Object snapping to ground plane

Objects in blender starts with half of it -z and half +z how can i move my object so it snaps on the ground plane from its bottom not center ?

I’m unaware of any blender setting to do that, but you may check at the link for a solution:

i just want to move my object so its bottom sits on the ground plane

To just move an object in the vertical direction, press g to move, press Z to lock the movement to the vertical direction, and move it by hand or typing a number.
Eg: if the object’s origin is in the center, and the object is high 2 meters, press G, press Z, press 1 and it will be moved up by 1 meter.

add:
the tricky part is: in blender, every object has a point called “origin”, that’s where its transformations start. If the origin is in the center of the geometry of the object, it will for example rotate around its center. If the origin is placed on a vertex, the object will rotate around that vertex.
All this applies to just the Object mode. To have the transformation don’t use the object origin, tab into edit mode and you will be able to modify the mesh parts (vertices, edges and faces) without moving the orign point.

what if the object is organic form … it doesn’t have a specific height what to do then ?

Interactive tools for blender or Originie, you can set custom origin points.

Then I think the easiest way is:
(steps to set the origin)

  • tab into edit mode
  • select the vertex where you want your object to sit
  • snap the 3d cursor to that vertex (the shortcut was shift - s in blender 2.79, no idea if it’s still the same in 2.8+).
  • go back in Object mode
  • press F3 and find Set Origin. In the next menu, select “Set origin to 3d cursor”.

(steps to set the object’s origin to the center)

  • still in object mode, shift - s -> snap selection to center