Normal map on only part of the material I'm trying to apply a normal map on a plane with two textures painted on it,
but in a way that it's only applied to the second one. It's a really simple 
scene of a crossroad going through a field (for testing the normal map).

The thing is that I can't figure out how to apply the normal map so that
it uses only the road texture and not the whole plane. I have done my researches 
about this problem with no luck, so I'd really appreciate any help.

Not sure if I'm using correct terms to describe this but here's a figure and
link to the .blend file to give you a better idea what I mean.

.blend: https://drive.google.com/folderview?id=0B4NHszZHPR_1TFlfdThBMjF6MzA&amp;usp=sharing
There were a couple of things I changed with your file, so here's my edited version: Edited .blend

First off there were a few things you did wrong with your normal map:


When using an image for the normal map you have to set the Texture node to non-color data mode.
Then, you don't plug the image texture into the Displacement input of the material node(only takes B/W data), but instead into the Normal input of your respective shader.


Now that we've covered that it becomes very easy to achieve what you want:

You plug the normal map only into your pebble shader, so that only the pebbles get bump mapping effects, and the mask you already created does the rest. For that I added a second vector input to you group nodes so that they can receive normal mapping data as well.

Also, if you are using only one UV map, a single UV map node is usually enough.