Wildflowers are not fussy, and generally do not need rich soil, fertilizers, or other soil amendments. If anything is already growing in your planting area, even if it's just grasses or weeds, the soil will be fine for wildflowers.
If you're planting in a problem area where nothing is currently growing, or where there was a chemical spill, or new construction where the topsoil was removed, you may need to amend your soil or find a new site for the meadow. Wildflowers are quite adaptable but will not grow on a sterile site. If you’re not sure about your soil, it’s easy to get a soil test with your local Extension program, or by purchasing a soil testing kit from a local or online hardware store.
Although compost or other organic fertilizers are not necessary for growing wildflowers, they certainly can be useful to enhance soil quality if you have very challenging soil. For example, if your soil is heavy clay, adding some organic matter can help loosen its structure and help it drain better to encourage better success with your meadow, since most wildflowers prefer well-drained soil.