The Prophet (s) used to pray on earth and is also reported to have shown reverence towards the soil of Karbala in his lifetime with the foreknowledge of what would happen upon it.

So although it is not compulsory for the Shia to pray on a turba made from the soil of Karbala, it is considered meritorious to do so.

You can read more details with full references here:

https://www.al-islam.org/articles/laws-and-practices-why-do-shiah-prostrate-turbah