Agricultural aircraft
An agricultural aircraft is an aircraft that has been built or converted for agricultural use – usually aerial application of pesticides ( crop dusting ) or fertilizer ( aerial topdressing ) ; in these roles they are referred to as "crop dusters" or "top dressers" . 
<triplet> fertilizer <subj> agricultural <obj> use <triplet> aerial topdressing <subj> aerial application <obj> subclass of <subj> crop dusting <obj> subclass of