I would first decide what color the dirt is during the summer,dark brown, light brown, redish brown.
Farms get muddy during the spring and rainy season. I would add a thicker wash of browns near the bottom. I would use the browns to complement the dirt color. I would then put a lighter tone of pigments over the entire thing, farms and country buildings are dusty things. I would go back with some shadow washes and add extra pin washes here and there to accent high use areas. Then another coat of pigments.
One thing I would do is to touch up the edges of the outter boards. There are some light streaks coming through. There are some brown areas showing through the lighter plaster areas too. Touch those up and it will look a lot better.