I took your advice, ravells, and played around a bit. I first tried duplicating the grass layers procedure to see what kind of effect I'd achieve. In the end it was a wash. While I managed to create a really nice mottled dirt color map I couldn't find a way to blend the dirt and grass layers to give me a mottled effect. I stepped through all the layer blending options, overlay, dodge, burn, difference, multiply, etc. So, I went back to RobA's original dirt layers procedure and played around with the settings. I tried applying a color gradient to the dirt color layer. Well, all I accomplished there was making the overall layer quite dark and not blending well at all. In the end, I found myself playing with the color levels for the dirt layer mask and was able to produce something more akin to what I wanted. While the difference is subtle it is noticeable. The result is shown below. The key here is to save your work and then save a copy. Play around with the settings on the copy and go from there.