We can't get to the root (pun intended) of the problem
Each year the tree puts out new candles, they grow well, then in August parts of the tree (inner and outer needles randomly) turn brown and eventually turn to mulch. A few years ago I noticed many roots were exposed due to soil runoff, so I added a few bags of topsoil to cover the roots. I do not overwater the tree, yet I constantly monitor the root area to prevent dry conditions. Fertilizing does not seem to make any difference...some years I skip it, others I add it. This summer I built a low retaining wall and filled in with just enough dirt to insure no more erosion. Originally, the tree was not set very deep into a hole by the landscaper, but the rootball was partially covered with soil. I believe this was the beginning of the problem, but I have taken the steps outlined above to correct the erosion and bare roots yet I still get the branch/needle dieoff. I know that many fir trees will shed needles on the inner branches (like a white pine), but this is more severe and occurs in late summer each year. The new growth at the top looks good and there is a healthy leader, but overall the color is very pale and the tree si looking very sparse. I've run out of ideas...any tips greatly appreciated...i can send pics if needed.