An important question about draw-call

Dear all, I wonder that what factors is the draw-call related to in U3D.
When I create a terrain with no textures. The draw-call is 1. If I raise up the textures a bit more, then the number of draw-call will add to about 40. As I know,the draw-call is related to the numbers of materials. But there is no material in the scene but just a little raised terrain that moment. So I want the reasons, thanks for help me!

I think that depending on the number of polygons it splits the Terrain between different draw calls.

Also make sure it’s marked as static.

Check this out: http://unity3d.com/support/documentation/Manual/iphone-DrawCall-Batching.html