Yellowing and leave drop in transplanted Yew
I have volunteer yews in my yard (from a very large Yew) and after they get 2-3 feet tall I transplant them. Usually there are no problems, but my last transplant, made in November, 2014 directly into healthy soil, probably sandy loam, started to show vigorous growth beginning in late Feb., but in the last weeks that new growth is yellowing and the needles dropping. The plant gets irrigated about twice a week, but doesn't typically get a good soaking. Planting preparation included composted chicken manure well mixed with soil. The plant gets morning shade and afternoon shade. Does it perhaps need more fertilizer?
Yews have few problems, but they do not like soil that does not drain. They really don't like being waterlogged. This article, Yew Problems, may give you some ideas. You can test for drainage in an area by filling a dug-out area with water and seeing how fast the water disappears. Trees should be planted shallowly in the soil, so the flair (where the trunk flairs out just above the roots) is just above the soil line. The width of the hole should be 3x that of the pot or root ball. Keep the base of the plant above the soil with just the roots buried. Since you have prepared the soil, mixing in chicken manure, that may not be the problem, unless the plant sank into the soil. Watering deeply once a week would be preferable to watering shallowly twice a week - and maybe not watering at all in winter. Two other problems yews experience are Yew Root Rot, and Yew Scale. These articles will help you decide if your yew has either of these conditions. I don't think fertilizer would help.