Tertill the weeding robot works best on different types of bare soil because it enables Tertill's specially designed wheel to churn up the top layer of the soil disrupting germinating weeds at their most vulnerable state. The soil should be free from large debris such as rocks, twigs, root clumps, etc.
Here are examples of different types of bare soils that Tertill can work in:
Soil with small rocks (under 1-inch).
Note: Rake out large rocks with a leaf rake to help maximize Tertill's maneuverability.
Note that clay soil can clump up on Tertill's wheels after a rainstorm - so monitor the mud and just remove it by brushing it off if it doesn't naturally fall off as the robot moves around.