Air Space: Federal law provides that the United States government has exclusive sovereignty of airspace of the United States; the Federal Aviation Administration (FAA) makes the plans and policy for the use of the navigable airspace; and any citizen has a public right of transit through the navigable airspace. 49 U.S. Code § 40103.
Ownership no longer extends endlessly into space. The FAA defines and regulates navigable airspace, through which the public has a right of transit. Today, air rights extend to the airspace above the surface that could reasonably be used in connection with the land. Airplanes soaring above your property are not trespassing because they are flying in what Congress has declared as the public highway.
The landowner "owns at least as much of the space above the ground as he can occupy or use in connection with the land." (US v Causby at p. 264) Everything above is navigable air space, available to the public.
The Government considers the public highway to begin around 500 feet in uncongested areas, and 1000 feet otherwise. Aerial Applicators, however, may operate below 500 feet over 'persons, vessels, vehicles, and structures,' if no hazard to people or property is created. How close someone needs to get near someone's property can be decided by which direction the landowner cultivates their rows, and wind is also a factor.
If an aerial applicator is in the 'middle of an operation' they may need to make turns, possibly over property, to accomplish the task. There are regulations to abide by. When an applicator leaves the airport to do a job, they need to be 500 feet. As they transition to do the work, the regulations don't apply as they will need to be lower to the ground. If you were to experience frequent flyovers, the local sheriff's department is the person who would assist. If you are not in a 'congested area,' no notice will be given (unlike over a city for mosquitos, for example).