you might want to create a much higher resolution map then for the ground, and dont tile the map, make some patches uneven such as darker dirt like you got their exepct dont tile it cause you can see it and it doesnt look good, take the dimension of your plane and make a map for it in photoshop but about 2-3 times bigger so you wont have to tile it and it will look good, just make sure you experiment.
If you dont know how to create a dirt map in photoshop from scratch maybe you can look for tutorials. One thing with dirt or grass that you will never see in real life is that one area of grass will not look extactly like another, dont tile scenery, its not meant to be tiled