Rendering object outline on top only for one object

Hi, i want to show outline of an abject when it is under the ground in game. I'm using one of this shaders http://wiki.unity3d.com/index.php/Silhouette-Outlined_Diffuse but its rendering always on top and i need it on top only for ground.

Here is example which i made in photoshop to show result i'm looking for. The sphere is under the ground and box can cover sphere outline but ground can't. How can i achive this effect ? I'm very new to writing shaders.