Artjak: so sorry to hear of your farmer problem. There is a large field adjoining my back garden which from time to time is sprayed or the corn gets cut sending great clouds of corn dust into the air. I may have imagined it but it seems that the wind direction is more often than not blowing away from our row of back gardens - as though the farmer or his labourer is checking it out before embarking on a job that could impact on our gardens.
Whilst I'm not naive enough to believe any farmer can or will arrange his schedule to accommodate neighbours all the time I would have thought that they do have some choice some of the time to control when their work will affect nearby residents.
I'm wondering whether the psychology of an approach which seems to reflect a high expectation of him, eg of him being a reasonable man, might be worth trying before getting at loggerheads - at least that way he can't pretend you've upset him.
If that doesn't work you can always have a solicitor's letter as a Plan B.
Also, what about approaching your local parish council (or equivalent) by telephoning the Clerk of the Parish Council (or equivalent) for advice. These organisations exist for the local inhabitants but often have close ties with local farmers too. Depending how your particular local council works, that might be a way of getting advice and having more influence on the situation - especially if other residents have had similar problems.