Answer
$$1.07 \space kg$$
Work Step by Step
$$21\% \space by \space mass:\frac{21 \space g \space (nitrogen)}{100 \space g}$$
$$225 \space g \space (nitrogen) \times \frac{100 \space g}{21 \space g \space (nitrogen)} \approx 1.1 \times 10^3 \space g$$
$$m \space (fertilizer) = 1.1 \times 10^3 \space g \times \frac{1 \space kg}{1000\space g} = 1.1 \space kg$$
