I also use a 50/50 mixture of white glue and water, but no dish detergent in the glue mix. Instead, I use a large, hand operated sprayer loaded with plain tap water and 4 or 5 drops of dish detergent. Simply spray the area with this "wet" water before you apply the glue mixture. If you're using lightweight ground foam as ground cover, aim the first few spritzes of "wet" water upward, so as to not blow the foam all over the place. Once it becomes dampened by the falling mist, it will stay in place better and can be sprayed directly. And when you apply the glue, it should soak in immediately. If it beads up on the surface, you need more "wet" water. Also, don't skimp on the application of the glue mixture either, especially if you've got a thick layer of scenic materials. When ballasting track, I usually do, where required, rip-rap, coarse stone fill, cinder under-ballast, and the finished ballast layer in one application. It sometimes takes 2 or 3 days for the glue to set up, and even longer for it to fully harden. I use a small plastic applicator bottle, which allows me to work much faster than with an eyedropper. If you're going to be doing lots of this type of work, it's much cheaper to buy your white glue by the gallon, at any hardware store or home improvement centre.
Wayne