Weak Yew Pines (Podocarpus)

[Ryan & Carol Spicer]

Hi,

I have some Yew Pines (Podocarpus Macrophyllus) that I planted a couple
of years ago as a screen in our back yard.  They were around four or
five feet high when I planted them, they are now anywhere from seven to
ten feet high.  They have good green growth however they are kind of
thin and not filling out like I expected.  Also the limbs on some of
them hang almost straight down like they're weak or something.

I haven't put a lot of effort into this (not much of a gardener I'm
afraid).  I water them once a week and Miracle Grow about once a month. 
Any suggestions on what I might add to help these guys out?  The soil is
mostly clay by the way.  When I planted them I used some potting soil
that was around the house.  (I know, I know - I need some serious
education in this area!)

Thank you,
Ryan