For general purposes, shoot for between 20-30 kg/m2 for a lightly loaded airfoil.  In your case, a 3m span with a 50cm chord would give you 1.5 square meter area.  Including your airframe weight plus your 30kg payload, you could see better than a 30 kg/m2 wing loading.  Depending on your airfoil selection you should see pretty docile handling and slow enough speed for crop dusting.  Make sure your tail moments are adequate for the wingspan you use as this will affect handling as well.  Tail volume should be calculated utilizing aerodynamic practices which can be found online.

For general purposes, shoot for between 20-30 kg/m2 for a lightly loaded airfoil.  In your case, a 3m span with a 50cm chord would give you 1.5 square meter area.  Including your airframe weight plus your 30kg payload, you could see better than a 30 kg/m2 wing loading.  Depending on your airfoil selection you should see pretty docile handling and slow enough speed for crop dusting.  Make sure your tail moments are adequate for the wingspan you use as this will affect handling as well.  Tail volume should be calculated utilizing aerodynamic practices which can be found online.