For a first diorama that is looking very good. I can't think of anything more useful to add to the comments alrady posted except to maybe suggest breaking up the colour and tone of the dirt track a bit. It looks very even at the moment and so a bit unrealistic.
I have used real dirt from the garden in the past which worked really well. Dry it out completely and you will have a rich source of textures and colours at your disposal which will bring your model to life.
Have a read through this thread as well, the early pictures seem to have dissappeared, which is a shame, but the later ones are there and the text is very interesting anyway:
http://www.scale-models.co.uk/showth...=diorama+tiger