IF you have rich top soil, fine. If not, buy some good quality potting soil, and try to find some without a lot of perlite. Hopefully, you will grow the plants thick enough that the tortoises won't be digging around. You can do it, plants have some basic needs and you provide them or you don't... The plants I listed are quite hardy. And you should consider "weeds" that grow wild in your area. Dandelions are probably the single healthiest thing I can think of for tortoises. Chicory is another easy to grow plant.