Any compost for tomatoes in pots needs to retain moisture and also be free draining. A multipurpose compost will contain all the nutrients required for tomatoes to grow into healthy plants. To add extra nutrients, add some garden compost or well-rotted manure into the lower half of the mix and, as well as boosting nutrients, this can also help with soil structure and also boost drainage. Always ensure any manure used in the garden is well-rotted, as fresh manure can kill plants as it is too high in nitrogen and salts.