The type of soil that can become as hard as stone when it dries up is clay soil.

Here's why:

* Clay particles are very small: This means they pack tightly together, leaving little space for air or water.

* Clay absorbs water: When wet, clay soil feels smooth and pliable.

* Clay shrinks when it dries: As the water evaporates, the clay particles draw closer together, becoming extremely dense and hard. This process is called shrink-swell.

This hardening effect can make it difficult for plants to grow in clay soil, as their roots struggle to penetrate the hardened surface.