To find out how much potting soil Henry uses, we need to calculate \( \frac{1}{5} \) of \( \frac{9}{10} \) kilogram.
We can do this by multiplying the two fractions:
\[ \frac{1}{5} \times \frac{9}{10} = \frac{1 \times 9}{5 \times 10} = \frac{9}{50} \]
So, Henry uses \( \frac{9}{50} \) kilogram of potting soil to plant the roses.