Greetings, Dimitar, and thanks for prompt and constructive reply!
It would seem to me that aerodrome=agricultural or perhaps aerodrome:type=agricultural would be quite appropriate. Be bold, go ahead :) !
NB there used to be a Bulgarian state service for agro-flying, as I seem to remember; if this is still the operator (if indeed it still exists at all) it could well be mentioned, as "operator=xxx"
Kindly yours,
