MSP, for grass and plants, get various types of WS ground cover, (I use Soil, Earth, Burnt Grass and Summer Grass, sprinkled in that order with a large spoon). That gives you a varied natural look and you can vary the amounts based on your preference of color. Then I add various foliage types, in different thicknesses (fine, medium, coarse) as desired. I first spray with Matte Medium so the ground cover has something to stick to and is less likely to move around. Then sprinkle, then saturate again until it's all wet. Make sure your sprayer has a fine spray adjustment (try it with plain water to test it). Also, clean out the sprayer each time you use it so the glue doesn't clump up inside. If you use white glue, you can buy a whole gallon of it and dilute it anywhere from 1:2 to 1:4. Very inexpensive way to do it. The Matte Medium comes diluted or for a cheaper way, you can get the product that then gets diluted 1:4. It's from Scenic Express. The mountainous areas should be painted a base color (earth or something dark) so you're not trying to cover all that white with ground cover/foliage. You can dilute the paint to it's more of a wash if you like that effect. Also, you can add some alcohol/india ink mix to bring out some highlights before adding the ground cover.
For ballast, I wouldn't use a sprayer. Use squeeze bottles which are much more precise. But, first, squeeze on "wet" water (tap water with a few drops of dish detergent), then squeeze on the diluted glue. The "wet" water reduces the surface tension so the glue doesn't ball up and you'll find it flows very easily throughout the ballast wherever you've added the wet water.
Since the lower portion seems to be yards, you don't really need much grass/foliage between the tracks. You probably want to use cinders or very fine N or Z scale black ballast. Then add a little greenery on top.
Hope this helps some.
