Mimic nature. Ballast doesn’t just come to an abrupt end and hit a perfect defining line. It makes a transition to the surrounding ground, as shown in this photo. I used some transition materials including Brennan’s Model Railroading’s superior sand in both fine and coarse textures from the end of the ballast to the ground. Lastly, I sprinkled a bit of Woodland Scenics no. T41 fine turf, which is jet black and mimics the oily residue from passing trains.
Dare to be dull. Avoid bright colors for track. Big dirty trains carrying big dirty loads run on these rails. You don’t go from tracks to a green pasture. Oil, grease, and other materials suppress the growth of greenery. Your growth near the tracks should be more burnt out and dying than lush and green. Make transition areas using various ground covers as you work your way outward from the tracks.
Avoid perfection. Always remember you are striving for realism – the way things look in the real world. Being too perfect, with a perfect ballast line, doesn’t mimic the real world. The more I worked on layout building, the less meticulous I became. In theory the work of ballasting O gauge track was done by O gauge people. Like all of us, they have off days. Be a little haphazard and your work will look more real as a result.