To find out how much superphosphate is in two tonnes of the mixture, we first convert the weight from tonnes to kilograms.
1 tonne is equal to 1,000 kilograms, so:
\[ 2 \text{ tonnes} = 2 \times 1,000 \text{ kg} = 2,000 \text{ kg} \]
Next, we calculate the amount of superphosphate in the mixture. Since 25% of the fertiliser is superphosphate, we can calculate it as follows:
\[ \text{Amount of superphosphate} = 25% \times \text{Total weight of the mixture} \]
This can be calculated as:
\[ \text{Amount of superphosphate} = 0.25 \times 2,000 \text{ kg} = 500 \text{ kg} \]
Therefore, there are 500 kg of superphosphate in two tonnes of the mixture.