relatively selective, like killing plants more than they kill soil life. Ammonia would be very selective. Since its mode of action is probably just caustic destruction of plant tissue, it would probably kill soil microbes faster than it kills roots. Certainly fungi would be more vulnerable than higher plants, and maybe even bacteria would be killed. You might wind up with sterile soil even after the ammonia came back. But you can re-inoculated soil with compost etc.
I don't know for sure that ammonia is worse than 2,4-D, Agent Orange, or large amounts of salt, I'm just saying that even if it can be made to work, such a high concentration seems likely to kill other things, until it's diluted.
Until you get enough concentration to be toxic, it is a fertilizer. And once it's diluted, it will again be a fertilizer.
Maybe a blow torch would be more effective, if it has enough range to reach past the fence.
If you soil used to be very acid, ammonia will certainly cure that!
But ammonia is very soluble in water, and the first few strong rains will wash "lots" away. When the local concentration is less than it takes to be a poison, remember that what is left is high-nitrogen fertilizer, and any plants that survived or re-colonized it will take the ammonia up and be stimulated.
If you apply enough ammonia to kill plants, some will evaporate and smell very bad. That's local air pollution, but what evaporates into the atmosphere will be washed out of the air by the first rain, where it will again fertilize whatever it falls on. Maybe it would be more of an air pollutant in a very arid region.
I don't think you can put enough ammonia into the air to be a serious pollutant one block away, starting with a few gallons of household ammonia (5-10%). If you have enough, concentrated enough, to annoy your neighbors a block away, I think your eyes and lungs would have stopped you from pouring the third or fourth gallon.
Good luck whichever way you go! I know I'm mostly nay-saying and being unhelpful, except for the idea of a weed torch. It's not like using a flamethrower - you don't even need to get steam from the weeds, just heat them to 160. Would it kill roots? I'm guessing not.
http://www.ehow.com/how_866629...
http://www.youtube.com/watch?v...
