Heard of layers? I don't know if Photoshop supports layers (never used or really seen it), but it involves using two textures. The dirt texture is one, a lower layer (grass sticks up higher and closer objects are the first seen objects), and the grass layer, the higher layer, is used on top. The best I can come up with is to make use of transparency and partially erase the grass texture layer for fading, fully erase it for dirt only, and leave it as is for grass only. If that doesn't work, I've got other tricks and shortcuts. One of which is to create a temporary, special layer with shades of gray to use as alpha, smooth it out when done, convert the shades of gray to the equivelent alpha (color to alpha with black as the color (in GIMP)), then use alpha to selection, and erase everything at full blast as needed. If that doesn't work, there's likely at least another way or two of doing so.