After doing some research, I've come to the conclusion that your game warden does NOT know the law he was hired to uphold. You need to go over his head ASAP and contact the DNR directly at the highest level they will allow. The following is an excerpt from the harrassment clause of the 2007 Iowa Fish And Game Laws:
OBSTRUCTING HUNTING OR TRAPPING:
No one may intentionally obstruct the participation of another in the lawful activity of hunting or trapping, including but not limited to:
A.) Intentionally placing oneself in a location where human presence may affect the behavior of a furbearing animal, game bird or other game or the feasibility of killing or taking a furbearing animal, game bird or other game with the inent of obstructing or harassing another person who is lawfully hunting or trapping.
B.) Intentionally crateing a visual, aural, olfactory (the soap you mentioned) or physical stimulus for the purpose of effecting the behavior of a furbearing animal, game bird or other game with the intent of obstructing or harassing another person who is lawfully hunting or trapping.
C.) Intentionally affecting the condition or altering the placement of personal property used for the purpose of taking furbearing animals, birds or other game with the intent of obstructing or harassing another person who is lawfully hunting or trapping.
A person shall not interfere with the lawful hunting or trapping activities of another person where hunting or trapping is authorized by a custodian of public property or an owner or lessee of private property. This rule does not prohibit a landowner, tenant or an employee of the landowner from performing normal agricultural operations or a law enforcement officer from performing official duties.
So it's beyond clear that you have a case against these guys. You just had the wrong guy on your side. Push the issue to a higher source and you will come out on top. NO WAY they can argue normal agricultural practice as their excuse and it is NOT allowable simply because they're on their own property. Your warden needs to recheck his handbook and get with the program.
Regards,
Iowabowtech