I am confused about the solution to the Dirac equation and how it corresponds to left-/right-handed Weyl spinors. In Srednicki, page 242, it is stated that taking the ultrarelativistic limit ($|\bar{p}|>>m$), projects the solutions of the Dirac equation to purely left- or right-handed spinors.
From the Dirac equation: $$(p_\mu \gamma^\mu + m)u_s(\bar{p})=0$$
we get the solutions (for simplicity, I set p_x=p_y=0):
$$ u_+ = \begin{pmatrix}1 \\ 0 \\ \frac{p_z}{E+m} \\ 0 \end{pmatrix} , ~~u_- = \begin{pmatrix}0 \\ 1 \\ 0 \\ \frac{p_z}{E+m} \end{pmatrix} $$
Now, since $E=\pm \sqrt{p_z^2+m^2}$, the limit where $p_z >> m$ should give:
$$ u_+ = \begin{pmatrix}1 \\ 0 \\ 1 \\ 0 \end{pmatrix} , ~~u_- = \begin{pmatrix}0 \\ 1 \\ 0 \\ 1 \end{pmatrix} $$
but I do not understand how these two solutions are purely left- or right-handed?
In Srednicki, he showed it by using $u_s(\bar{p})\bar{u}_s(\bar{p})\rightarrow \frac{1}{2}(1+s\gamma_5)(-\gamma^\mu p_\mu)$, but should it not be possible to show the same thing by going directly from the two solutions?