the shader (you can look at the source in the <terrain multitexturing shader> thread) takes three textures as input.

the first texture of the terrain on the screenshot was a tillable grass texture and the second one a tillable texture with soil and some dry grass.

the third texture's alpha channel (which doesn't get tiled) specifies how the first and second texture get blended. for example if a pixel in the alpha channel has a value of 64, the shader mixes 25% of texture one and 75% of texture two together at this location.