I think 1200 $/tonne is not a bad estimate, see recent quotes. Shipping is expensive so you have to take that into account. Note that the 1200 could be per unit of nutrient, not product. NH3 is 82% N so you must divide by .82 to convert product price into nutrient price. Lots of people were paying $0.60 per lb-N this spring. That's 1323 $/(tonne N).

I don't know which part of Canada you farm in but we probably spend more than that figure. It comes out to 52 $/acre. Using some spring 2009 prices: 60 lbs/acre of N, 25 lbs of P2O5, and 9 lbs of K comes to about 59 $/acre.

If that exhaust system worked it would be nice. Unfortunately there are no studies that show that it does. Probably the manufacturers are making out okay at $40,000 per system. Hmm.

Synthesizing nitrogen is very expensive (in energy and in monetary price). If this exhaust idea worked you can be sure farmers would snatch it up. Unfortunately it is snake oil. AFAIK, there is no serious study showing any effect.

Using legumes to fixate nitrogen is something that *does* work and farmers are happy to do so if there is a market for the crop (we grow yellow peas as much as reasonably possible). Because organic farmers can get a premium for their other crops, they sometimes grow legumes purely for the residual nitrogen and plow them down instead of harvesting. Unfortunately organic farms requires quite a bit more fossil fuel than modern conventional farming (something most shoppers are probably unaware of).

Yes, the amount of NOx is much too small to affect plant growth (even assuming 100% of it stays in the ground and converts into a plant available form). After people pointed this fact out, the snakeoil salesman dreamed up the idea of the CO2 (or something) in the exhaust stimulating microbes in the soil.