You are right; It's tractor exhaust injection. Here are a couple of links from a quick google search.
http://www.fwi.co.uk/Articles/2008/01/08/108904/exhaust-gases-cut-fertiliser-requirement-in-canada.html
http://www.wipo.int/pctdb/en/wo.jsp?IA=WO2006130965&wo=2006130965&DISPLAY=DESC