Stair stringers should always land on a level and well-drained surface. If you are landing on a concrete pad or a patio, this should not be a problem. However, if you are landing on the yard, you may want to install paver blocks or fill the area with compacted gravel to promote drainage. Make sure you are using .60 retention pressure-treated wood that is rated for ground contact.
You should install footings at the bases of the stringers to prevent frost heave. This is not always required, but we seriously recommend it. With frost footings in place, you can use a concrete anchor, base plate and short post in order to create a solid ground connection. A 3' x 3' level landing area is required at the base of all staircases. The ground may qualify if it meets the requirements.
Below are steps to create a base, if needed:
Notch out the bottom front of the center stair stringers to accept a 2x4 base plate.