已知\( \overrightarrow{a}=( \sqrt {3},-1)\),\( \overrightarrow{b}=( \dfrac {1}{2}, \dfrac { \sqrt {3}}{2})\),且存在实数\(k\)和\(t\),使得\( \overrightarrow{x}= \overrightarrow{a}+(t^{2}-3) \overrightarrow{b}\),\( \overrightarrow{y}=-k \overrightarrow{a}+t \overrightarrow{b}\),且\( \overrightarrow{x}⊥ \overrightarrow{y}\),试求\( \dfrac {k+t^{2}}{t}\)的最值.