Han is probably not referring to Tatooine
Because of the extreme environment, crops were mainly grown underground on Tatooine, and irrigated using moisture from moisture farms, like the one owned by Owen and Beru.
Han is likely drawing instead from his upbringing on Corellia — this is an Earth-like world with farmland outside of its major cities, and agriculture is one of the main industries. (See here.)
There would likely be a need for crop dusting and topdressing on Corellia — this includes seed planting on a wide scale, fertilizer distribution, and pesticide application.
Regarding the latter, Corellia seems to teem with wildlife and insects. A selection of the native species is listed here. Some of them certainly have the potential to damage crops.
In any case, crop dusting is an efficient technique associated with industrial-scale agriculture. The Star Wars galaxy might have better (or worse) technology for this than we do, depending on the planet, but there is no reason for it to not occur on a planet like Corellia, which would inform Han's perspective on the matter.