Doesn’t have to be. But usually the very first layer of the blend - and you also need to fill the landscape with it and paint other stuff on top.
which to me is a red flag. You pay the cost of 2 layers on everything if you look at the shader cost. I prefer artifacts to that 
Make a near empty layer with a neutral grass. Only 1 texture. And use that as the base with no blend so it’s the cheapest cost possible without using a solid color.